Author Topic: Linear lab power supply (Read 101219 times)

JuanGg · « **on:** November 09, 2018, 09:23:04 pm »

I am doing the "Learning the Art of Electronics" course and I am in need for a lab power supply. All I have is a single-channel one (discussed here: https://www.eevblog.com/forum/beginners/power-supply-ripple/) and I get by with that and a couple batteries.

I have read in multiple places that a lot can be learnt from building a power supply, so I thought I'll have a go at it.

I have a couple 15V 1A transformers and a 2x 10V 2 A one that I salvaged. All were enclosed into sealed plastic cases, so I guess current ratings will be a little higher with better dissipation. Not planing on exceding them, but good to know. I also have two 6800uF 50V caps also salvaged, which I could use for the filter caps. I will buy some new ones anyway just in case.

The specs I am aiming at are:
-Two chanels (just two identical circuits on the same enclosure) at 0-15..20 V , 0-0.5 A
-Hopefully arduino controlled (PWM DAC...), although I don't mind using pots for current and voltage setting, I will just use an arduino and
some lcd / 7-segments to display voltage and current, as it's cheaper than panel meters. That would be one arduino per channel to keep both
isolated, maybe some opto- isolated communication between them so I can do tracking or whatever.
- 0.01 V and 1mA resolution.

What I thought:

I could maybe connect each 15 V transformer with each 10 V tap on the other one, so I will get two 25V ac outputs. Maybe taps could be switched depending on the set voltage (say 10 V tap for under 10V and the 25 V one for over that). However maybe it's not worth it given the max power disipation of +- 20 W.

My plan is to test some schematics that I found online and see how it works. I will be posting the results I get here.

I would like to start with the one attached, which I found here: https://www.electro-tech-online.com/threads/lm723-based-psu-with-min-voltage-of-0-00v.150216/page-3 (AGND just connected do GND).
I had read through the thread and also the voltage regulation chapter on The Art of Electronics and I think I got a light grasp on how the LM723 ic works. I will order the components that I haven't got around and test it out. I will only use one series pass transistor given the smaller current. Any ideas or suggestions are welcome.

Equipment that I have: The said power supply, an atx one for higher current stuff, a rigol 1054z scope, an UT61E multimeter, a 5200a function generator and recently an electronic load which is described here: https://www.eevblog.com/forum/projects/arduino-based-electronic-load/

EDIT: ALL FILES AND A PROJECT SUMMARY ARE AVAILABLE HERE:
Schematics, PCBs, firmware, 3d printed parts, front panel overlay... :
https://github.com/Juan-Gg/Linear-lab-bench-power-supply/tree/master
A short description:
https://juangg-projects.blogspot.com/2019/05/linear-lab-power-supply.html

Many thanks to everyone that helped me out. I hope I can give this back.
Juan
/EDIT

Mr. Scram · « **Reply #1 on:** November 09, 2018, 09:26:36 pm »

A middle ground option could be digital pots.

W2DML · « **Reply #2 on:** November 09, 2018, 09:37:21 pm »

I recently designed a 0-12V, 0-500mA high accuracy power supply that has a computer interface for my job. I would share the design but I can't,

. I recommend you look at this article for digital control of any type of regulator (linear, switching, etc.) which has resistive feedback with a DAC, might help with your design. https://www.microchip.com/forums/m688260.aspx

lordvader88 · « **Reply #3 on:** November 10, 2018, 04:12:05 am »

the LM723 is a great chip to learn from, read up on it, like I should do more of.

That design looks better than the one I've been trying to get together, I should try it too

spec · « **Reply #4 on:** November 10, 2018, 11:22:07 am »

Quote from: JuanGg on November 09, 2018, 09:23:04 pm

... I have read in multiple places that a lot can be learnt from building a power supply, so I thought I'll have a go at it...

My plan is to test some schematics that I found online and see how it works. I will be posting the results I get here.

I would like to start with the one [PSU] attached, which I found here: https://www.electro-tech-online.com/threads/lm723-based-psu-with-min-voltage-of-0-00v.150216/page-3 Any ideas or suggestions are welcome.

+ JuanGg

The wheel goes round and round

I did a lot of the design work on that PSU for the OP, so if you need to know how the various bits work, just post.

Much of the extra circuitry is there to cater for the high input voltage from the reservoir capacitors which exceeds the max VCC of the LM723.

The other complication is that the OP wanted a precision current limit rather than the standard LM723 approach and have a LED illuminate at the instant the PSU went into constant current. He also wanted the LED to have constant brightness.

My advice to you is to build a standard LM723 power supply, with standard current limiting and with no more than 35V raw voltage. Then you would only need an LM723 and three transistors: driver transistor, two power transistors.

The physical layout is critical and you must use star point techniques and thick wires for the high current paths. Whatever you do, keep the wires to the power transistors short to avoid parasitic oscillations so you need to think about the physical layout of your PSU from day 1. I have a general physical layout that achieves all this, so if you feel you would like to take that approach just shout.

Also make sure that all components are readily accessible and can be easily changed. You can gaurentee that any component that is inaccesable will give you the most problems, even a simple resistor.

Apart from good wire routing, always remember that you can never have too big a heatsink and pay particular attention to the thermal washers between the power transistors and the heatsink. Aluminum oxide have the lowest thermal resistance, but are expensive and prone to short, ceramic is good, but the best all round choice is still thin mica. Stay clear of foam, plastic etc. Use power transistors with T03 or larger cases, and look for a low thermal resistance, junction to case. The good old 2N3055s are dirt cheap and, while not the best, are not bad for this application.

The OP had a fan on his PSU to cool the heatsink. This has a big advantage.

By the way, the OP ended up with a nice PSU with 10 turn pots for the voltage and current controls.

Have fun- the OP did building that PSU.

spec · « **Reply #5 on:** November 10, 2018, 12:35:14 pm »

Just a bit of general advice:

When you are just starting out in electronics do not try to aim too high as many of us have done. Go with something relatively simple and achievable and then build on that experience.

Newbees often think that PSUs are dead simple to build and get working: they are not. Think of a PSU as a precision, high power amplifier and remember that the LM723 has a very high voltage gain and will oscillate if you give it the chance.

The other thing is to forget about the electronics until you have built the chassis, and mounted the heatsink with fan, transformer, bridge rectifier, reservoir capacitors, controls, etc, in fact, all the big items.

The next stage is to wire up the raw supply and make sure that it is working correctly under full current load (check ripple voltage). In my experience, about 60% of cases where there have been problems with stabilized PSUs, it has been due to inadequate wiring and shortcomings of the raw supply.

When you design your power supply, one fundamental thing is to do a thermal budget to ensure that you are well below the transistor's maximum junction temperature and also check the transistors safe operating area graphs on the datasheet.

The other fundamental calculation is to do a voltage overhead budget, not forgetting ripple voltage.

The worst thing you could do for a prototype is to have bits of circuit lying around on the bench and connected by long wires.

In conclusion, do not let my lecturing put you off- with the correct approach you can build a very nice lab power supply that will last you for years. And non of the procedures that I have described are difficult. Also, there are many experienced EEV members who I am sure would be glad to advise you.

JuanGg · « **Reply #6 on:** November 10, 2018, 11:35:59 pm »

First off all, thank you everyone for answering. This is really valuable to me and I am and will be learning a lot.

Quote

A middle ground option could be digital pots.

Could be. I will try getting the thing working with regular pots, then add the microcontroller.

Quote

I recently designed a 0-12V, 0-500mA high accuracy power supply that has a computer interface for my job. I would share the design but I can't, . I recommend you look at this article for digital control of any type of regulator (linear, switching, etc.) which has resistive feedback with a DAC, might help with your design. https://www.microchip.com/forums/m688260.aspx

That article was interesting indeed. Will take note of that.

Quote

The other complication is that the OP wanted a precision current limit rather than the standard LM723 approach and have a LED illuminate at the instant the PSU went into constant current. He also wanted the LED to have constant brightness.

My advice to you is to build a standard LM723 power supply, with standard current limiting and with no more than 35V raw voltage. Then you would only need an LM723 and three transistors: driver transistor, two power transistors.

I am more that happy with standard current limiting and no led indicator.
With 25 Vac from the transformer that would be 25*1.41 = 35 V minus a couple volts drop on the rectifier bridge, that would be 33V raw voltage.

Quote

The physical layout is critical and you must use star point techniques and thick wires for the high current paths. Whatever you do, keep the wires to the power transistors short to avoid parasitic oscillations so you need to think about the physical layout of your PSU from day 1. I have a general physical layout that achieves all this, so if you feel you would like to take that approach just shout.

Also make sure that all components are readily accessible and can be easily changed. You can gaurentee that any component that is inaccesable will give you the most problems, even a simple resistor.

I have attached a photo of the layout I am thinking off, just the size of an A4 page. Heatsinks (probably pc ones, which I have access to) and a transformer are missing, and I only "populated" one side with random components just to make provision for the board. Transformers are put in a way that evenly distributes the weight.
I am thinking of 3d printing front and back panels, joined together with aluminum profiles, to which I can attach components to in a way I can chage things easily inside later on. A sheet metal cover would complete the enclosure. I am glad to hear any suggestion.

Quote

Apart from good wire routing, always remember that you can never have too big a heatsink and pay particular attention to the thermal washers between the power transistors and the heatsink. Aluminum oxide have the lowest thermal resistance, but are expensive and prone to short, ceramic is good, but the best all round choice is still thin mica. Stay clear of foam, plastic etc. Use power transistors with T03 or larger cases, and look for a low thermal resistance, junction to case. The good old 2N3055s are dirt cheap and, while not the best, are not bad for this application.

The OP had a fan on his PSU to cool the heatsink. This has a big advantage.

I was thinking of using two separate heatsinks, one per chanel, so I could bolt directly the power transistors to them, no thermal washer needed as they would be inside the case, just some thermal paste. I have 5 3055 in TO-220 packages, will those be good enough or do I buy TO-3 ones? (not expensive at all, so I will probably get some anyway). I intend to use one temperature controlled (via the micro) fan per chanel.

Quote

By the way, the OP ended up with a nice PSU with 10 turn pots for the voltage and current controls.

Have fun- the OP did building that PSU.

I hope I will!

Quote

When you are just starting out in electronics do not try to aim too high as many of us have done. Go with something relatively simple and achievable and then build on that experience.

Newbees often think that PSUs are dead simple to build and get working: they are not. Think of a PSU as a precision, high power amplifier and remember that the LM723 has a very high voltage gain and will oscillate if you give it the chance.

The other thing is to forget about the electronics until you have built the chassis, and mounted the heatsink with fan, transformer, bridge rectifier, reservoir capacitors, controls, etc, in fact, all the big items.

The next stage is to wire up the raw supply and make sure that it is working correctly under full current load (check ripple voltage). In my experience, about 60% of cases where there have been problems with stabilized PSUs, it has been due to inadequate wiring and shortcomings of the raw supply.

I will try to make something simple enough, that can be later upgraded when I know better what I'm doing.
I will build up the enclosure and raw supply, and test it, leaving provision to add the remaining circuitry later, so I can test designs and make changes easily.

Quote

When you design your power supply, one fundamental thing is to do a thermal budget to ensure that you are well below the transistor's maximum junction temperature and also check the transistors safe operating area graphs on the datasheet.

Will take that into account.

Quote

The other fundamental calculation is to do a voltage overhead budget, not forgetting ripple voltage.

The worst thing you could do for a prototype is to have bits of circuit lying around on the bench and connected by long wires.

With 33 V raw voltage input, a 0-25 V -ish will leave plenty of overhead, even with substantial ripple. As I said, I will first build the enclosure so I can test stuff on it.

Quote

In conclusion, do not let my lecturing put you off- with the correct approach you can build a very nice lab power supply that will last you for years. And non of the procedures that I have described are difficult. Also, there are many experienced EEV members who I am sure would be glad to advise you.

Thank you very much for taking the time to answer and for all the valuable advice and sorry for not responding properly, it's late over here.

Juan

spec · « **Reply #7 on:** November 11, 2018, 04:11:24 am »

Excellent replies: you have a good solid engineering approach.

Mr. Scram · « **Reply #8 on:** November 11, 2018, 02:33:47 pm »

Quote from: spec on November 10, 2018, 12:35:14 pm

Just a bit of general advice:

When you are just starting out in electronics do not try to aim too high as many of us have done. Go with something relatively simple and achievable and then build on that experience.

Newbees often think that PSUs are dead simple to build and get working: they are not. Think of a PSU as a precision, high power amplifier and remember that the LM723 has a very high voltage gain and will oscillate if you give it the chance.

The other thing is to forget about the electronics until you have built the chassis, and mounted the heatsink with fan, transformer, bridge rectifier, reservoir capacitors, controls, etc, in fact, all the big items.

The next stage is to wire up the raw supply and make sure that it is working correctly under full current load (check ripple voltage). In my experience, about 60% of cases where there have been problems with stabilized PSUs, it has been due to inadequate wiring and shortcomings of the raw supply.

When you design your power supply, one fundamental thing is to do a thermal budget to ensure that you are well below the transistor's maximum junction temperature and also check the transistors safe operating area graphs on the datasheet.

The other fundamental calculation is to do a voltage overhead budget, not forgetting ripple voltage.

The worst thing you could do for a prototype is to have bits of circuit lying around on the bench and connected by long wires.

In conclusion, do not let my lecturing put you off- with the correct approach you can build a very nice lab power supply that will last you for years. And non of the procedures that I have described are difficult. Also, there are many experienced EEV members who I am sure would be glad to advise you.

Aren't your warnings exactly why it's a good first project? It looks simple, yet it isn't simple and you'll learn lots. Failing is part of the process.

spec · « **Reply #9 on:** November 12, 2018, 07:44:29 pm »

Quote from: Mr. Scram on November 11, 2018, 02:33:47 pm

Quote from: spec on November 10, 2018, 12:35:14 pm
Just a bit of general advice:

When you are just starting out in electronics do not try to aim too high as many of us have done. Go with something relatively simple and achievable and then build on that experience.

Newbees often think that PSUs are dead simple to build and get working: they are not. Think of a PSU as a precision, high power amplifier and remember that the LM723 has a very high voltage gain and will oscillate if you give it the chance.

The other thing is to forget about the electronics until you have built the chassis, and mounted the heatsink with fan, transformer, bridge rectifier, reservoir capacitors, controls, etc, in fact, all the big items.

The next stage is to wire up the raw supply and make sure that it is working correctly under full current load (check ripple voltage). In my experience, about 60% of cases where there have been problems with stabilized PSUs, it has been due to inadequate wiring and shortcomings of the raw supply.

When you design your power supply, one fundamental thing is to do a thermal budget to ensure that you are well below the transistor's maximum junction temperature and also check the transistors safe operating area graphs on the datasheet.

The other fundamental calculation is to do a voltage overhead budget, not forgetting ripple voltage.

The worst thing you could do for a prototype is to have bits of circuit lying around on the bench and connected by long wires.

In conclusion, do not let my lecturing put you off- with the correct approach you can build a very nice lab power supply that will last you for years. And non of the procedures that I have described are difficult. Also, there are many experienced EEV members who I am sure would be glad to advise you.
Aren't your warnings exactly why it's a good first project? It looks simple, yet it isn't simple and you'll learn lots. Failing is part of the process.

It is all a matter of degree

Yes, you can learn by failure, but on the other hand, you can also be discouraged and learn nothing. It depends on your character. It also depends on your objectives: just learn, or learn and end up with a low-cost lab power supply to use for experiments. And don't forget, if the basic construction is good you can always add advanced features, like precision current control and digital meters, for example.

That is a complex power supply and the chap who built it, who was an engineer, expended quite a lot of effort to get it working.

The engineering department where I worked used to mentor the under grads and apprentices. Over the years probably 1000 went through the system, and maybe 250 started their own projects, mainly audio amplifiers but some power supplies. I think out of all those starters there was only about five audio amps completed, and they were Texan kits and no power supplies to speak off.

There are five gotchas with any power supply: star point wiring, frequency stability, max junction temperature, SOA, voltage overhead. If you learn about all of those you will have achieved an awful lot. But that is just the electronics. There is also the mechanical side: heatsink, chassis, controls, meters, fuse holder, etc etc.

So, I would advise anyone just starting in electronics to aim at the achievable, and as I said, that would be a straight forward LM723 type PSU. And, believe me, that is plenty enough of a challenge and a valuable learning experience.

Wolfgang · « **Reply #10 on:** November 13, 2018, 01:50:57 am »

Quote from: JuanGg on November 09, 2018, 09:23:04 pm

I am doing the "Learning the Art of Electronics" course and I am in need for a lab power supply. All I have is a single-channel one (discussed here: https://www.eevblog.com/forum/beginners/power-supply-ripple/) and I get by with that and a couple batteries.

I have read in multiple places that a lot can be learnt from building a power supply, so I thought I'll have a go at it.

I have a couple 15V 1A transformers and a 2x 10V 2 A one that I salvaged. All were enclosed into sealed plastic cases, so I guess current ratings will be a little higher with better dissipation. Not planing on exceding them, but good to know. I also have two 6800uF 50V caps also salvaged, which I could use for the filter caps. I will buy some new ones anyway just in case.

The specs I am aiming at are:
-Two chanels (just two identical circuits on the same enclosure) at 0-15..20 V , 0-0.5 A
-Hopefully arduino controlled (PWM DAC...), although I don't mind using pots for current and voltage setting, I will just use an arduino and
some lcd / 7-segments to display voltage and current, as it's cheaper than panel meters. That would be one arduino per channel to keep both
isolated, maybe some opto- isolated communication between them so I can do tracking or whatever.
- 0.01 V and 1mA resolution.

What I thought:

I could maybe connect each 15 V transformer with each 10 V tap on the other one, so I will get two 25V ac outputs. Maybe taps could be switched depending on the set voltage (say 10 V tap for under 10V and the 25 V one for over that). However maybe it's not worth it given the max power disipation of +- 20 W.

My plan is to test some schematics that I found online and see how it works. I will be posting the results I get here.

I would like to start with the one attached, which I found here: https://www.electro-tech-online.com/threads/lm723-based-psu-with-min-voltage-of-0-00v.150216/page-3 (AGND just connected do GND).
I had read through the thread and also the voltage regulation chapter on The Art of Electronics and I think I got a light grasp on how the LM723 ic works. I will order the components that I haven't got around and test it out. I will only use one series pass transistor given the smaller current. Any ideas or suggestions are welcome.

Equipment that I have: The said power supply, an atx one for higher current stuff, a rigol 1054z scope, an UT61E multimeter, a 5200a function generator and recently an electronic load which is described here: https://www.eevblog.com/forum/projects/arduino-based-electronic-load/

Hi, welcome to the world of PSUs !

I have some comments to make about the design:

- much of your circuitry is busy to create a "mini"-PSU for the LM723, taking care to not exceed the 40V permanent 723 supply voltage.
This part could be handled by a single 317HV regulator, with less components and safe input up to 60V.
- Another alternative would be a floating design (see 723 datasheets or appnotes). This would not need special precautions,
you could implement better current sensing and the design is not voltage limited. To make this work well, however, the
LM723 needs a little stabilized Vc supply, best derived from a small PCB transformer and an 7812 or 7815 chip.
- Current limiting will work, but is very inexact due to the temperature and manufacturing tolerances of the 723s current
sense transistor. You could sense this by a real op-amp and then use the FC pin to shutdown the 723 in case of overcurrent.
The current control loop also needs frequency compensation.

If you want, some 723 circuits can be found here:

https://electronicprojectsforfun.wordpress.com/power-supplies/a-collection-of-proper-design-practices-using-the-lm723-ic-regulator/

and some silly applications here:

https://electronicprojectsforfun.wordpress.com/silly-circuits/silly-circuits-a-heated-lm723-reference/

Have fun. Dont be scared off by experimenting or by committing errors. If you dont make errors, you will learn very slowwwwly.

spec · « **Reply #11 on:** November 13, 2018, 03:00:43 am »

Quote from: Wolfgang on November 13, 2018, 01:50:57 am

I have some comments to make about the design:

- much of your circuitry is busy to create a "mini"-PSU for the LM723, taking care to not exceed the 40V permanent 723 supply voltage.
This part could be handled by a single 317HV regulator, with less components and safe input up to 60V.
- Another alternative would be a floating design (see 723 datasheets or appnotes). This would not need special precautions,
you could implement better current sensing and the design is not voltage limited. To make this work well, however, the
LM723 needs a little stabilized Vc supply, best derived from a small PCB transformer and an 7812 or 7815 chip.
- Current limiting will work, but is very inexact due to the temperature and manufacturing tolerances of the 723s current
sense transistor. You could sense this by a real op-amp and then use the FC pin to shutdown the 723 in case of overcurrent.
The current control loop also needs frequency compensation.

If you want, some 723 circuits can be found here:

https://electronicprojectsforfun.wordpress.com/power-supplies/a-collection-of-proper-design-practices-using-the-lm723-ic-regulator/

and some silly applications here:

https://electronicprojectsforfun.wordpress.com/silly-circuits/silly-circuits-a-heated-lm723-reference/

Have fun. Dont be scared off by experimenting or by committing errors. If you don't make errors, you will learn very slowwwwly.

Hmm, some interesting comments. I was not able to find any PSU schematics in the two links that you posted- just LM723 general comments and application notes. There is a world of difference between an outline schematic, like you see in application notes, and a real working PSU.

Good idea to use the LM317HV but there are still some design issues and, from what you say, to get the performance you would end up with more complexity with extra supplies mains transformers etc. But you obviously have some experience of PSU design, so would you please post a schematic with all the features that you mention. I am talking about a practical design with decoupling etc. I would be genuinely interested to see your design.

Just to put the design in context: there were a few givens- perhaps take a look at the thread on ETO. The first is that the PSU was already built but not operating to the OP's satisfaction. The second thing is that you have over simplified the purpose of the 'busy' components which are not just there to cater for the LM723 40V Vcc limit. And finally the schematic is difficult to follow and thus difficult to understand so looks more complicated than it is. I have a rationalised schematic but the OP wanted to use his own version.

There is one problem with the posted PSU: it will not go to precisely 0V, as there is no mechanism for getting rid of the leakage current from the power transistors. I did a mod to fix that, but the OP was quite happy without it.

You over emphasize the the shortcomings of the LM723 current limit- if you look at the data sheet you will see that the tempco is -1.8mV/degC in 650mV = -0.3% degC, and that is adequate for a general purpose bench power supply, as thousands of LM723 user will testify.

As for bench power supplies in general, and given a free hand, I certainly would not use an LM723, there are much better approaches these days.

Finally, you overstate the benefit for making errors. Good design is not based on stumbling from one error to another. I would say that you learn best by taking a logical and thorough approach and learning by your successes rather than failures. And many people just make the same mistake over and over again without learning a thing.

But perhaps you are thinking about the saying, 'Anyone who has never made a mistake, never made anything'

m3vuv · « **Reply #12 on:** November 13, 2018, 08:16:18 am »

Hi has anyone a schematic for a SG3532 based psu?,i cant find any online,cheers m3vuv.

spec · « **Reply #13 on:** November 13, 2018, 09:17:34 am »

+ m3vuv

The SG3532 is essentially a greatly improved version of the LM723.

Because the SG3532 has the same architecture as the LM723, with a few changes here and there, you can use the SG3532 in most LM723 circuits. The main differences are a Vref of 2.5V compared to 7.15V, current limit sense voltage of 80mv rather than 650mV. Also the SG3532 has a shutdown input, which the LM723 does not have: simply disable it by connecting it to V-

The datasheet for the SG3532, which includes some PSU circuits, is attached below:

By the way, you do know that the SG3532 is obsolete (unfortunately).

m3vuv · « **Reply #14 on:** November 13, 2018, 07:31:44 pm »

yes i know its obsolete,are still available on ebay tho,the lower current sense voltage is the reason for needing one as can use a lower value current sense resistor for low current outputs.

JuanGg · « **Reply #15 on:** November 13, 2018, 08:11:53 pm »

I can't possibly respond to every single point stated, so here is a summary:

-I am a 17 year-old with next to none electronics design experience, have been playing around with breadboards, soldering iron and multimeter since I can remember, but no actual knowledge of what I am doing, hence I want to learn as much as possible. What I have some experience on is CAD (LibreCAD, FreeCAD and Fusion 360), 3d printing and coding (here is some stuff I've done: https://www.thingiverse.com/JuanGg/designs) . I am not too worried about the mechanical aspect of the power supply, I should be able to work it out.

-I want to build my own power supply because I need one and looks like there is lots to learn from it. I do not mind making mistakes and letting the magic smoke out if that teaches me something. However, I would rather do things properly, knowing what I'm doing (and the magic smoke will come out anyways...).

-I don't have any preference for the LM723 ic (although I have some on the way), just what I found online and in The Art of Electronics. I'll go for whatever you guys recommend.

-Again, the specs I am aiming at are:
Two chanels (identical separate circuits) at 0 - 20...25 ish V and 0.5 A
I would like to go down to 0 V, if possible.
Some form of adjustable current limiting, doesn't need to be precise.
Be able to controll it from a DAC, although I am ok with potentiometers if it makes it simpler.

-I have two 15 Vac 1 A and one 2x 10 V 2A transformers. I was thinking of connecting them in series so I get two isolated taps at 25 Vac, one per chanel. That would be 25 * sqrt(2) = around 35 V, minus a couple diode drops, input to the circuit.

-As spec suggested, I will start building the enclosure and the raw power supply. That is going to take me weeks for components to arrive and to design and manufacture the thing. In the meantime, some of the schematic design can be done. I will go over the linked application notes and try to make sense out of it.

Again, thank you all for the replies.

JuanGg · « **Reply #16 on:** November 14, 2018, 03:10:53 pm »

I read spec's last reply and it suddenly vanished so I can't quote it.

Don't be sorry, it's me the one that should be for not being clear. I will gladly reply to any questions asked.

What I want is two isolated supplies on the same case, no common ground, so I can connect them as I need to.

Thanks again

spec · « **Reply #17 on:** November 14, 2018, 03:34:08 pm »

I deleted the last post because I realized that you had already given the answer to my questions

But thanks for confirming any way.

Another post underway.

spec · « **Reply #18 on:** November 14, 2018, 03:42:37 pm »

Quote from: JuanGg on November 13, 2018, 08:11:53 pm

-I don't have any preference for the LM723 ic (although I have some on the way), just what I found online and in The Art of Electronics. I'll go for whatever you guys recommend.

-Again, the specs I am aiming at are:
Two chanels (identical separate circuits) at 0 - 20...25 ish V and 0.5 A
I would like to go down to 0 V, if possible.
Some form of adjustable current limiting, doesn't need to be precise.
Be able to controll it from a DAC, although I am ok with potentiometers if it makes it simpler.

-I have two 15 Vac 1 A and one 2x 10 V 2A transformers. I was thinking of connecting them in series so I get two isolated taps at 25 Vac, one per channel. That would be 25 * sqrt(2) = around 35 V, minus a couple diode drops, input to the circuit.

That configuration of the three transformers to give two isolated rectified raw DC supplies is optimum, I would say, given the transformers that you have.

I would suggest that, once the metalwork is complete, wire up just one of the transformer/rectifier/ reservoir circuits and test just one raw supply. That will be a major step forward in your PSU build: hopefully without any blue smoke

Best, to complete just one of the stabilized supplies, then once that is working to your satisfaction start on the second PSU circuit. That way you will have just one ball in the air, rather than two.

I would aim for 1 amp output current and a ripple voltage of no more than 2V peak to peak, so a 4700uF, or higher, reservoir capacitors will be required with a working voltage of 50V or higher. The reservoir capacitors must be able to handle the ripple current, so they will be fairly large. You would need a minimum ripple current rating of 1.8A or greater.

You can work out the value of reservoir capacitor (in Farads) for yourself if you like, by using the following formula: C=IT/Vripple, where I is the current drain (in amps), T is the half period time of your mains supply (in seconds) (10ms for Spain on 50Hz), and Vripple is the peak to peak ripple voltage (in volts). This formula is approximate, but it is close enough for this type of PSU.

When the raw supplies are built and tested, they will be suitable for almost any stabilizing circuit, vanilla or strawberry, so they will be a valuable asset in your electronic adventures.

Hopefully, that covers the raw supplies. I will post some thoughts about the stabilizing circuits, maybe in a couple of hours time.

Kleinstein · « **Reply #19 on:** November 14, 2018, 04:33:10 pm »

With these 3 transformers it might get a little tricky to get 2 separate identical supplies. I would more look at 2 slightly different versions: one using the 2x10 V transformer - this could end up at some 0-18 V at 1 A max. The other would than used the two 15 V transformers and could deliver up to 25,maybe 30 V, but only 0.5 A or 12 V up to 1A. Due to the filtering and not so good power factor one can expect to get a maximum DC current of about 50-65% of the AC rating.

There are some lab supply kits available like these:
https://ru.aliexpress.com/item/2016-NEW-Free-Shipping-Red-0-30V-2mA-3A-Continuously-Adjustable-DC-Regulated-Power-Supply-DIY/32660068947.html
The original plan circuit has some weak points / bugs, but it is possible to fix the worst ones (especially not using a 24 V transformer and lower current limit at some 1-1.5 A). With a few changes the circuit can give a reasonable lab supply. There are already some threads here on this type of circuit.

A LM723 circuit is also possible, but usually has a current limit that is not really accurate. Current adjustment over a large span can be tricky and might need switching the shunt. If one can live with this slight limitation it's also a possible path.

JuanGg · « **Reply #20 on:** November 14, 2018, 05:28:07 pm »

Quote

That configuration of the three transformers to give two isolated rectified raw DC supplies is optimum, I would say, given the transformers that you have.

I would suggest that, once the metalwork is complete, wire up just one of the transformer/rectifier/ reservoir circuits and test just one raw supply. That will be a major step forward in your PSU build: hopefully without any blue smoke

Best, to complete just one of the stabilized supplies, then once that is working to your satisfaction start on the second PSU circuit. That way you will have just one ball in the air, rather than two. ...

That is what I was thinking of, I will first build one raw supply, test it, then build one regulated supply on matrix board, test it, throubleshoot it...
If that works out, I may get some PCB's done or etch them myself, two identical ones.

According to Learning the art of electronics, I calculated the capacitor value just as you suggested, but aiming for 1 V ripple (as was in the example) This resulted in 10000 uF cap value. I have two 6800 uF 50 V caps that I salvaged I could use (It says CE Japan on them, no clue on the datasheet). Anyway, I went and bought a couple 10000 uF 50V Rubicon MXR caps of ebay (not sure if they are legit...). According to the datasheet, they have a 2.6 A ripple current rating. I also got some 8 A bridge rectifiers, LM723s and some misc stuff.

Quote

With these 3 transformers it might get a little tricky to get 2 separate identical supplies. I would more look at 2 slightly different versions: one using the 2x10 V transformer - this could end up at some 0-18 V at 1 A max. The other would than used the two 15 V transformers and could deliver up to 25,maybe 30 V, but only 0.5 A or 12 V up to 1A. Due to the filtering and not so good power factor one can expect to get a maximum DC current of about 50-65% of the AC rating. ...

I thought about doing what you propose, but I prefer having two identical ones for now, I can always change that later. Why would it be tricky to get 2 identical supplies?
I'll have a look at that kit's threads, as I said, anything that fits the purpose will be fine. However, I would like to build the thing myself, not to buy a pre-made kit. Again, I don't mind crude current limiting.

Juan

Kleinstein · « **Reply #21 on:** November 14, 2018, 06:05:15 pm »

Having a series connection of a 10 V 2 A turn and a 15 V 1A transformer limits the current to 1 A AC and thus some 0.5-0.6 A DC.
It also needs the 3 transformers to be use together, as opposed to 2 totally separate units.

With a very large cap it would be closer to the lower value. Usually one has to accept a little more ripple, more like 5-10%.
I would consider on of the 6800 µF caps plenty for 1 A.

The circuits could be still essentially the same (e.g. same board), just a higher possible current limit for the lower voltage one, or a lower maximum voltage if the 15 V transformers in parallel.

I can understand of wanting to build the supply without a kit. However the general type of circuit would be used as a starting point. It is a little odd in some aspects, but also has some good points.

rstofer · « **Reply #22 on:** November 14, 2018, 08:47:40 pm »

There certainly seem to be some naysayers! Add me to the list... Longer explanation deleted except to note that the probability of success likely decreases exponentially with parts count.

And give up the idea that it's easy to cut a fat hog and produce a $500 supply for a buck ninety-eight.

ArthurDent · « **Reply #23 on:** November 14, 2018, 08:48:43 pm »

JuanGg - I followed your first power supply project with interest and although I haven't commented on this one yet, I'm watching it as well. There are a lot of suggestions from posters with many different points of view and you've done a good job of evaluating the pros and cons of the suggestions to see how they would work for what parts you have to work with.

You're making good progress.

spec · « **Reply #24 on:** November 14, 2018, 09:08:28 pm »

+ JuanGg

Attached is an outline schematic of your proposed three transformer, two power supply circuit. Could you check it to ensure that it reflects what you intend? Don't worry about the component values etc, just the architecture.

I do not know how much electronics knowledge you have, but you seem to know a lot more than I first thought. So please bear with me while I go through some points which are essential to get an accurate and stable PSU.

Just like audio power amplifiers, it is vital to connect components in the correct order and to use star points. A star point is a point of reference for certain functions in a circuit. These are indicated by SP # 1-1 etc. Also indicated by the thickness of the lines is an indication of the gauge of wire. A thick line indicated heavy current which requires low resistance wire. This is not for current handling though, it is to limit the voltage drops due to the current flowing.

I would suggest then that when you wire up the raw DC supply circuit that you follow the the same routing indicated on the schematic. The importance of the component order and the star points will be more evident when the stabilizing circuits are in place. Star point 1 is the most important as it is the voltage reference point for the whole PSU circuit.

I have also attached another schematic which illustrates the way that ordering and star points are used in a complete power supply. It also indicates the importance of decoupling (schematic is currently work in progress for another application).

JuanGg · « **Reply #25 on:** November 14, 2018, 09:26:09 pm »

Quote

Having a series connection of a 10 V 2 A turn and a 15 V 1A transformer limits the current to 1 A AC and thus some 0.5-0.6 A DC.
It also needs the 3 transformers to be use together, as opposed to 2 totally separate units. ...

I know, I've been thinking about it. I don't really need much curent, 0.5 A would be plenty. Plus, I don't have much space in my 'lab', so I would prefer a compact solution. And I am running out of power outlets! It seems that you can never have too many.

Quote

With a very large cap it would be closer to the lower value. Usually one has to accept a little more ripple, more like 5-10%.
I would consider on of the 6800 µF caps plenty for 1 A.

What do you mean "closer to the lower value", closer to 0.5 A than to 0.6 A ? I will use the 6800 uF ones I have.

Quote

The circuits could be still essentially the same (e.g. same board), just a higher possible current limit for the lower voltage one, or a lower maximum voltage if the 15 V transformers in parallel.

I can understand of wanting to build the supply without a kit. However the general type of circuit would be used as a starting point. It is a little odd in some aspects, but also has some good points.

Sure, just maybe a couple different component values between boards. That one came from here : http://www.electronics-lab.com/project/0-30-vdc-stabilized-power-supply-with-current-control-0-002-3-a/ didn't it?

Quote

There certainly seem to be some naysayers! Add me to the list... Longer explanation deleted except to note that the probability of success likely decreases exponentially with parts count.

And give up the idea that it's easy to cut a fat hog and produce a $500 supply for a buck ninety-eight.

Constructive criticism is always welcome! I do understand that. I am not aiming for a high-end supply nor for building it for free.

Quote

JuanGg - I followed your first power supply project with interest and although I haven't commented on this one yet, I'm watching it as well. There are a lot of suggestions from posters with many different points of view and you've done a good job of evaluating the pros and cons of the suggestions to see how they would work for what parts you have to work with.

You're making good progress.

Thanks! Good to see you here. I have the other supply on hold, it works for what I need for now in its current state...

Quote

Attached is an outline schematic of your proposed three transformer, two power supply circuit. Could you check it to ensure that it reflects what you intend? Don't worry about the component values etc, just the architecture...

Yes, that is precisely what I thought. As I said, I have not much electronics knowledge, just what I have adquired as I had the need to implement into my rather simple projects. Any explanations are welcome.

I really appreciate you taking the time to do this. I am surprised at the help I am receiveing. Thank you.

spec · « **Reply #26 on:** November 15, 2018, 12:15:40 pm »

Quote from: JuanGg on November 14, 2018, 09:26:09 pm

Quote
Attached is an outline schematic of your proposed three transformer, two power supply circuit. Could you check it to ensure that it reflects what you intend? Don't worry about the component values etc, just the architecture...

Yes, that is precisely what I thought. As I said, I have not much electronics knowledge, just what I have acquired as I had the need to implement into my projects. Any explanations are welcome.

I really appreciate you taking the time to do this. I am surprised at the help I am receiving. Thank you.

No sweat

It is good that the circuit for the raw supply is now pretty much defined and that the overall architecture of the twin PSU is also defined. That is the first major milestone in the design.

It is also good that you respond quickly and intelligently to the various posts- it is not always the case on all threads

There is one point that I forget to mention: heatsinks. I would suggest the lowest thermal resistance heatsink you can get. Also low thermal resistance insulating washers, with clamps on the power transistor/regulator case. I would also advise fan cooling.

The insulating washer has a surprisingly negative impact, so one approach is to mount directly to the heatsink. This does mean that the heatsink will be at the same voltage as the power component so the heatsink would need to be insulated from the other components and chassis/PSU case.

Another problem with a live heatsink is that it acts a huge capacitor and can cause parasitic oscillations by coupling to other components. With a bit of simple screening this is easy to fix though.

About the stabilizing circuit, here, as I see it, are the four options.
[1] LM723 based

[2] Three terminal regulator based

[3] Design available from books, internet etc

[4] Custom

LM723 BASED
The LM723 is a great PSU building block, but it has some limitations which mean that a LAB PSU using the LM723 gets complicated. Going down to 0V requires extra components. The voltage overhead is high, the absolute voltage is limited and the current limiting is basic, but probably adequate.

THREE TERMINAL REGULATOR BASED
This is the simplest and quickest approach, if the standard non-adjustable current/temperature limiting is acceptable. As you can only dissipate about 12W in a LM317 or LM337 (with a good thermal washer, case clamp, and high performance heatsink + fan cooling), before the current limiting kicks in. The PSU would produce around 1A with an input/output voltage difference (IOD) of about 12V, but as the IOD increased the current output would limit proportionally reducing to 400mA at 0V output. I can probably post a schematic of this approach if you like.

Of course you could select a 10A regulator which would give a touch more current but not that much more because of the same thermal factors.

DESIGN AVAILABLE FROM INTERNET ETC
There are 100s of designs available, but I have not been able to find one that is well proven or simple. Maybe the other EEC members could advise here.

CUSTOM
This approach has been suggested in other EEV threads about PSU construction. I think I have read them all. The general feeling is that this approach has the potential for the best performance with the simplest circuit. It promises precise voltage control, precise current control, and a simple technique to allow the output voltage range to include 0V. Overall it is probably the cheapest approach too.

Out of interest, I outlined a circuit which should do the job (famous last words). If you are at all interested in this approach I could post the schematic for you to consider. One big factor to take into account with new designs, that have not been prototyped and tested is that they may not work at first and may need some development work.

So there it is. Could you give the approach some thought and post what you think?

And finally a lecture about protection. Many of the circuits that you see around have no protection and are not worst case designs. There is often no reverse voltage protection, no protection for the emitter/base junction of the output transistor, and no over-current or over-voltage protection on the input. How do you feel about protection? Some protection can be bolted on later, but some needs to be incorporated at the architecture stage.

ArthurDent · « **Reply #27 on:** November 15, 2018, 03:37:09 pm »

Quite often the simplest circuits work well enough to cover almost all your needs. Here is a 0-30VDC/1A uA723 circuit I found that looks pretty good. You could scale it for other voltages or just use it as a starting point for your own design. They use a fairly simple circuit to generate the necessary negative voltage the uA723 needs to allow the output to go down to zero volts.

As has been pointed out trying to use common grounding points is a good idea but if you are using adequate size wire at 1A any voltage errors caused by drops along a lead probably won't be significant compared to temperature drift, etc.. If you are using an adjustable current limit where you have sub-ohm sensing resistors with higher currents that's where you really have to be careful about adding stray lead resistance that could have a large effect on your readings. Some of the better power supplies use 4-terminal resistors to get higher precision current adjustments.

I might use a larger filter capacitor than the 1000/63 shown in the attached schematic but one of the previous circuits showed 4x4700 used which I believe to be overkill. All regulators also acts as capacitance multipliers by a factor of the combined hfe of the series transistors and that allow you to use smaller capacitors.

Wolfgang · « **Reply #28 on:** November 15, 2018, 03:40:52 pm »

Quote from: ArthurDent on November 15, 2018, 03:37:09 pm

Quite often the simplest circuits work well enough to cover almost all your needs. Here is a 0-30VDC/1A uA723 circuit I found that looks pretty good. You could scale it for other voltages or just use it as a starting point for your own design. They use a fairly simple circuit to generate the necessary negative voltage the uA723 needs to allow the output to go down to zero volts.

As has been pointed out trying to use common grounding points is a good idea but if you are using adequate size wire at 1A any voltage errors caused by drops along a lead probably won't be significant compared to temperature drift, etc.. If you are using an adjustable current limit where you have sub-ohm sensing resistors with higher currents that's where you really have to be careful about adding stray lead resistance that will could have a large effect on your readings. Some of the better power supplies use 4-terminal resistors to get higher precision current adjustments.

.. you could further simplify this by using an 7905 regulator for the negative rail. It would also improve stability.

JuanGg · « **Reply #29 on:** November 15, 2018, 05:35:40 pm »

Quote

No sweat

It is good that the circuit for the raw supply is now pretty much defined and that the overall architecture of the twin PSU is also defined. That is the first major milestone in the design.

It is also good that you respond quickly and intelligently to the various posts- it is not always the case on all threads

There is one point that I forget to mention: heatsinks. I would suggest the lowest thermal resistance heatsink you can get. Also low thermal resistance insulating washers, with clamps on the power transistor/regulator case. I would also advise fan cooling. ...

It would be unwise not to reply quickly, as I am the first one interested in getting this thing working

I was thinking of using CPU cooling heatsinks, which usually have a fan attached to. I think the easisest approach is no thermal washers and mount them inside the case. I can esily make some plastic brackets for that purpose. (abs, so they won't melt, even if the heatsink is at 100 ºC, which it shouldn't be...). Insulating washers can be added later on if needed. I will hopefully have news about the heatsinks in a couple days time.

On the stabilizing circuit, as you present it, the "Custom" option seems the best, if it delivers good performance on a simple circuit, and thus more likely for me to better understand what is going on. I don't mind prototyping and testing. Going through the whole design process is something I would like to do.

Regarding protection, it being a lab psu it should be a litle bit foolproof (to use). If adding some is a matter of including a couple diodes here and there, a fuse or an SCR, then for sure.

Quote

Quite often the simplest circuits work well enough to cover almost all your needs. Here is a 0-30VDC/1A uA723 circuit I found that looks pretty good. You could scale it for other voltages or just use it as a starting point for your own design. They use a fairly simple circuit to generate the necessary negative voltage the uA723 needs to allow the output to go down to zero volts. ...

That looks simple enough and it will fit my purpose just by changin a couple resitors for the 0.5 A current. About capacitors I have two salvaged 6800uF /50V and another couple 10000 uf / 50V on the way.

Regarding components, could you guys propose some common ones that will likely be used in any circuit I will end up building? I have 5x 3055 transistors, some op-amps, assorted resistors, diodes and so on, but nothing about zeners or other things. This is so I can order stuff now so it arrives when I need it. Anything I don't use can go the the parts bin, and else I can resort to the local elcetronics shop.

Juan

ArthurDent · « **Reply #30 on:** November 15, 2018, 06:35:05 pm »

"Regarding protection, it being a lab psu it should be a litle bit foolproof (to use). If adding some is a matter of including a couple diodes here and there, a fuse or an SCR, then for sure."

One problem with power supplies with crowbar protection circuits is that some people use their power supplies for battery charging. I've seen spikes on the output lines or limit adjustment errors cause the crowbar to trigger causing the output leads to short. The battery will then deliver a large amount of current into the supply blowing the SCR circuitry and quite often burning runs on circuit boards.

I have mixed feelings about having crowbar circuits in adjustable bench type power supplies, I have some with and most without. If I were using a fixed supply built into a piece of equipment where I was not changing the load then I'd be comfortable using a crowbar but probably not in an adjustable bench supply I was building.

A lot of the newer supplies have MOSFETs for the pass elements and sense if the output is over a set point and turn the output off rather than shorting the output and this is much nicer. Here is how two of my supplies allow you to set OVP/OCP points.

JuanGg · « **Reply #31 on:** November 15, 2018, 08:47:42 pm »

Quote

One problem with power supplies with crowbar protection circuits is that some people use their power supplies for battery charging. I've seen spikes on the output lines or limit adjustment errors cause the crowbar to trigger causing the output leads to short. The battery will then deliver a large amount of current into the supply blowing the SCR circuitry and quite often burning runs on circuit boards.
...

Given that, we can rule crowbar protection out for now. It is something that can be added later, and it will only prevent the output from going ,say, + 25 V. If the series pass transistor fails short, and I am powering something at 5 or 12 V, it will kill it anyway...

spec · « **Reply #32 on:** November 15, 2018, 10:00:35 pm »

Quote from: ArthurDent on November 15, 2018, 03:37:09 pm

Quite often the simplest circuits work well enough to cover almost all your needs. Here is a 0-30VDC/1A uA723 circuit I found that looks pretty good. You could scale it for other voltages or just use it as a starting point for your own design. They use a fairly simple circuit to generate the necessary negative voltage the uA723 needs to allow the output to go down to zero volts.

Brilliant

Quote from: ArthurDent on November 15, 2018, 03:37:09 pm

As has been pointed out trying to use common grounding points is a good idea but if you are using adequate size wire at 1A any voltage errors caused by drops along a lead probably won't be significant compared to temperature drift, etc.. If you are using an adjustable current limit where you have sub-ohm sensing resistors with higher currents that's where you really have to be careful about adding stray lead resistance that could have a large effect on your readings. Some of the better power supplies use 4-terminal resistors to get higher precision current adjustments.

That is true about the voltage drops to an extent, but there is also frequency stability to consider. Using star points costs nothing so why not do it? (don't forget the huge gulps of current flowing into the reservoir capacitor)

Quote from: ArthurDent on November 15, 2018, 03:37:09 pm

I might use a larger filter capacitor than the 1000/63 shown in the attached schematic.

Yeah, 1mF is too small and will give a large ripple voltage. 4.7mF upwards would be better and give a peak to peak ripple of 2v rather than 10V which is not workable. Reservoir capacitors normally have a tolerance of -40% +100% so you would need to factor that in. So 6.8mF upwards would be good. 10mF would be ideal, giving a ripple voltage of 1V peak to peak.

Quote from: ArthurDent on November 15, 2018, 03:37:09 pm

one of the previous circuits showed 4x4700 used which I believe to be overkill. All regulators also acts as capacitance multipliers by a factor of the combined hfe of the series transistors and that allow you to use smaller capacitors.

I would say that 10mF would be ideal: low ripple voltage, giving more voltage overhead margin and less ripple voltage on the output.

Just one point about the PSU circuit- there is no decoupling.

spec · « **Reply #33 on:** November 15, 2018, 10:05:44 pm »

Quote from: Wolfgang on November 15, 2018, 03:40:52 pm

Quote from: ArthurDent on November 15, 2018, 03:37:09 pm
Quite often the simplest circuits work well enough to cover almost all your needs. Here is a 0-30VDC/1A uA723 circuit I found that looks pretty good. You could scale it for other voltages or just use it as a starting point for your own design. They use a fairly simple circuit to generate the necessary negative voltage the uA723 needs to allow the output to go down to zero volts.

As has been pointed out trying to use common grounding points is a good idea but if you are using adequate size wire at 1A any voltage errors caused by drops along a lead probably won't be significant compared to temperature drift, etc.. If you are using an adjustable current limit where you have sub-ohm sensing resistors with higher currents that's where you really have to be careful about adding stray lead resistance that will could have a large effect on your readings. Some of the better power supplies use 4-terminal resistors to get higher precision current adjustments.

.. you could further simplify this by using an 7905 regulator for the negative rail. It would also improve stability.

There is a bit more to starpoints than just voltage drops. Bad wiring can also lead to frequency instability, just like in audio amps. And there is no reason not to do a proper job - it costs nothing

Good suggestion about using a three terminal regulator for the -5V reference supply.

Kleinstein · « **Reply #34 on:** November 15, 2018, 10:17:26 pm »

One can protect the crowbar circuit with an extra fuse. So there would be a fuse between the crowbar and the output terminal. Someone adding a strong external supply would only blow the fuse. This type of fuse might be a good idea anyway to avoid a "short" through the diode across the output - in case the battery is the other way around.
Well set the crowbar can be closer to the actually set voltage and actually protect the circuit, if no too sensitive.

The LM723 circuit with the negative supply shown below has 2 slight problems:
There is a supply voltage limit for the 723 - so with 30 V output voltage this would likely cause too much voltage at least under no load conditions. With an output level above about 25 V it might be a good idea to have some voltage limitation / regulation for the LM723. Often an extra filter cap is also a good idea, as this allows some extra 2 V more ripply on the main supply current and thus maybe half the capacitor there.

The 723 only needs a rather low negative voltage, if at all. So something like -2 V should be enough.

In that circuit the negative voltage is part of the reference. One of the good points of the 723 is it's usually low noise reference.

The simpler way to use the 723 in a supply that goes all the way down to 0 V (or at least very close) is to have the feedback divider no towards 0 V but towards a small fraction (e.g. around 1.5 V) of the reference level and no extra negative supply. The little current flowing back from the divider might limit the lowest voltage to get open circuit to some 50 mV, depending on the minimum internal load.

It is also better to adjust the reference level and keep the feedback divider constant. I know many old supplies do it the other way round, but it's not that clever, because it changes the loop gain. Adjusting the reference side is also more convenient if a digital control is used.

spec · « **Reply #35 on:** November 15, 2018, 10:33:26 pm »

Quote from: JuanGg on November 15, 2018, 05:35:40 pm

I was thinking of using CPU cooling heatsinks, which usually have a fan attached to. I think the easiest approach is no thermal washers and mount them inside the case. I can esily make some plastic brackets for that purpose. (abs, so they won't melt, even if the heatsink is at 100 ºC, which it shouldn't be...). Insulating washers can be added later on if needed. I will hopefully have news about the heatsinks in a couple days time.

Using a CPU heatsink and fan is a good idea but it will limit your PSU. I am thinking that you should be able to make a 0V to 20V supply (x2) with the transformers and reservoir capacitors that you have already.

Quote from: JuanGg on November 15, 2018, 05:35:40 pm

On the stabilizing circuit, as you present it, the "Custom" option seems the best, if it delivers good performance on a simple circuit, and thus more likely for me to better understand what is going on. I don't mind prototyping and testing. Going through the whole design process is something I would like to do.

Arthur Dent's circuit is really neat and with a few mods: 6.8mF reservoir capacitor, three terminal -5V regulator, decoupling, a low thermal resistance heatsink, and to say it yet again, star point wiring, you will have a pretty good power supply that will be a useful piece of lab equipment for the future.

But, as you are still interested in a custom approach, I will post an outline schematic to give you a feel for the custom PSU design mentioned in reply #26. As well as an adjustable precision voltage output, it also has adjustable precision current limiting. The OV output point is pretty accurate too.

iMo · « **Reply #36 on:** November 15, 2018, 11:02:11 pm »

I like 723 but I think you can do the same with discrete parts easily (and maybe better). Also from my experience the important things your PSU should manage well are:
. it has to sustain a time unlimited short at its output, and,
. produce 0V (or close to zero) when your output voltage potentiomer's wiper looses its contact.

For example a cheapo LM358 (or similar) with 78L05/8/9 as the ref source will do the same. And the over-current protection is done by a single NPN in 723, you may do it better with the second opamp in the package. And you may "filter" the ref source with RC in the same manner as with 723, in case you require uV noise

spec · « **Reply #37 on:** November 15, 2018, 11:04:44 pm »

NOTE: In the attached PSU schematic, D8 is the wrong way around. Thanks to Arthur Dent for spotting (2018_11_18).

The circuit is not ready for prototyping and may have some errors- but it is the principle that matters at this stage. The known shortcomings are:

[1] Too much loop gain

[2} No optimization

[3] Inadequate decoupling

[4] Frequency compensation components not shown

[5] Resistor values not normalised

spec · « **Reply #38 on:** November 15, 2018, 11:16:22 pm »

Wow, this thread is attracting a lot of interest, as is normally the case with PSUs. 39 posts so far, and a lot more to come, no doubt

JuanGg · « **Reply #39 on:** November 16, 2018, 08:31:47 pm »

Quote

One can protect the crowbar circuit with an extra fuse. So there would be a fuse between the crowbar and the output terminal. Someone adding a strong external supply would only blow the fuse. This type of fuse might be a good idea anyway to avoid a "short" through the diode across the output - in case the battery is the other way around.
Well set the crowbar can be closer to the actually set voltage and actually protect the circuit, if no too sensitive.

That would do it then. I had no idea the crowbar voltage could be adjusted to the set voltage. I am not planing on using the supply to charge batteries or anything like it, but better safe than sorry.

Quote

The simpler way to use the 723 in a supply that goes all the way down to 0 V (or at least very close) is to have the feedback divider no towards 0 V but towards a small fraction (e.g. around 1.5 V) of the reference level and no extra negative supply. The little current flowing back from the divider might limit the lowest voltage to get open circuit to some 50 mV, depending on the minimum internal load.

It is also better to adjust the reference level and keep the feedback divider constant. I know many old supplies do it the other way round, but it's not that clever, because it changes the loop gain. Adjusting the reference side is also more convenient if a digital control is used.

Going close enough to 0 V should be fine. My original intent was to also be able to controll it from a micro, as I am going to use one for the displays. That would do.

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Using a CPU heatsink and fan is a good idea but it will limit your PSU. I am thinking that you should be able to make a 0V to 20V supply (x2) with the transformers and reservoir capacitors that you have already.

I just got hold of the heatsinks! (photo attached). Two identical ones with a 12 V 0.28 A fan each. They look pretty substantial, don't know if it would be enough. No markings or anything, so no idea about thermal resistance.

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Arthur Dent's circuit is really neat and with a few mods: 6.8mF reservoir capacitor, three terminal -5V regulator, decoupling, a low thermal resistance heatsink, and to say it yet again, star point wiring, you will have a pretty good power supply that will be a useful piece of lab equipment for the future.

But, as you are still interested in a custom approach, I will post an outline schematic to give you a feel for the custom PSU design mentioned in reply #26. As well as an adjustable precision voltage output, it also has adjustable precision current limiting. The OV output point is pretty accurate too.

It surely will do, plus the modifications proposed by Kleinstein for it to be digitally controlled and removing the negative rail. I am doubt which way to go. I can maybe have a go at both approaches...

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Attached, below, is an outline schematic for the custom PSU mentioned in reply #26. ...

Thank you. I kind of understand what everything does in the circuit. Not sure about micro control with the negative 12V rail though. Again, don't know wich way to go, the LM723 one or something more like this one. I may as well try both. For now, I will model the transformers and the heatsinks on the computer so I can start designing the case. I hope the AC mains connector and the bridge rectifier arrive soon (a week or two...) so I can assemble the raw supply.

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Wow, this thread is attracting a lot of interest, as is normally the case with PSUs. 39 posts so far, and a lot more to come, no doubt

It certainly is. Thank you to anyone that is taking part.

Kleinstein · « **Reply #40 on:** November 16, 2018, 09:32:46 pm »

The circuit in #38 has still quite a few weak points. So it is a starting point at best. It's still in between the 2 main circuit types.

With a lab supply designed essentially from scratch it is a good idea to use a simulation (e.g. LTspice) to do the first tests and adjustment if the loop. It is a really powerful tool for this.

Starting from 25 V AC is just at the border, where one can still use the LM723 or an OP as voltage mode controller and emitter follower. One might still need to limit the supply for the LM723 or the OP, but the voltage swing to some 25 V is sufficient. The LM723 is not that bad: it has a reasonable good reference and the difference amplifier with access to the transconductance part can also be handy. The main drawback it the limited voltage and crude current limit - so not really a lab supply in this respect.

The other circuit type uses an output stage that is more current controlling. However this usually needs an extra auxiliary supply for the control part and thus another transformer. This type of circuit is commonly used in many commercial supplies, as it is very flexible. However it usually needs a larger capacitor at the output.

spec · « **Reply #41 on:** November 17, 2018, 02:00:50 am »

Quote from: Kleinstein on November 16, 2018, 09:32:46 pm

The circuit in #38 has still quite a few weak points. So it is a starting point at best. It's still in between the 2 main circuit types.

There is no circuit in answer #38. I take it that you mean answer #37. I really cannot see that your general and sweeping statements from on high are of any help. And your negative statements are unwarranted. In fact, they are disruptive.

What are these weak points- please be specific. If you are thinking about the points I have already listed, what is the objective of mentioning them again. Besides that circuit, as clearly stated, is not complete. It is an outline circuit posted at this stage to show the architecture and scope. Just for the record, that architecture, which has been recommended a number of times on EEV and elsewhere has a lot going for it in terms of flexibility, cost and, most of all, performance. The other thing about it is that it can easily be built in a negative version, which is usefull when you need plus and minus supplies in one PSU, using a single transformer.

What does the statement "It's still between the 2 main circuit types mean". What 2 main circuit types? Anyway, this sounds like another instance of opinion being stated as fact. Besides which, it is up to the OP to decide which circuit to go for.

"So it is a starting point at best" I cannot see the point of this statement.

Finally, I take it from your tone that you are an expert in PSU design and construction. So if you can post a schematic for a PSU, that is complete, practical, and meets your own undefined criteria? That would be a big help. Perhaps include a simulation too.

xavier60 · « **Reply #42 on:** November 17, 2018, 02:24:21 am »

Quote from: spec on November 17, 2018, 02:00:50 am

What does the statement "It's still between the 2 main circuit types mean". What 2 main circuit types?

Too me the two types are whether the voltage control op-amp has to tolerate the full unregulated supply voltage or not.
With the type often referred to as the "floating type" , everything is referenced to the positive output terminal including the +/- control power supply rails. The op-amps see only the control rail voltages regardless of what the unregulated and output voltages are.

xavier60 · « **Reply #43 on:** November 17, 2018, 02:40:53 am »

I see that the circuit in #37 is the exception. The op-amps have there own supply rail that is independent of the unregulated voltage.

spec · « **Reply #44 on:** November 17, 2018, 03:33:31 am »

Quote from: JuanGg on November 16, 2018, 08:31:47 pm

… Not sure about micro control with the negative 12V rail though. Again, don't know which way to go, the LM723 one or something more like this one. I may as well try both.

I would not worry about controlling a PSU with a negative supply- it is dead easy and not an issue.

All linear stabilized PSUs are essentially voltage feedback power amplifiers in their normal mode and you can alter the voltage feedback arrangements to suit your requirements. But when the PSU goes into the constant current mode you need to consider the frequency stability in that mode too. There is a quite difficult situation when both voltage feedback and current feedback are operating at the same time (there are techniques to sort this), especially with the LM723 single transistor approach.

I would not be put off by the general statements about circuit design, which sound very learned but often do not take in the whole picture. A circuit needs to be analyzed in it's entirety to reach a proper conclusion. One example is K's treatise where he warns of the danger of varying the feed back voltage rather than the reference voltage. Well, perhaps the millions of three terminal regulators in use ought to be withdrawn and redesigned. You simply cannot design by platitudes,

As to the lecture about referencing the feedback on an LM723 back to the reference voltage, that has already been shown in the schematic in the original post. Besides which it is a well known technique in general.

K has also voiced some undefined warnings about your three transformer approach, which is difficult to fathom.

There seems to be a lot of fear and loathing about having a negative supply, but a negative supply is a low technical risk and costs little in money and complexity terms. A negative supply has a lot of advantages, especially as it allows the PSU to go to exactly to 0V which is a big issues for some lab testing. It can also be used (as in PSU #37) to keep the temperature of the control electronics more constant as they will not be exposed to all the fire and brimstone going on above them with the raw supply and the power transistors heating up and cooling down.

Don’t forget that the output transistors will have quite a high leakage current, especially when the junction heats up and you need to get rid of that current.

Another advantage of a negative supply is that it allows you to have some current sinking which is useful.

Some PSUs without a method of draining off the various standing currents get into an unhappy state around zero volts output.

Having said all this, I am not advocating the answer #37 PSU. I leave that up to you? But, as mentioned before, the Arthur Dent circuit #27 (with modifications) has a lot going for it in my book.

spec · « **Reply #45 on:** November 17, 2018, 04:04:46 am »

Quote from: xavier60 on November 17, 2018, 02:24:21 am

Quote from: spec on November 17, 2018, 02:00:50 am
What does the statement "It's still between the 2 main circuit types mean". What 2 main circuit types?
Too me the two types are whether the voltage control op-amp has to tolerate the full unregulated supply voltage or not.
With the type often referred to as the "floating type" , everything is referenced to the positive output terminal including the +/- control power supply rails. The op-amps see only the control rail voltages regardless of what the unregulated and output voltages are.

Yes, that is a good general categorization.

spec · « **Reply #46 on:** November 17, 2018, 04:10:50 am »

Quote from: xavier60 on November 17, 2018, 02:40:53 am

I see that the circuit in #37 is the exception. The op-amps have there own supply rail that is independent of the unregulated voltage.

Yes again

That is one of the major advantages of that architecture and it brings huge performance benefits, as I mention in answer #44

Interestingly, with a few mods here and there, that architecture can be turned into a high precision calibration type PSU/voltage source.

spec · « **Reply #47 on:** November 17, 2018, 06:09:59 am »

JuanGg

The PSU in answer #36 may be suitable for your needs: https://www.eevblog.com/forum/beginners/help-me-design-a-psu/msg1971227/#msg1971227

It is certainly the cheapest and simplest approach.

Once you complete the chassis and raw power supply you could quickly build this circuit. Then, as you go along, you could enhance the performance in a number of ways which we could discuss on EEV.

One enhancement would be to fit a bypass power transistor to improve the current output at PSU low output voltages.

Another enhancement would be to replace the LM317 with a better three terminal regulator.

spec · « **Reply #48 on:** November 17, 2018, 06:38:57 am »

There is a topic which I have been meaning to mention.

With the transformers arrangement you have with 10V and 15V windings it may be an advantage to have a range switch which selects just one of the windings for PSU low output voltages and both windings for high output voltages. This is common practice on many linear supplies, either manually or automatically. On my linear lab PSUs the switching is automatic by a relay.

The reason for this switching is to reduce the power dissipation in the output power transistors.

not1xor1 · « **Reply #49 on:** November 17, 2018, 08:08:57 am »

Quote from: spec on November 17, 2018, 03:33:31 am

I would not be put off by the general statements about circuit design, which sound very learned but often do not take in the whole picture. A circuit needs to be analyzed in it's entirety to reach a proper conclusion. One example is K's treatise where he warns of the danger of varying the feed back voltage rather than the reference voltage. Well, perhaps the millions of three terminal regulators in use ought to be withdrawn and redesigned. You simply cannot design by platitudes,

Hi
Are you referring to Kleinstein's message?

Sorry... but he is right and you are wrong.
Platitudes???
It doesn't matter if a theory is fancy of fashionable, but if it is confirmed by the facts.
And if you change the loop gain the phase margin does change while it generally doesn't if you just change the input signal level (in this case the voltage reference).

Regarding 3 terminal regulators I guess you are referring to the ubiquitous LM317 ?!?!
Well its feedback is fixed, not variable.
You just program a fixed current via the resistor between output and adjust which sets the output voltage via the resistor between adjust and the ground reference (just the plain, old, boring and unfashionable Ohm Law

).

For that reason you can add a 10µF or even greater capacitor in parallel to that resistor (adj-ground) without affecting stability, while the output capacitance may cause ringing or oscillations.

spec · « **Reply #50 on:** November 17, 2018, 08:56:16 am »

Quote from: not1xor1 on November 17, 2018, 08:08:57 am

Quote from: spec on November 17, 2018, 03:33:31 am

I would not be put off by the general statements about circuit design, which sound very learned but often do not take in the whole picture. A circuit needs to be analyzed in it's entirety to reach a proper conclusion. One example is K's treatise where he warns of the danger of varying the feed back voltage rather than the reference voltage. Well, perhaps the millions of three terminal regulators in use ought to be withdrawn and redesigned. You simply cannot design by platitudes,

Hi
Are you referring to Kleinstein's message?
Sorry... but he is right and you are wrong.
Platitudes???
It doesn't matter if a theory is fancy of fashionable, but if it is confirmed by the facts.
And if you change the loop gain the phase margin does change while it generally doesn't if you just change the input signal level (in this case the voltage reference).

Regarding 3 terminal regulators I guess you are referring to the ubiquitous LM317 ?!?!
Well its feedback is fixed, not variable.
You just program a fixed current via the resistor between output and adjust which sets the output voltage via the resistor between adjust and the ground reference (just the plain, old, boring and unfashionable Ohm Law ).

For that reason you can add a 10µF or even greater capacitor in parallel to that resistor (adj-ground) without affecting stability, while the output capacitance may cause ringing or oscillations.

It is not a case of right or wrong it is a matter of usefulness. You are doing the same thing- going on about theory in isolation without looking at the overall picture. There are many power supplies that use both kinds of feedback.

By the way, not that it matters, but are you sure about the LM317 feedback being fixed?

Can you explain your mention of capacitors- I don't understand what you are saying?

Kleinstein · « **Reply #51 on:** November 17, 2018, 09:58:53 am »

The Forum software seems to have a small bug: depending on the settings (Show most recent posts at the top. under Forum Look and layout), the numbering of the answers is off by 1. This causes the confusion with the numbering.

The circuit under Reply 37 (with recent post at bottom) has the output stage in between a low impedance (voltage setting) and high impedance (current setting). These are the 2 main classes I spoke off. The low impedance output stages are like the conventional voltage regulators, while the current setting stages are the low drop ones. Generally these two types use a different type of compensation, which might make the still missing compensation a real challenge. Getting the compensation right is about the main difficulty in a lab supply.

The voltage adjustment in the feedback divider is another problem. This changes the loop gain depending on the set voltage, so the regulation would be fast at low set voltage and slower at a higher voltage. There are other supplies working this way, but it's still a weak point. It is relatively easy to solve by having a fixed divider and adjusting the voltage on the other side of the OP.

For a precision current regulation the OPs should get a separate negative supply from the reference: the current through the "GND / ADJ" Pin of the -12 V regulator also flows through the current shunt and depending on the regulator this current depends on the load current and thus the controlling current from the OPs. Besides precision this might also effect stability of the current regulation.

As shown the ripple rejection could be a problem, because the collector side of Q2 would be heavily influenced by the raw voltage. It depends on the details of the compensation however.

The high loop gain due to the transistor Q2 could be a real problem: the OPs kind of need some extra gain, as there supply is smaller than the output range. This also makes the compensation really tricky.

The current regulator works with the OP at its upper supply edge. This can be tricky with some RR OPs. So one has to be careful with the choice of OP here. Especially at some 1-2 V below the upper limit RR OPs may behave a little odd. The voltage regulator also works at it upper supply - though this might change anyway.

For the relatively low power level in question here (e.g. 0.5 A and up to about 30-35 V raw voltage) I don't think one would need switching of the transformer tap. It could be a good idea with higher power (e.g. more than 1-2 A) though.
At some 15 W of worst case power loss, cooling it not that difficult either and one could getaway without a fan.

nemail2 · « **Reply #52 on:** November 17, 2018, 11:10:38 am »

Take a look at my Lab PSU - so far everything working as intended. Tried to make it oscillate but wasn't successful yet.
https://github.com/mamama1/LabPSU_Darlington/blob/master/Hardware/schematics.pdf

It uses a DAC to drive a voltage control opamp at a rather hilarious gain (17.14something) which controls the bases of two BD139/TIP3055 Darlingtons.
The voltage control loop is coupled to the current control loop through a diode and does high-side current measuring with a 100mOhm shunt and a MAX4080 current measuring IC. the current control loop is controlled via a DAC as well. Because the DAC can only output 1,195V at max, I had to up that a bit using another opamp and a gain of 1.713something.

In the end, this PSU goes from 50mV to 20.48V in 5mV steps and from ~10mA (that's pretty much what the fixed minimum load which is at the output consumes) to 4.096A in 1mA steps. Precision is about 0.1% over the whole range, however I didn't verify that thoroughly.

I really like the idea of the high side shunt + the MAX4080, took that from Daves µSupply so credits to him.

JuanGg · « **Reply #53 on:** November 17, 2018, 12:47:31 pm »

I can't keep up with the pace this is going at!

Quote

With a lab supply designed essentially from scratch it is a good idea to use a simulation (e.g. LTspice) to do the first tests and adjustment if the loop. It is a really powerful tool for this.

Learning LTSpice has been in my to do list for a while, I guess it's time to do it now.

Quote

I would not worry about controlling a PSU with a negative supply- it is dead easy and not an issue. ...

Ok. Good to know. Between the lm723 and the custom approach, if the custom one provides more flexibility and better performance, I' ll go for it. I can give the lm723 a try if the other doesn't work out.

Quote

JuanGg

The PSU in answer #36 may be suitable for your needs: https://www.eevblog.com/forum/beginners/help-me-design-a-psu/msg1971227/#msg1971227

It is certainly the cheapest and simplest approach.

I could very well start by doing something like that, as I dont have any experience on power supplies, but it doesn't have adjustable current limit and I don't know how would it perform. As I am planing on spending quite some time making a decent case, adding micro-controlled voltage and current displays, I would like to do something more sophisticated. However, I can always upgrade it later.

Quote

There is a topic which I have been meaning to mention.

With the transformers arrangement you have with 10V and 15V windings it may be an advantage to have a range switch which selects just one of the windings for PSU low output voltages and both windings for high output voltages. This is common practice on many linear supplies, either manually or automatically. On my linear lab PSUs the switching is automatic by a relay.

The reason for this switching is to reduce the power dissipation in the output power transistors.

I mentioned that in my first post, maybe a relay driven by a comparator or even the microcontroller would do. At a maximum power disipation of around 15W/channel I don't know if it's worth it, but it wouldn't be hard to implement.

Other thing I was thinking of is that I am going to need a +12V rail to power the fans, a possible relay and the Arduino. One way I found to do this is as in the schematic attached (https://electronics.stackexchange.com/questions/153399/using-all-the-taps-of-the-transformer-at-the-same-time), using the two transformers instead of a center tapped one, and a 7812 regulator. This way the fan and anything else would draw current from the 10 V 2 A transformer. A 10V rail would do, getting regulated 12V from 10 V ac does't seem easy.

Quote

Take a look at my Lab PSU - so far everything working as intended. Tried to make it oscillate but wasn't successful yet.
https://github.com/mamama1/LabPSU_Darlington/blob/master/Hardware/schematics.pdf

That is a bit complicated-looking. I will have a read of the schematics and documentation when I am able to.

I fould a bridge rectifier I took out of a broken power supply a while ago. I will start making the case and try the raw power supply soon.
Juan

nemail2 · « **Reply #54 on:** November 17, 2018, 12:58:52 pm »

Quote from: JuanGg on November 17, 2018, 12:47:31 pm

Quote
Take a look at my Lab PSU - so far everything working as intended. Tried to make it oscillate but wasn't successful yet.
https://github.com/mamama1/LabPSU_Darlington/blob/master/Hardware/schematics.pdf

That is a bit complicated-looking. I will have a read of the schematics and documentation when I am able to.

In fact, it is as simple as it gets. I have to admin, however, that the readability of the schematics isn't too good because I didn't draw it very well.
But the concept is really easy and basic. It is a microcontroller controllable PSU with constant voltage and constant current mode. Whether it is very precise or not, simply depends on the parts chosen (high precision resistors for shunt, voltage dividers, feedback loop and high precision opamps).

spec · « **Reply #55 on:** November 17, 2018, 06:42:26 pm »

UPDATED: 2018_11_18

Quote from: Kleinstein on November 17, 2018, 09:58:53 am

The circuit under Reply 37 (with recent post at bottom) has the output stage in between a low impedance (voltage setting) and high impedance (current setting). These are the 2 main classes I spoke off. The low impedance output stages are like the conventional voltage regulators, while the current setting stages are the low drop ones. Generally these two types use a different type of compensation, which might make the still missing compensation a real challenge. Getting the compensation right is about the main difficulty in a lab supply.

I have no idea what you mean here. The voltage and current opamps drive Q2 emitter, and hence the output darlington transistor in the same way for both voltage and current sense modes. Of course you have to compensate a feedback circuit. That is a standard procedure for all PSUs, in fact all feedback systems.

Incidentally, the handover from the constant voltage mode to the constant current mode is done by a diode AND gate and uses the reducing gm technique to help prevent the two control loops fighting with each other. The way that this is done is one of the features of this architecture.

Quote from: Kleinstein on November 17, 2018, 09:58:53 am

The voltage adjustment in the feedback divider is another problem. This changes the loop gain depending on the set voltage, so the regulation would be fast at low set voltage and slower at a higher voltage. There are other supplies working this way, but it's still a weak point. It is relatively easy to solve by having a fixed divider and adjusting the voltage on the other side of the OP.

That is not true. The open loop voltage gain will be around 110dB (when the voltage gain of Q2 is reduced to around 1 in the future) whereas the change in feedback ratio amounts to a mere 2.5dB. You would get that much gain change between individual opamps of the same type, and with temperature changes. You also seem to be missing the point that the current through the 12K resistor is constant at 1mA.

I would like to see your calculations that prove that the voltage stabilization would be inadequate. I suggest that you would find the voltage stabilization would be highly accurate, way better than the average designs you see on the net etc.

Incidentally, PSU #27 has the same feedback arrangement, yet you said not a word about that.

Quote from: Kleinstein on November 17, 2018, 09:58:53 am

For a precision current regulation the OPs should get a separate negative supply from the reference: the current through the "GND / ADJ" Pin of the -12 V regulator also flows through the current shunt and depending on the regulator this current depends on the load current and thus the controlling current from the OPs. Besides precision this might also effect stability of the current regulation.

The three terminal regulator is not specified so how can you make any comments about its performance. Besides which, you greatly exaggerate the situation in a negative way. There are three terminal regulators with a ground current of 10uA. Anyway you are talking in terms of a precision voltage calibration supply, not a lab supply.

Quote from: Kleinstein on November 17, 2018, 09:58:53 am

As shown the ripple rejection could be a problem, because the collector side of Q2 would be heavily influenced by the raw voltage. It depends on the details of the compensation however.

Of course, just the same as millions of other PSUs that use that architecture. Remember this is not intended to be a precision reference PSU. It is a general purpose lab PSU. Once again can I stress that this is not a complete optimized design. You must know that it is a relatively simple matter to reduce the effects of ripple, if necessary.

But I do agree that this area could be improved.

Quote from: Kleinstein on November 17, 2018, 09:58:53 am

The high loop gain due to the transistor Q2 could be a real problem: the OPs kind of need some extra gain, as there supply is smaller than the output range. This also makes the compensation really tricky.

Everything is a problem with you. But what you are talking about are normal design procedures to solve so called problems. Besides which, the gain controlling elements are not included in the circuit- it is an outline circuit.

Quote from: Kleinstein on November 17, 2018, 09:58:53 am

The current regulator works with the OP at its upper supply edge. This can be tricky with some RR OPs. So one has to be careful with the choice of OP here. Especially at some 1-2 V below the upper limit RR OPs may behave a little odd. The voltage regulator also works at it upper supply - though this might change anyway.

This is just general lecturing. The opamp that is specified, OPA191, is quite happy working at VCC. Incidentally both the current and voltage opamps are working at VCC. Besides, it would be a simple matter to change the opamp for one of the many over the rail types.

You do not know what the regulator is, so how can you make such a statement. Besides which, it is not a problem that can't be easily fixed. But you are not correct even if you are assuming an LM337, which has a maximum Vin to Vout limit of 40V.

The maximum voltage possible on the negative regulator input is -1* ((25V * 1.414) - (1V +1V)) = -33.35V DC. That in itself is withing the 40V ipV/opV rating of an LM337. But as the LM337 output voltage would be -12V, this means the voltage across the LM337 would be -1 *(33.35V -12V) = -21.35V. So well within an LM337 ipV/opV limit.

Quote from: Kleinstein on November 17, 2018, 09:58:53 am

For the relatively low power level in question here (e.g. 0.5 A and up to about 30-35 V raw voltage) I don't think one would need switching of the transformer tap. It could be a good idea with higher power (e.g. more than 1-2 A) though.
At some 15 W of worst case power loss, cooling it not that difficult either and one could getaway without a fan.

I am suggesting going for 1A. Besides which, if the facility is there, why not be aware of it as a possibility.

I find your comments completely negative and general. You could make the same sort of critisisems about practically any PSU circuit. And none of your claims are backed by any calculations or figures- just sweeping statements.

You are also picking holes in a circuit which has been categorically stated as work in progress and for demonstrating the principle only.

But as I said before, as you have all this knowledge about PSU design, please lets see your design for his application. Or, if not at least recommend a PSU circuit that meets your criteria and the OPs.

Finally, on a general point, not only are you wasting my time, which could be spent on something more constructive than warding of your unwarranted criticisms of the circuit but, worse sill you will be discouraging the OP who may not have the technical knowledge to establish the degree and relevance of your list of problems as you put it.

After all that, I do appreciate you posting all the above information. It does give me a feel for how you think. And I am sure that your comments are well intended.

spec · « **Reply #56 on:** November 17, 2018, 07:31:03 pm »

Quote from: JuanGg on November 17, 2018, 12:47:31 pm

I can't keep up with the pace this is going at!

Neither can I

Quote

I would not worry about controlling a PSU with a negative supply- it is dead easy and not an issue. ...

Quote from: JuanGg on November 17, 2018, 12:47:31 pm

Ok. Good to know. Between the lm723 and the custom approach, if the custom one provides more flexibility and better performance, I' ll go for it. I can give the lm723 a try if the other doesn't work out.

Very wise. As mentioned before, once you have the chassis and raw supplies built, tested and the performance established you can pretty much go anyway that you want

Quote

The PSU in answer #36 may be suitable for your needs: https://www.eevblog.com/forum/beginners/help-me-design-a-psu/msg1971227/#msg1971227

It is certainly the cheapest and simplest approach.

Quote from: JuanGg on November 17, 2018, 12:47:31 pm

I could very well start by doing something like that, as I dont have any experience on power supplies, but it doesn't have adjustable current limit and I don't know how would it perform. As I am planing on spending quite some time making a decent case, adding micro-controlled voltage and current displays, I would like to do something more sophisticated. However, I can always upgrade it later.

As you say you have no experience of building PSUs, this simple approach may be a good idea. By the way, you will need a practical circuit with decoupling capacitors etc. You know that even simple power supplies can be problematic. And even three terminal regulators can play up. You would not believe the the things that can go wrong.

Quote

There is a topic which I have been meaning to mention.

With the transformers arrangement you have with 10V and 15V windings it may be an advantage to have a range switch which selects just one of the windings for PSU low output voltages and both windings for high output voltages. This is common practice on many linear supplies, either manually or automatically. On my linear lab PSUs the switching is automatic by a relay.

The reason for this switching is to reduce the power dissipation in the output power transistors.

Quote from: JuanGg on November 17, 2018, 12:47:31 pm

I mentioned that in my first post, maybe a relay driven by a comparator or even the microcontroller would do. At a maximum power dissipation of around 15W/channel I don't know if it's worth it, but it wouldn't be hard to implement.

Other thing I was thinking of is that I am going to need a +12V rail to power the fans, a possible relay and the Arduino. One way I found to do this is as in the schematic attached (https://electronics.stackexchange.com/questions/153399/using-all-the-taps-of-the-transformer-at-the-same-time), using the two transformers instead of a center tapped one, and a 7812 regulator. This way the fan and anything else would draw current from the 10 V 2 A transformer. A 10V rail would do, getting regulated 12V from 10 V ac doesn't seem easy.

Ahh missed that!

Another nice touch, that is easy to do, would be to have the fans temperature controlled, or have you already mentioned that too.

Getting any DC voltage converted to any other DC voltage, within reason, is dirt cheap and easy these days.
But you could probably just use a bridge rectifier and reservoir cap off the 10V 2A winding. If necessary, you could put a power resistor in series with the fan to get the right current.

I am hoping to see the test results of your raw supply and also some pictures

ArthurDent · « **Reply #57 on:** November 18, 2018, 12:10:58 am »

I think we've seen now that there are as many ways to build a power supply as there are posters here. The circuit I found and posted in #27 is an old design but should work fine and replacing the 4.7V zener in the negative supply with a 7905 as Wolfgang suggested should improve stability slightly. The circuit spec designed in #37 with some fine tuning should be better yet (NOTE-a slight oversight, D8 was drawn backwards).

As much as I love to try to continually improve any circuit, if the power supply you make is to allow you to test other circuits you are more interested in, I'd say decide on one circuit you think is adequate for your needs and after you've built it, get on to constructing the other stuff you want to play with that will be powered by this supply.

fsr · « **Reply #58 on:** November 18, 2018, 02:39:21 am »

Several years ago, i built the "5A Constant Voltage/Constant Current Regulator" circuit in the LM317 datasheet, with some modifications. Of course the 5A and 35v input are bullshit, i went with far lower input voltage and current limits. But it seems to work fairly ok (didn't test it seriously, however). There are probably better, simpler circuits posted here, anyways.

But what i was going to, is that i searched for some power transistors with low thermal resistance and found the MJ15003/MJ15004 complementary pair. They have crazy specs. Thermal resistance 0.70 degrees C Junction-Case. Nice stuff.

spec · « **Reply #59 on:** November 18, 2018, 04:18:38 am »

Quote from: ArthurDent on November 18, 2018, 12:10:58 am

(NOTE-a slight oversight, D8 was drawn backwards). [PSU # 37]

Well spotted AD: will correct

spec · « **Reply #60 on:** November 18, 2018, 04:28:09 am »

Quote from: fsr on November 18, 2018, 02:39:21 am

But what i was going to, is that i searched for some power transistors with low thermal resistance and found the MJ15003/MJ15004 complementary pair. They have crazy specs. Thermal resistance 0.70 degrees C Junction-Case. Nice stuff.

One of my all-time favorites too.

ON (Motorola) Do some pretty good power devices in general. They second source a few of the Toshiba beauties, like the MJL3281A/MJL1302A, which were no longer available at one time.

JuanGg · « **Reply #61 on:** November 18, 2018, 11:44:41 am »

Quote

I think we've seen now that there are as many ways to build a power supply as there are posters here. ...

I think I am going with the circuit in #37. I am building this as a tool, but as a learning project as well. When I have something that works for me, I'll use it until I have the need to upgrade it.

Just noticed that the fans on the heatsinks blow air into them, instead of out. If they are inside the case, this would heat everything up (not good). I am thinking on taking the fans out and flip them so they blow air from vents on the sides/front, across the supply and out of the case, cooling everything. It's a pity because they are nicely fitted to the heatsinks... Or I could have the heatsinks sit at the back of the case, using thermal pads (switching taps to reduce power dissipation). Any ideas on thermal design etc?
Juan

ArthurDent · « **Reply #62 on:** November 18, 2018, 03:15:00 pm »

In the 'Beginners' section there is a thread called "Help me design a PSU" and post #17 has another power supply design that has all the basic building blocks we are discussing here. It appears that the idea for the negative supply using 2 caps and 2 diodes was gleaned from the same old schematic I had found and posted here but the rest is different and might be of interest in the power supply you are thinking of building. There aren't a lot of parts values shown but the circuit uses a different approach than what has been discussing. It's always good to see as many variations as you can before making a decision.

https://www.eevblog.com/forum/beginners/help-me-design-a-psu/?action=dlattach;attach=571409;image

spec · « **Reply #63 on:** November 18, 2018, 05:09:32 pm »

Quote from: ArthurDent on November 18, 2018, 03:15:00 pm

It appears that the idea for the negative supply using 2 caps and 2 diodes was gleaned from the same old schematic
https://www.eevblog.com/forum/beginners/help-me-design-a-psu/?action=dlattach;attach=571409;image

That technique goes back much further than that... 1920s I would guess $:-\$

ArthurDent · « **Reply #64 on:** November 18, 2018, 05:13:45 pm »

JuanGg - "Just noticed that the fans on the heatsinks blow air into them, instead of out."

The way a processor heat sink fan is used is it removes the heat from the immediate area of the processor heatsink inside the PC case then they have a large fan or two on the back of the case to remove the heat from the case. If you can design your power supply so the top of the fan/heatsink you're using is against a hole in the case you're using or you have something like a piece of duct work pipe to go from the fan to a hole in the case, that would work to force in/remove the heat from the case. I would look for an aluminum can the correct diameter and cut that to the correct length and attach that if you need small duct work round pipe.

As far as sucking air in or blowing air out I have wrestled with this question for years and concluded it probably doesn't make much difference. One piece of equipment I have, an Array 3711A a max 360V/30A/300W electronic load, solved the problem by using 4 fans, 2 sucking in and 2 blowing out.

spec · « **Reply #65 on:** November 18, 2018, 05:24:01 pm »

Quote from: JuanGg on November 18, 2018, 11:44:41 am

Quote
I think we've seen now that there are as many ways to build a power supply as there are posters here. ...
I think I am going with the circuit in #37. I am building this as a tool, but as a learning project as well. When I have something that works for me, I'll use it until I have the need to upgrade it.

Just noticed that the fans on the heatsinks blow air into them, instead of out. If they are inside the case, this would heat everything up (not good). I am thinking on taking the fans out and flip them so they blow air from vents on the sides/front, across the supply and out of the case, cooling everything. It's a pity because they are nicely fitted to the heatsinks... Or I could have the heatsinks sit at the back of the case, using thermal pads (switching taps to reduce power dissipation). Any ideas on thermal design etc?
Juan

The heatsink is fundamental to the success of all power supplies, even very simple low-power types.

I did a rough thermal budget for your PSU, assuming 0V to 20V and 0A to 1A and based on that you will need a heatsink of 2 degC/W or lower if you are using a single output power transistor.

You can do a very simple test to establish the actual thermal resistance of a heatsink. Clamp one of those gold-colored high-power, wire-wound resistors directly to your heat sink where the transistor will fit. Then dissipate, say 10W in the resistor.

After no less than 15 minutes, measure the room temperature and measure the temperature of the heatsink as close to the resistor clamp area as possible. With the three parameters: power dissipation, heatsink temperature, room temperature (ambient), we can calculate the thermal resistance of your heatsink.

spec · « **Reply #66 on:** November 18, 2018, 05:43:46 pm »

Online JuanGg,

Just a quick question: on PSU #37 do you want to control the voltage and current by MCU, or just voltage?

I take it that the control signal will be a positive going analog signal originating at 0V: 0V to 5V or 0V to 2V5 say.

spec · « **Reply #67 on:** November 18, 2018, 05:58:29 pm »

Quote from: spec on November 18, 2018, 05:24:01 pm

Quote from: JuanGg on November 18, 2018, 11:44:41 am
Quote
I think we've seen now that there are as many ways to build a power supply as there are posters here. ...
I think I am going with the circuit in #37. I am building this as a tool, but as a learning project as well. When I have something that works for me, I'll use it until I have the need to upgrade it.

Just noticed that the fans on the heatsinks blow air into them, instead of out. If they are inside the case, this would heat everything up (not good). I am thinking on taking the fans out and flip them so they blow air from vents on the sides/front, across the supply and out of the case, cooling everything. It's a pity because they are nicely fitted to the heatsinks... Or I could have the heatsinks sit at the back of the case, using thermal pads (switching taps to reduce power dissipation). Any ideas on thermal design etc?
Juan
The heatsink is fundamental to the success of all power supplies, even very simple low-power types.

I did a fag packet thermal budget for your PSU, assuming 0V to 20V and 0A to 1A and based on that you will need a heatsink of 2 degC/W or lower if you are using a single output power transistor.

You can do a very simple test to establish the actual thermal resistance of a heatsink. Clamp one of those gold-colored high-power, wire-wound resistors directly to your heat sink where the transistor will fit. Then dissipate, say 10W in the resistor.

After no less than 15 minutes, measure the room temperature and measure the temperature of the heatsink as close to the resistor clamp area as possible. With the three parameters: power dissipation, heatsink temperature, room temperature (ambient), we can calculate the thermal resistance of your heatsink.

As you say, suck or blow, makes little difference, provided the hot air is expelled from the equipment case, the cooling air impinges on the heatsink, and there are no stagnant areas around the heatsink

JuanGg · « **Reply #68 on:** November 18, 2018, 08:24:41 pm »

Quote

The way a processor heat sink fan is used is it removes the heat from the immediate area of the processor heatsink inside the PC case then they have a large fan or two on the back of the case to remove the heat from the case. If you can design your power supply so the top of the fan/heatsink you're using...

Quote

As you say, suck or blow, makes little difference, provided the hot air is expelled from the equipment case, the cooling air impinges on the heatsink, and there are no stagnant areas around the heatsink

Will post a screenshot of the CAD when I have it in a decent state. I am thinking both heatsinks at the back, two holes for the fans directly on the back panel and some vents.

Quote

The heatsink is fundamental to the success of all power supplies, even very simple low-power types.

I did a rough thermal budget for your PSU, assuming 0V to 20V and 0A to 1A and based on that you will need a heatsink of 2 degC/W or lower if you are using a single output power transistor.

You can do a very simple test to establish the actual thermal resistance of a heatsink. Clamp one of those gold-colored high-power, wire-wound resistors directly to your heat sink where the transistor will fit. Then dissipate, say 10W in the resistor.

After no less than 15 minutes, measure the room temperature and measure the temperature of the heatsink as close to the resistor clamp area as possible. With the three parameters: power dissipation, heatsink temperature, room temperature (ambient), we can calculate the thermal resistance of your heatsink.

Thanks again for the effort!. Not sure about going to 1 A with the 1 A transformer (although it was on a hermetically sealed plastic case, so current rating would be a bit more I suspect...). I don't have any of those resistors nor a way to acurately measure temperature (I have some k thermocouples around, but my DMM does not support it

). What I do have is a 3d printer with a heated bed that i can bring up to 95 ºC. That may work.

However, I found an Intel thermal desing guide recomending this heatsink for aplications requiring about 0.6 ºC /W case to ambient. (here it is: http://download.intel.com/design/intarch/designgd/27370403.pdf , there is a graph in page 28 and a table with the heatsink on the following page, assuming ambient temperature of 42 ºC). (With the fan running I would assume). It should be more than adecuate.

Quote

Online JuanGg,

Just a quick question: on PSU #37 do you want to control the voltage and current by MCU, or just voltage?

I take it that the control signal will be a positive going analog signal originating at 0V: 0V to 5V or 0V to 2V5 say.

I would like to control both if posible, so I can set both say using an encoder/keypad. I intended to use a filtered 12 bit PWM, from 0 to 5 V. I did this on my electronic load and it worked quite well (more details here: https://www.eevblog.com/forum/projects/arduino-based-electronic-load). If I need to, maybe an i2c DAC with a voltage reference.

Juan

spec · « **Reply #69 on:** November 18, 2018, 08:56:37 pm »

All sounds good. You have a handle on the heatsink.

You may be correct about the transformer, but on paper it should do 20V at 1A and that is what I suggest we should aim for. If the transformer won't cut it, that is just bad luck, but nothing lost.

Thanks for info about the V and I control inputs.

Just to restate then, the signal will be analog and ground referenced. 0V input will be zero volts and zero amps output.

I don't care too much about the maximum input voltage (that can be changed at any time in the future) but I will work with 5V in both cases.

We have not discussed input resistance. Would around 100k Ohms be suitable?

JuanGg · « **Reply #70 on:** November 19, 2018, 03:14:08 pm »

Quote

All sounds good. You have a handle on the heatsink.

You may be correct about the transformer, ...

Perfect.

Quote

We have not discussed input resistance. Would around 100k Ohms be suitable?

No idea on input resistance on power supplies. Looked for it on the internet but no luck. How can it be calculated?
Juan

spec · « **Reply #71 on:** November 19, 2018, 06:26:41 pm »

Quote

We have not discussed input resistance. Would around 100k Ohms be suitable?

Quote from: JuanGg on November 19, 2018, 03:14:08 pm

No idea on input resistance on power supplies. Looked for it on the internet but no luck. How can it be calculated?
Juan

Think of the input resistance of a scope (1M Ohm), Audio power amplifier (10k Ohm), Muiltimeter set to 10V DC range (1G Ohm). The only criteria is, will your analog signal drive the input resistance.

Don't worry about it, 100K will be fine. And if not it can be changed anyway.

JuanGg · « **Reply #72 on:** November 19, 2018, 06:46:13 pm »

Quote

Think of the input resistance of a scope (1M Ohm), Audio power amplifier (10k Ohm), Muiltimeter set to 10V DC range (1G Ohm). The only criteria is, will your analog signal drive the input resistance.

Don't worry about it, 100K will be fine. And if not it can be changed anyway.

Ok. Sorry, just realized now. Input resistance of the control signals

. Alright. As they will probably be driven from an op amp buffer, 100k should be more than enough.

spec · « **Reply #73 on:** November 19, 2018, 07:24:53 pm »

Quote from: JuanGg on November 19, 2018, 06:46:13 pm

Quote
Think of the input resistance of a scope (1M Ohm), Audio power amplifier (10k Ohm), Muiltimeter set to 10V DC range (1G Ohm). The only criteria is, will your analog signal drive the input resistance.

Don't worry about it, 100K will be fine. And if not it can be changed anyway.
Ok. Sorry, just realized now. Input resistance of the control signals . Alright. As they will probably be driven from an op amp buffer, 100k should be more than enough.

All good

JuanGg · « **Reply #74 on:** November 23, 2018, 07:51:34 pm »

Haven't been able to do much since last time I posted (exam week!, and more to come), here is what I have on the case so far. Hopefully I will finish designing the back panel and print it this weekend. I will leave the front panel to last, when meters and binding posts arrive. Attached are a couple of screenshots, one showing the airflow. Excuse the crudity of the pictures. There is yet a lot of work to be done.

spec · « **Reply #75 on:** November 23, 2018, 11:50:51 pm »

Looking good. Your case has a sort of military look to it.

Just one thing: I would suggest having the fans sucking or blowing out. You don't want to have the hot air from the heatsinks blowing into the case.

Apart from the power transistor/s, the components including transformers, rectifiers and reservoir capacitors will be OK with convection cooling and do not need fan cooling, providing you have sufficient vents on the case. The bridge rectifier may need a heatsink, but the chassis will do for that.

PS: Don't let anything get in the way of your studies. Good exam results are essential- you can do PSUs anytime.

JuanGg · « **Reply #76 on:** November 24, 2018, 09:55:17 am »

Quote

Looking good. Your case has a sort of military look to it.

Just one thing: I would suggest having the fans sucking or blowing out. You don't want to have the hot air from the heatsinks blowing into the case.

Apart from the power transistor/s, the components including transformers, rectifiers and reservoir capacitors will be OK with convection cooling and do not need fan cooling, providing you have sufficient vents on the case. The bridge rectifier may need a heatsink, but the chassis will do for that.

It must be the green I guess. It just happens to be the only filament I have around.
Having the fans blow the hot air out is what I wanted on the first place. I will try to cut the fans out of their support and mount them reversed.
Can't the bridge rectifiers be mounted on the same heatsink as the power transistor? I won't have much metal chassis, but just a cover. (Not shown).

Quote

PS: Don't let anything get in the way of your studies. Good exam results are essential- you can do PSUs anytime.

No worries. I have my priorities right. I don't work on this on weekdays.

spec · « **Reply #77 on:** November 24, 2018, 04:35:15 pm »

Quote from: JuanGg on November 24, 2018, 09:55:17 am

Can't the bridge rectifiers be mounted on the same heatsink as the power transistor? I won't have much metal chassis, but just a cover. (Not shown).

Not advisable- look upon the power transistor heat sinks as heaters as far as other components are concerned.

Normally an aluminum bracket or sheet is sufficient. The bridge rectifiers will only be dissipating around 500mW each anyway. Rectifier cooling is not an issue.

Quote

PS: Don't let anything get in the way of your studies. Good exam results are essential- you can do PSUs anytime.

Quote from: JuanGg on November 24, 2018, 09:55:17 am

No worries. I have my priorities right. I don't work on this on weekdays.

That's the way

perieanuo · « **Reply #78 on:** November 24, 2018, 08:32:10 pm »

The ventilation for even 1 amp is overkill,I did more with 1 40x40 quiet one.with those and 2-3 power transistors you can go to 10 amps.I end up putting an termistor on the heatsink with pwm control at 3 different temps(I have atmega328 board monitoring U-I with ina sensor showed on oled white,looks fine).And the atmega can easily monitor between U and I the temp and put some pwn on the ventilation, cost bucks and works fine, coding is simple.
Regards,pierre

Envoyé de mon iPad en utilisant Tapatalk

fsr · « **Reply #79 on:** November 24, 2018, 11:08:48 pm »

When i buit mine, i went fanless. It had 34W max dissipation. I put the transistor with mica and carefully applied thermal grease on the heatsink. The MJ15003 transistor did most of the magic. Rth J-C of 0.7 °C/W and 200 °C max junction temperature. I don't really know how much i used it at full load + low voltage, but the magic smoke remains contained up to this day

Look at the attached datasheet for details of how to calculate this stuff. Page 10, "Heatsink Requirements". Just substitute the PD and other characteristics with the ones for your transistor.
TO-3's are hard to mount, but with that thermal figures, it's worth the effort for me! Check other power packages, anyways, but it's probably hard to beat a metal can mounted with two screws.

Remember that CPUs sometimes reach power dissipations up to 135 watts (but obviously not all CPUs). So that's probably too much cooling in this case.

Also remember that hot air moves up, so you need to put ventilation holes or fans near the top of the case blowing air out. That's why good vertical computer cases have the PSU on the top, and input ventilation holes on front, near the bottom.

In my case, i just put the heatsinks on the outside, so i didn't have to deal with that. Just natural convection, and it goes up unimpeded.

spec · « **Reply #80 on:** November 25, 2018, 06:06:12 am »

Quote from: perieanuo on November 24, 2018, 08:32:10 pm

The ventilation for even 1 amp is overkill,I did more with 1 40x40 quiet one.with those and 2-3 power transistors you can go to 10 amps.I end up putting an termistor on the heatsink with pwm control at 3 different temps(I have atmega328 board monitoring U-I with ina sensor showed on oled white,looks fine).And the atmega can easily monitor between U and I the temp and put some pwn on the ventilation, cost bucks and works fine, coding is simple.
Regards,pierre

Envoyé de mon iPad en utilisant Tapatalk

I take it that by ventilation, you mean fan cooling. I don't like the term overkill- it is a sweeping statement that can be used to criticize pretty much anything. After all, your house, car, PC are all overkill.

Although I tend to agree with you that fan cooling is probably is not necessary, you cannot say for sure without doing a thermal budget and to do that you would need to know the thermal resistance of the heatsink being used. Besides which, the OP already has the fans and heatsinks which were pulled from another unit.

The other point is that, not only will this PSU design and construction result in a usable PSU, but it will be a very good experience for the OP, who is just starting in his electronics career. It is probable that this PSU will be further developed, maybe to have more output voltage and current, at a later date, so a low thermal resistance heat sink would then be an advantage.

You mention about your own power supply- feel like posting a schematic? There is always great intrest in PSUs on EEV.

spec · « **Reply #81 on:** November 25, 2018, 06:32:06 am »

Quote from: fsr on November 24, 2018, 11:08:48 pm

When i buit mine, i went fanless. It had 34W max dissipation. I put the transistor with mica and carefully applied thermal grease on the heatsink. The MJ15003 transistor did most of the magic. Rth J-C of 0.7 °C/W and 200 °C max junction temperature. I don't really know how much i used it at full load + low voltage, but the magic smoke remains contained up to this day

That is real class. The MJ15003 is a beauty. Unfortunately the more mundane, but cheaper, 2N3055 has a thermal resistance junction to case of 1.52 DegC/W, more than twice that of the MJ1503. But the 2N3055 safe operating area (SOA) is better.

https://www.onsemi.com/pub/Collateral/MJ15003-D.PDF
https://www.onsemi.com/pub/Collateral/2N3055-D.PDF

Would be great to see a schematic of your PSU.

Update: just checked prices from DigiKey UK, the MJ15003 costs £6.36 and the 2N3055 costs £4.60, so not a vast price difference.

spec · « **Reply #82 on:** November 25, 2018, 07:49:00 am »

There has been quite a bit of discussion about heat sinking, both on this thread and elsewhere on EEV.

Just to give give an overview, here is a description of a thermal budget using an actual heatsink and power transistor.

Assume, due to space and cost considerations, that the following heatsink must be used in an equipment: https://www.apexanalog.com/resources/mechdrawings/hs14.pdf. This heatsink is quite expensive at £51.11UK and fairly large at 5 by 3 by 1.3 inches. Its thermal resistance in free air is 2 degC/W.

You have one 2N3055 transistor and you want to know how much power you can theoretically dissipate.

So the list of parameters is:

2N3055 maximum junction temperature (Tjmax): 200 degC
2N3055 thermal resistance junction to case (ThRjc): 1.52 degC/W
Insulating washer between 2N3055 case and heat sink thermal resistance (ThRwasher): 1 degC/W
Heat sink thermal resistance to air (ThRhs): 2 degC/W
Air temperature in the vicinity of heatsink (Tamb): 70 degC

Armed with this information, a thermal budget can be calculated from the following formula:

Tjmax = P (ThRjc + ThRwasher + ThRhs) + Tamb

P is required, so transposing gives, P = (Tjmax-Tamb) / (ThRjc + ThRwasher + ThRhs)

Inserting actual values gets, P = (200 - 70) / (1.52 + 1 + 2) = 28.76W

That is the maximum theoretical dissipation and does not include, a safety margin, SOA considerations, and temperature effects on SOA, for example.

At the end of day, you would probably end up with a practical maximum power dissipation of 20W to 25W.

fsr · « **Reply #83 on:** November 25, 2018, 03:57:45 pm »

Quote from: spec on November 25, 2018, 06:32:06 am

Would be great to see a schematic of your PSU.

I have some screen captures back from the days i built it. It's just a circuit it used to be on the LM317 datasheet i attached before, with some modifications, and also a negative version. I didn't really ever tested it, but seems to do the job. I had to build it in 3 boards, because of limitations on Eagle PCB.

If someone is interested on the PCBs, i can open Eagle and PDF them, or upload the files.

I posted it on another forum back in tha day, but it didn't catch much attention, so i never released much about it.

Again, the power transistors are MJ15003 and MJ15004.

JuanGg · « **Reply #84 on:** November 25, 2018, 08:28:52 pm »

I have printed and glued the back panel together. I won't bore you with the details, but sufices to say that big abs pieces don't get 3d printed without putting up a fight. I just printed it in pieces and acetone welded it together. Attached are some photos.
I think I am keeping the fan and heatsinks as they are, just blocking the inner sides, so air enters the back, goes through the heatsink and exits through vent holes on the sides, without going into the case.

Juan.

spec · « **Reply #85 on:** November 26, 2018, 05:40:57 am »

Quote

so air enters the back, goes through the heatsink and exits through vent holes on the sides, without going into the case.

JuanGg · « **Reply #86 on:** November 27, 2018, 09:15:44 pm »

Thinking about the user interface, I saw the power supply attached, which is was very similar to what I had thought (withoght the middle chanel). I just think it is very simple and functional. The downside is that it would require isolated comunication between chanels (serial via optocouplers between arduinos?). Each chanel would have its display, and only one of them would read the encoder and buttons, sending information to the other via isolated serial. This way I could do tracking or whatever. Other way I thought of doing is using a central arduino that reads the encoder and keys, and sends the information to the other two, also providing an isolated USB, so the PSU can be interfaced with a computer. (See diagram attached). Is it feasible? Or maybe too much for a first power supply? I don't think it is very hard to acomplish, and given the arduinos are very cheap, I don't mind including three of them in the unit.

Juan

wasyoungonce · « **Reply #87 on:** November 27, 2018, 10:05:04 pm »

Quote from: fsr on November 25, 2018, 03:57:45 pm

.............
If someone is interested on the PCBs, i can open Eagle and PDF them, or upload the files.I posted it on another forum back in tha day, but it didn't catch much attention, so i never released much about it...Again, the power transistors are MJ15003 and MJ15004.

Actually yes please...I need a basic fixed rail supply and some limited variable outputs. I already have qty 2 PS-3005Ds 30V/5A but I really need something simple to drive small motors (want to monitor their start voltage currents) and logic ccts. The others will do this but far easier to have a small supply with fixed and some variable outputs. Eagle .sch .brd etc would be great...also whats the I/P transformer ~ ~~15VAC?~~

In VA? I do have some decent 30VAC ~@120VA but they are probably a little too high in output.

I'm supposed to be trying to use KICad but its so difficult to leave something you already know so well!
Many thanks

edit.....Oh I love your PSU you have done.....:

https://easyeda.com/asokolsky/Analog-Lab-Power-Supply

I have to ask...why a 56V out as there is a 0-30V and other fixed O/P?. Maybe a bit of self gratification and promotion is due your way. I see you designed it in with your own Amp/Voltmeter and fan controller.

https://easyeda.com/asokolsky/AmpVolt-Meter-with-Fan-Control

Well done. Looking intently at them for an unashamed and undeserved diy for myself.

Mr. Scram · « **Reply #88 on:** November 28, 2018, 02:46:17 am »

Quote from: JuanGg on November 27, 2018, 09:15:44 pm

Thinking about the user interface, I saw the power supply attached, which is was very similar to what I had thought (withoght the middle chanel). I just think it is very simple and functional. The downside is that it would require isolated comunication between chanels (serial via optocouplers between arduinos?). Each chanel would have its display, and only one of them would read the encoder and buttons, sending information to the other via isolated serial. This way I could do tracking or whatever. Other way I thought of doing is using a central arduino that reads the encoder and keys, and sends the information to the other two, also providing an isolated USB, so the PSU can be interfaced with a computer. (See diagram attached). Is it feasible? Or maybe too much for a first power supply? I don't think it is very hard to acomplish, and given the arduinos are very cheap, I don't mind including three of them in the unit.

Juan

Ask yourself whether you want to multiplex the controls.

JuanGg · « **Reply #89 on:** November 28, 2018, 07:12:34 am »

Quote

Ask yourself whether you want to multiplex the controls.

I don't really know if it's worth it. I can try different ways and see how it works out. Anyway, the important thing is to get the analog PSU working. I just have a tendency to overcomplicate stuff. As I said, I'll leave the front panel to last.

perieanuo · « **Reply #90 on:** November 28, 2018, 09:49:45 am »

Quote from: spec on November 25, 2018, 06:06:12 am

Quote from: perieanuo on November 24, 2018, 08:32:10 pm
The ventilation for even 1 amp is overkill,I did more with 1 40x40 quiet one.with those and 2-3 power transistors you can go to 10 amps.I end up putting an termistor on the heatsink with pwm control at 3 different temps(I have atmega328 board monitoring U-I with ina sensor showed on oled white,looks fine).And the atmega can easily monitor between U and I the temp and put some pwn on the ventilation, cost bucks and works fine, coding is simple.

I take it that by ventilation, you mean fan cooling. I don't like the term overkill- it is a sweeping statement that can be used to criticize pretty much anything. After all, your house, car, PC are all overkill.

Although I tend to agree with you that fan cooling is probably is not necessary, you cannot say for sure without doing a thermal budget and to do that you would need to know the thermal resistance of the heatsink being used. Besides which, the OP already has the fans and heatsinks which were pulled from another unit.

The other point is that, not only will this PSU design and construction result in a usable PSU, but it will be a very good experience for the OP, who is just starting in his electronics career. It is probable that this PSU will be further developed, maybe to have more output voltage and current, at a later date, so a low thermal resistance heat sink would then be an advantage.

You mention about your own power supply- feel like posting a schematic? There is always great intrest in PSUs on EEV.

hi,
for me overkill means too much.
I attached my schematic, it's not the final version, I upgraded current sense.But the idea is, I put also temperature measurement and tested the thing at 3.000 Amps with variable load.
The temp stabilised to 44-48 deg C if I remember right.I sense with 10K NTC thermister and I was calibrated with Volcraft PL-125-T2USB.
I pwm to fan starting at 30 deg C (33%, then 66% at 40deg then 100% at 50degC).
If my 40*40 fan succeed to cool the 2xTIPL790A transistors delivering 3amps at 5V from 24 Vac transformer that means those 2x80*80 fans will be toooo much for what OP wants( he has 2 amps rated transformers).
Imho, he can think of more current with those fan/cooler combination and he MUST reduce fans speed, a variable lab supply MUST be quiet, it's not just fancy stuff, when you're focused doing electronics and coding some dsp last thing you need is a stupid vent crying.
Same cooling technique I did for other people in scopes for example (one of my bosses had enough of his oscillo and power supply doing noise when he was working on research stuff).
I take seriously the cooling, I'm not perfect at this but even implement a fan cooling controled by a thermistor and a fet who starts at a defined temperature for me is a must.
Attached I'll put schematics of one of my bench ps with power stage.just for example.I had a case and stuffed inside toroidal, power stage, simple opamp regulation (V/I control) and measurement with custom atmega 328 board, current module down to 0.1mA resolution is not in schematics, but it's in the picture

).
regards,Pierre

perieanuo · « **Reply #91 on:** November 28, 2018, 10:05:59 am »

Quote from: JuanGg on November 27, 2018, 09:15:44 pm

Thinking about the user interface, I saw the power supply attached, which is was very similar to what I had thought (withoght the middle chanel). I just think it is very simple and functional. The downside is that it would require isolated comunication between chanels (serial via optocouplers between arduinos?). Each chanel would have its display, and only one of them would read the encoder and buttons, sending information to the other via isolated serial. This way I could do tracking or whatever. Other way I thought of doing is using a central arduino that reads the encoder and keys, and sends the information to the other two, also providing an isolated USB, so the PSU can be interfaced with a computer. (See diagram attached). Is it feasible? Or maybe too much for a first power supply? I don't think it is very hard to acomplish, and given the arduinos are very cheap, I don't mind including three of them in the unit.

Juan

yes it's feasible, but you can do it with only 2 arduinos not 3, or better even 1 if you put galvanic separations, but costs a little more, you can calculate.what you economise on arduino you put in isolation, but switch to clean and pro solution.I did arduino-like board and ina219 voltage/current sense (mA resolution) but only 1 channel, I had only a small box for the PS (see my earlier post).
When I'll find a bigger one, I'll do like you said, central supervision and screen on 2 oleds or a big LCD.take a look on my pdf's if you want, posted earlier.control as I sugested the vents speed, most of the time you'll need zero active cooling so zero noise...you can do it with microcontroller/dsp or a simple FET transistor+thermister, costs nothing...
regards,pierre

JuanGg · « **Reply #92 on:** November 28, 2018, 03:31:20 pm »

Quote

I pwm to fan starting at 30 deg C (33%, then 66% at 40deg then 100% at 50degC).
If my 40*40 fan succeed to cool the 2xTIPL790A transistors delivering 3amps at 5V from 24 Vac transformer that means those 2x80*80 fans will be toooo much for what OP wants( he has 2 amps rated transformers).
Imho, he can think of more current with those fan/cooler combination and he MUST reduce fans speed, a variable lab supply MUST be quiet, it's not just fancy stuff, when you're focused doing electronics and coding some dsp last thing you need is a stupid vent crying. ...

Better safe than sorry

. I will do temperature-controlled fans anyway. I did implement a pwm temperature controlled fan on my electronic load and it worked quite well. Plus, this fans are quiet enough (compared to the rigol 1054z's anyway...). In fact, one of the transformers I plan to use is 1A rated.

Quote

yes it's feasible, but you can do it with only 2 arduinos not 3, or better even 1 if you put galvanic separations, but costs a little more, you can calculate.what you economise on arduino you put in isolation, but switch to clean and pro solution.I did arduino-like board and ina219 voltage/current sense (mA resolution) but only 1 channel, I had only a small box for the PS (see my earlier post).
When I'll find a bigger one, I'll do like you said, central supervision and screen on 2 oleds or a big LCD.take a look on my pdf's if you want, posted earlier.control as I sugested the vents speed, most of the time you'll need zero active cooling so zero noise...you can do it with microcontroller/dsp or a simple FET transistor+thermister, costs nothing...
regards,pierre

I just thought it would be easier to have isolated digital comunications, so I don't have to deal with nonlinearities on analog optocouplers and such (any other way of getting galvanic isolation?). Plus, I would have to control two four-digit sevent segment displays for each chanel. Anyway, I will deal with that later.

I am waiting for the AC mains conector (it's taking ages to arrive!) so I can wire the raw psu and test it. Regarding that, do I go with tap-switching?

Juan

fsr · « **Reply #93 on:** November 30, 2018, 03:57:40 pm »

Quote from: wasyoungonce on November 27, 2018, 10:05:04 pm

Quote from: fsr on November 25, 2018, 03:57:45 pm
.............
If someone is interested on the PCBs, i can open Eagle and PDF them, or upload the files.I posted it on another forum back in tha day, but it didn't catch much attention, so i never released much about it...Again, the power transistors are MJ15003 and MJ15004.

Actually yes please...I need a basic fixed rail supply and some limited variable outputs.

I found the files, and posted them here: https://www.eevblog.com/forum/oshw/basic-lm317-based-cvcc-15v15v-1-5a-split-lab-psu/msg2003093/#msg2003093

wasyoungonce · « **Reply #94 on:** November 30, 2018, 05:10:04 pm »

Quote from: fsr on November 30, 2018, 03:57:40 pm

Quote from: wasyoungonce on November 27, 2018, 10:05:04 pm
Quote from: fsr on November 25, 2018, 03:57:45 pm
.............
If someone is interested on the PCBs, i can open Eagle and PDF them, or upload the files.I posted it on another forum back in tha day, but it didn't catch much attention, so i never released much about it...Again, the power transistors are MJ15003 and MJ15004.

Actually yes please...I need a basic fixed rail supply and some limited variable outputs.
I found the files, and posted them here: https://www.eevblog.com/forum/oshw/basic-lm317-based-cvcc-15v15v-1-5a-split-lab-psu/msg2003093/#msg2003093

Many thanks watching that thread now

coppercone2 · « **Reply #95 on:** November 30, 2018, 05:21:40 pm »

Exams more important then anything in engineering?? Set sail for the sales department!

Whats important is doing the job right at any cost.

JuanGg · « **Reply #96 on:** November 30, 2018, 09:03:56 pm »

Quote

Exams more important then anything in engineering?? Set sail for the sales department!

Whats important is doing the job right at any cost.

I try to do both things right.

Here is some progress with the case. I have attached the transformers, changed locations a little bit. My idea is to have the raw PSU section between transformers and the rest on the sides. I'll try to wire the bridge rectifier and filter cap for one side this weekend. It will temporarily take the full 25 Vac, I'll eventually add a relay and make a PCB.

fsr · « **Reply #97 on:** December 01, 2018, 12:10:41 am »

Quote from: wasyoungonce on November 30, 2018, 05:10:04 pm

Quote from: fsr on November 30, 2018, 03:57:40 pm
Quote from: wasyoungonce on November 27, 2018, 10:05:04 pm
Quote from: fsr on November 25, 2018, 03:57:45 pm
.............
If someone is interested on the PCBs, i can open Eagle and PDF them, or upload the files.I posted it on another forum back in tha day, but it didn't catch much attention, so i never released much about it...Again, the power transistors are MJ15003 and MJ15004.

Actually yes please...I need a basic fixed rail supply and some limited variable outputs.
I found the files, and posted them here: https://www.eevblog.com/forum/oshw/basic-lm317-based-cvcc-15v15v-1-5a-split-lab-psu/msg2003093/#msg2003093
Many thanks watching that thread now

JuanGg · « **Reply #98 on:** December 02, 2018, 07:30:58 pm »

Here are some photos of the wiring I have done so far. I just took the ac mains conector from my other power supply as it's running off a wall wart, I'll replace it when the new one arrives. I also changed the 10 A fuse that came with it with a 1 A one, which is the smallest I have around. I'll put a smaller one later.
I have wired all the transformers to the mains input, all conections heatshrunk and all exposed areas covered in electrical tape. (I know it's not the best, but it lessens the chance of me getting shocked). Transformer cores are grounded and I used reasonably thick wire. Wires coming out of the transformers are color-coded so I know hot to connect them so that the phases are in sync (I checked that with the scope). I will continue next weekend.

Juan

JuanGg · « **Reply #99 on:** December 02, 2018, 10:48:18 pm »

I have "bodged" the bridge rectifier and the filter cap just to try it out. Attached is a picture and some scope screenshots. Last one is the capacitor being charged and discharged by the 500 $\$\Omega\$$ bleeder resistor after turn off. My electronic load can't handle more than 30 V, so I was unable to test it with it. I also don't have power resistors around, so I paralleled a dozen 1/4 W ones, but drawing 250 mA they still got too hot. Voltage dropped from 38 V to 36 V. I'll look for means of testing it properly.

Juan

perieanuo · « **Reply #100 on:** December 03, 2018, 07:30:52 am »

hi,
for me the 2 transformers are a little on the edge for superior current limit.
Translation, I doubt they can take 1A for long time.
But for first PSU it's fine, if you want to power arduinos it will do fine.
Anyway changing them is 15' job, continue with pcb and all.
regards, Pierre

JuanGg · « **Reply #101 on:** December 03, 2018, 05:02:29 pm »

Quote

hi,
for me the 2 transformers are a little on the edge for superior current limit.
Translation, I doubt they can take 1A for long time.
But for first PSU it's fine, if you want to power arduinos it will do fine.
Anyway changing them is 15' job, continue with pcb and all.
regards, Pierre

On the wall - warts they came from said 15 V 1 A, but who knows. I am aiming at 500-600 mA anyways. I can always upgrade the transformers later. I'll make a PCB with the rectifier, cap, tap-switching relay and maybe an lm 317 for a 10 V rail (to power the Arduino, fans relays, etc.)Any thoughts on this?

Juan

perieanuo · « **Reply #102 on:** December 03, 2018, 05:47:04 pm »

Quote from: JuanGg on December 03, 2018, 05:02:29 pm

Quote
hi,
for me the 2 transformers are a little on the edge for superior current limit.
Translation, I doubt they can take 1A for long time.
But for first PSU it's fine, if you want to power arduinos it will do fine.
Anyway changing them is 15' job, continue with pcb and all.
regards, Pierre

On the wall - warts they came from said 15 V 1 A, but who knows. I am aiming at 500-600 mA anyways. I can always upgrade the transformers later. I'll make a PCB with the rectifier, cap, tap-switching relay and maybe an lm 317 for a 10 V rail (to power the Arduino, fans relays, etc.)Any thoughts on this?

Juan

Me,I always put a minimum 10 amps rectifier screwed to the bottom case, then main capacitors on separate pcb or glued to the case with 2-face adhesive like 3M stuff, the control board itself separate pcb.but you can do your thing

Of course fuses for every transformer output and 220V input.those days on outputs I tend to put rearmable fuses (ptc) for currents like 0-2 Amps, tired of fuse stocking...
Regards,pierre

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cdev · « **Reply #103 on:** December 03, 2018, 09:00:22 pm »

1 amp is a lot of power for low voltage stuff, most of the times I use my triple bench supply I use MUCH less than its rated 7v@6a and 20v @ 3 amps x2 . However, lots of the time I need two voltages at the same time with current limiting. If I were you I would make two identical supplies and make them well regulated with adjustable current limiting and some kind of notification (LED and/or beeper would be best) when the current draw reaches that limit. A nice multiturn pot for voltage and a digital 4 digit display of the voltage and current (so you can set the voltage + current limiting easily) with a floating decimal point (around $3 on ebay) is nice too.

You could probably get away with multiple common fixed voltages but make sure you have a nice pot to adjust the current limit.

fsr · « **Reply #104 on:** December 03, 2018, 11:01:17 pm »

Quote from: JuanGg on December 03, 2018, 05:02:29 pm

Quote
hi,
for me the 2 transformers are a little on the edge for superior current limit.
Translation, I doubt they can take 1A for long time.
But for first PSU it's fine, if you want to power arduinos it will do fine.
Anyway changing them is 15' job, continue with pcb and all.
regards, Pierre

On the wall - warts they came from said 15 V 1 A, but who knows. I am aiming at 500-600 mA anyways. I can always upgrade the transformers later. I'll make a PCB with the rectifier, cap, tap-switching relay and maybe an lm 317 for a 10 V rail (to power the Arduino, fans relays, etc.)Any thoughts on this?

Juan

And did the wall-warts had a bridge rectifier with capacitive filter? Because the maximum current output, ripple, and other characteristics varies with different rectifier circuits. Take a look:

JuanGg · « **Reply #105 on:** December 04, 2018, 06:50:05 am »

Quote

And did the wall-warts had a bridge rectifier with capacitive filter? Because the maximum current output, ripple, and other characteristics varies with different rectifier circuits. Take a look:

No, they were ac ones, just the tranformer. Thank you for the very informative sheet.

JuanGg · « **Reply #106 on:** December 04, 2018, 03:51:34 pm »

Quote

Me,I always put a minimum 10 amps rectifier screwed to the bottom case, then main capacitors on separate pcb or glued to the case with 2-face adhesive like 3M stuff, the control board itself separate pcb.but you can do your thing
Of course fuses for every transformer output and 220V input.those days on outputs I tend to put rearmable fuses (ptc) for currents like 0-2 Amps, tired of fuse stocking...
Regards,pierre

I was thinking of making a separate pcb with rectifier, cap, relay, even fan controller. I won't do it just for now, as I may be needing more voltage rails for the regulating pcb. The big transformer already has fuses on every winding, but I may add some more.

Quote

1 amp is a lot of power for low voltage stuff, most of the times I use my triple bench supply I use MUCH less than its rated 7v@6a and 20v @ 3 amps x2 . However, lots of the time I need two voltages at the same time with current limiting. If I were you I would make two identical supplies and make them well regulated with adjustable current limiting and some kind of notification (LED and/or beeper would be best) when the current draw reaches that limit. A nice multiturn pot for voltage and a digital 4 digit display of the voltage and current (so you can set the voltage + current limiting easily) with a floating decimal point (around $3 on ebay) is nice too.

You could probably get away with multiple common fixed voltages but make sure you have a nice pot to adjust the current limit.

That's precisely what I want to do. Dual 0-25 V @ 0.5 A. Arduino controlled, with filtered 12-bit pwm as DAC. I will be using the arduino's ADCs (10 bit) to measure voltage and current, showing the readings on 4-digit 7 segment displays or on an LCD. I can also display the set values as well.

I don't really know what to do now. I may start breadboard the circuit in #37.
Juan

perieanuo · « **Reply #107 on:** December 04, 2018, 05:06:49 pm »

37 may not work, missing reverse output diode, me I don't like it.
my favs are sokolsky one and mine course

regards,pierre

JuanGg · « **Reply #108 on:** December 04, 2018, 07:43:46 pm »

Quote

37 may not work, missing reverse output diode, me I don't like it.
my favs are sokolsky one and mine course
regards,pierre

Well that diode can be easily added.
I can't do the sokolsky one as-is as I don't have another transformer to derive the +-5V referenced to the output.Any ideas?

Juan

JuanGg · « **Reply #109 on:** December 04, 2018, 09:53:54 pm »

Connecting only the 15 Vac transformer, I had the raw PSU running at 1 A for 20 minutes, using my electronic load. The rectifier got warm, so did the transformer. Not too hot to keep one's hand on it for a while, so I guess it will be fine. DC voltage out dropped from 17 to 15 V and I was getting about 1 Vpp ripple.

Juan

perieanuo · « **Reply #110 on:** December 04, 2018, 10:13:29 pm »

Quote from: JuanGg on December 04, 2018, 07:43:46 pm

Quote
37 may not work, missing reverse output diode, me I don't like it.
my favs are sokolsky one and mine course
regards,pierre

Well that diode can be easily added.
I can't do the sokolsky one as-is as I don't have another transformer to derive the +-5V referenced to the output.Any ideas?

Juan

Dcdc converter (1.5 euros ebay) with capacitors aside or rcd to generate -5V from AC branch.see my schematic as guide,I use 24V and 12V outputs from transformer.
I doubt #37 works...

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fsr · « **Reply #111 on:** December 05, 2018, 02:22:34 am »

Quote from: JuanGg on December 04, 2018, 06:50:05 am

Quote
And did the wall-warts had a bridge rectifier with capacitive filter? Because the maximum current output, ripple, and other characteristics varies with different rectifier circuits. Take a look:

No, they were ac ones, just the tranformer. Thank you for the very informative sheet.

You're welcome.
If the wall-wart was AC, then it probably listed the transformer's AC output current, so if you use a diode bridge and capacitive filter, you can get about 620 mA of current from the 1A rated transformer. Any more current than that would be going over the maximum.

JuanGg · « **Reply #112 on:** December 05, 2018, 07:10:38 am »

Quote

Dcdc converter (1.5 euros ebay) with capacitors aside or rcd to generate -5V from AC branch.see my schematic as guide,I use 24V and 12V outputs from transformer.
I doubt #37 works...

Will look into that. As I previously said, I'll go with whatever you guys recommend.

Quote

If the wall-wart was AC, then it probably listed the transformer's AC output current, so if you use a diode bridge and capacitive filter, you can get about 620 mA of current from the 1A rated transformer. Any more current than that would be going over the maximum

As they were sealed up in a plastic enclosure, I suppose they would be able to handle a bit more. I am aiming for 500-600 mA output current anyway, so no problem there.

Juan

cdev · « **Reply #113 on:** December 06, 2018, 02:02:33 am »

Try an old ATX power supply as your power source. Do you have one?

JuanGg · « **Reply #114 on:** December 06, 2018, 09:49:31 am »

Quote from: cdev on December 06, 2018, 02:02:33 am

Try an old ATX power supply as your power source. Do you have one?

I have two in fact. But they are mains earth referenced, so not suitable for my purpose. One of them is fitted with some binding posts, and that's what I have been using for a while.
Plus, I already have the transformers mounted and the enclosure half made. I am not changing that unless I have to.

Juan

JuanGg · « **Reply #115 on:** December 06, 2018, 06:56:59 pm »

Some of the stuff I ordered just arrived, including 7-segment displays, 0.1 power resistors, some perfboard etc. I have been playing around with some of them, towards making the "easy" digital part. Who writes a 7-segment display library and doesn't support decimal points?

.
So far I have managed to write a function that displays a decimal number. Also I've made a simple test program that lets you set the value on the display via a rotary encoder. My idea is to have two of this displays per chanel, showing voltage and current.
(I know I should focus on the analog side first, just wanted to get something working)
Juan

JuanGg · « **Reply #116 on:** December 07, 2018, 01:50:47 pm »

Any thoughts on this schematic? (https://dangerfromdeer.com/2016/04/06/bench-power-supply-build-part-ii/). It seems simple enough and has 0-5 V inputs to set voltage and current, which can be easily changed from pots to DACs. No auxiliary transformers are needed as well. Current sensing can be changed.

Juan

xavier60 · « **Reply #117 on:** December 07, 2018, 10:38:08 pm »

It looks mostly fine to me.
I would do some experimenting with the compensation to reduce the size of C4 which would be causing slow reaction to sudden current overloads. The AC gain of the op-amps would have to be reduced with local feedback.

JuanGg · « **Reply #118 on:** December 08, 2018, 11:15:56 am »

Quote from: xavier60 on December 07, 2018, 10:38:08 pm

It looks mostly fine to me.
I would do some experimenting with the compensation to reduce the size of C4 which would be causing slow reaction to sudden current overloads. The AC gain of the op-amps would have to be reduced with local feedback.

Ok. I can breadboard the thing and try several capacitor values, loading it with my electronic load in pulse mode.
I suppose local feedback to reduce AC gain can be added by putting capacitors in the feedback loop, just between the output and the non inverting input of the op-amps.
I will order the current sense ic and the Darlington transistor, and maybe some DACs. I suppose I can use 2n3904's and 2n3906's instead of the BC337 and BC327.
Juan

xavier60 · « **Reply #119 on:** December 08, 2018, 11:48:35 am »

Quote from: JuanGg on December 07, 2018, 01:50:47 pm

Any thoughts on this schematic? (https://dangerfromdeer.com/2016/04/06/bench-power-supply-build-part-ii/). It seems simple enough and has 0-5 V inputs to set voltage and current, which can be easily changed from pots to DACs. No auxiliary transformers are needed as well. Current sensing can be changed.

Juan

I mocked up just the voltage regulation loop with different parts. I used a TIP35C and BC337 with 100Ω and 1KΩ B-E resistors for the Darlington. And a TLC072 op-amp.
The presence of C4 encourages the Darlington to oscillate at certain conditions. A 100Ω series Base resistor fixed that.
The loop was stable with C4 removed and a 1000pF compensating capacitor added to the op-amp, with or without an output capacitor.
The loop became unstable when I reduced the compensating capacitor to try to improve the load transient response.
This was mainly caused by high frequency phase reversal at Q2(BC548) caused by more signal being coupled through the Base to Collector capacitance than the expected amplified inverted signal. Bypassing R3 with 22pF made a big improvement.
I got the best load transient response of 5us by configuring Q2 as Common Base and swapping the op-amp inputs.
I can't do any testing of the current loop as I don't have the INA196 handy.

JuanGg · « **Reply #120 on:** December 08, 2018, 03:38:40 pm »

Quote

I mocked up just the voltage regulation loop with different parts. I used a TIP35C and BC337 with 100Ω and 1KΩ B-E resistors for the Darlington. And ...

Thanks a lot. I will draw a schematic with the changes you made and try it myself. Not sure about Q2 as common base, is it base to 3, emitter to 2 and collector to 1, as drawn on the schematic?
Juan

xavier60 · « **Reply #121 on:** December 08, 2018, 04:26:54 pm »

This shows Q2 configured as a Common Base amplifier. It is very good at transferring fast signals from ground referenced circuitry to circuitry referenced to a higher voltage.
Because the Base is tied to the 8V rail which is regarded as a signal ground, the Base acts like a shield between input and output. The Emitter is the signal input in a Common Base amplifier.
https://en.wikipedia.org/wiki/Common_base

xavier60 · « **Reply #122 on:** December 09, 2018, 02:01:22 am »

Q2 as a Common Base amplifier causes a possible complication.
Because it's non-inverting, the op-amp inputs had to be swapped.
When reference voltage is connected to the inverting input and the feedback divider to the non-inverting input, the op-amp can't ever function as a true Miller Integrator. The minimum gain can never be less than unity.
For the voltage regulation loop, a small amount of proportional gain can and does improved stability.
For the current loop it is an uncertainty.
With Q2 as a Common Emitter amplifier again and a bit of tweaking, the response and stability are near as good as the Common Base version.

JuanGg · « **Reply #123 on:** December 09, 2018, 08:20:16 pm »

Quote

Q2 as a Common Base amplifier causes a possible complication.
Because it's non-inverting, the op-amp inputs had to be swapped.

Thank you very much. I'll try to prototype it myself as soon as I can.

xavier60 · « **Reply #124 on:** December 09, 2018, 10:01:23 pm »

Because I don't have the IC for high-side current sensing. I have added low-side sensing.
I know it's not ideal because control circuit current flows through the shunt. It's only a few milliamps.
As expected, there is a massive current spike when the output is short circuited.
I do have a solution for this that is currently working in a bench supply that I had previously completed.
I'm trying to adapt it to the present design.
I have made small changes to the CV loop which is working very well.

not1xor1 · « **Reply #125 on:** December 10, 2018, 10:17:17 am »

Quote from: xavier60 on December 09, 2018, 10:01:23 pm

Because I don't have the IC for high-side current sensing. I have added low-side sensing.
I know it's not ideal because control circuit current flows through the shunt. It's only a few milliamps.
As expected, there is a massive current spike when the output is short circuited.
I do have a solution for this that is currently working in a bench supply that I had previously completed.
I'm trying to adapt it to the present design.
I have made small changes to the CV loop which is working very well.

I think you are looking for troubles with that design.
If you do not want to use an aux. supply voltage for the regulator circuit (Harrison design) the circuit below is much simpler, much easier to compensate (may even work with just a small cap across the feedback resistor) and might work up to 30-35 Vout.

So far I've just simulated lot of variations of this circuit in all possible conditions, but failed to find the time to test it (my lab is a mess because the walls are covered by mould and I had to move most of stuff elsewhere).

xavier60 · « **Reply #126 on:** December 10, 2018, 12:00:54 pm »

Quote from: JuanGg on December 09, 2018, 08:20:16 pm

Quote
Q2 as a Common Base amplifier causes a possible complication.
Because it's non-inverting, the op-amp inputs had to be swapped.
Thank you very much. I'll try to prototype it myself as soon as I can.

You might already know this. For the best results, the components shown below need to be wired directly together at the heat sink.
What part number Darlington have you decided to use?

perieanuo · « **Reply #127 on:** December 10, 2018, 12:01:43 pm »

After you do your first highside current sensing, you never come back to low side

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JuanGg · « **Reply #128 on:** December 10, 2018, 06:36:16 pm »

Quote

. Because I don't have the IC for high-side current sensing. I have added low-side sensing.
I know it's not ideal because control circuit current flows through the shunt. It's only a few milliamps.
As expected, there is a massive current spike when the output is short circuited.
I do have a solution for this that is currently working in a bench supply that I had previously completed.
I'm trying to adapt it to the present design.
I have made small changes to the CV loop which is working very well.

Looking good. Thanks again. I will see if I can order those current sense ICs. Not aiming for a precision PSU here and I can calibrate current error in software, so we may get away with low side current sense for starters.

Quote from: xavier60 on December 10, 2018, 12:00:54 pm

Quote from: JuanGg on December 09, 2018, 08:20:16 pm
Quote
Q2 as a Common Base amplifier causes a possible complication.
Because it's non-inverting, the op-amp inputs had to be swapped.
Thank you very much. I'll try to prototype it myself as soon as I can.
You might already know this. For the best results, the components shown below need to be wired directly together at the heat sink.
What part number Darlington have you decided to use?

I'll try to make it in a perfboard as compact as possible, right next to the heatsink. I don't have any darlingtongs at hand, only some 2n3055s and tip42s. I will have to order some, so whatever you guys recommend.

Quote

I think you are looking for troubles with that design.
If you do not want to use an aux. supply voltage for the regulator circuit (Harrison design) the circuit below is much simpler, much easier to compensate (may even work with just a small cap across the feedback resistor) and might work up to 30-35 Vout.

Thanks for the input. I'm going to need an 8-12V rail anyway to power the micro, displays, relays and fans.

Juan

xavier60 · « **Reply #129 on:** December 10, 2018, 11:04:30 pm »

Quote from: JuanGg on December 10, 2018, 06:36:16 pm

I'll try to make it in a perfboard as compact as possible, right next to the heatsink. I don't have any darlingtongs at hand, only some 2n3055s and tip42s. I will have to order some, so whatever you guys recommend.

It's not necessarily for the whole control circuit to be made compact and close to the heat sink. Mainly the high current rails, the input and output capacitors and those other components shown in the previous diagram that should be kept close together. I have about 15cm wires between the heat sink mounted components and the control circuit on the breadboard.
I'm using a TIP35C and BD137 for the Darlington at the moment. Using different transistors might alter the compensation.
I'm not familiar with common Darling types. What output voltage and current are you aiming for?
I'm worried that although that my mockup is working well, it might not work the same with different parts, mainly the transistors and the op-amp. I'm using the TLC072 which is very well suited.

What equipment do you have to test with?

I have included the current state of the circuit.
If you go ahead, don't bother with the CC loop until the CV loop is working properly.
I really like the topology of this power supply. I hope that it is successful. There doesn't seem to be much other viable alternatives with single control rail referenced to the - output terminal.

JuanGg · « **Reply #130 on:** December 11, 2018, 03:37:28 pm »

Quote

It's not necessarily for the whole control circuit to be made compact and close to the heat sink. Mainly the high current rails, the input and output capacitors and those other components shown in the previous diagram that should be kept close together. I have about 15cm wires between the heat sink mounted components and the control circuit on the breadboard.

All right. I was thinking of doing one board with the rectifier, filter cap, relay, and 8V reg, then another one with the control circuit, close to the heatsink. I may do a separate small one with the components shown in #126. The microcontroller would be closer to the front panel, as it has deal with displays, encoders and such.

Quote

I'm using a TIP35C and BD137 for the Darlington at the moment. Using different transistors might alter the compensation.
I'm not familiar with common Darling types. What output voltage and current are you aiming for?
I'm worried that although that my mockup is working well, it might not work the same with different parts, mainly the transistors and the op-amp. I'm using the TLC072 which is very well suited.

I may order those same ones, they are not expensive and if I don't use them they will go to the parts bin, same for the op amps. I'll see if I have some with similar characteristics as the TL072. I'll try to prototype the thing with what I have anyway.

I am aiming at 20-25 V at 0.6 A. The thing is my transformer arrangement 10Vac in series with 15 Vac gives me about 38 V wich drops to 36 V whend drawing 1 A. Would that be too much input voltage? I just want to make sure.

Quote

What equipment do you have to test with?

-Rigol 1054z scope
-Cheap arbitrary function generator
-24V 4A electronic load (https://www.eevblog.com/forum/projects/arduino-based-electronic-load/)
-Two multimeters (a $5 one and a UT-61E), DPS3005 power supply module running of a wall wart.

Quote

I have included the current state of the circuit.
If you go ahead, don't bother with the CC loop until the CV loop is working properly.
I really like the topology of this power supply. I hope that it is successful. There doesn't seem to be much other viable alternatives with single control rail referenced to the - output terminal.

Agreed. Thank you for taking the time.

Juan

xavier60 · « **Reply #131 on:** December 11, 2018, 04:18:15 pm »

It would be good to find a suitable op-amp that's cheaper than the TLC072. It needs to have at least 15v/us slew rate and 10Mhz, and most important, the input common mode range has to include ground. Odd thing is the data sheet shows that the common mode includes ground only when the IC is powered by 12V. When I tested mine, I found no problem even down to 5V.

JuanGg · « **Reply #132 on:** December 11, 2018, 06:25:20 pm »

Quote

It would be good to find a suitable op-amp that's cheaper than the TLC072. It needs to have at least 15v/us slew rate and 10Mhz, and most important, the input common mode range has to include ground. Odd thing is the data sheet shows that the common mode includes ground only when the IC is powered by 12V. When I tested mine, I found no problem even down to 5V.

I have "breadboarded" the circuit shown in #129 (only the CV loop), but using a 2n3055 as the series pass transistor, 2n3904s and 2n3906s for the remaining transistors and a lm358 op-amp. (which only has 1 MHz bandwidth, but its common mode range does include ground). I used whatever I had at hand, so no big expectations. Not the best conditions either with intermittent contacts and long wires. I powered the thing from a 14 V supply and used a 9V battery for the 8V rail (voltage was near 8V). A 100K pot served as the CV ref.

With no load appart from the feedback divider, it regulates without problems from 0 to about 12V. Using a 1K resistor as load made it oscillate, see scope screenshot.
Haven't had time for further testing. Also tried out the 6N137 opto couplers that I plan to use to comunicate both chanels of the psu.They seem to work just fine.

Juan

wasyoungonce · « **Reply #133 on:** December 11, 2018, 07:30:58 pm »

Quote from: JuanGg on December 11, 2018, 06:25:20 pm

Quote
It would be good to find a suitable op-amp that's cheaper than the TLC072. It needs to have at least 15v/us slew rate and 10Mhz, and most important, the input common mode range has to include ground. Odd thing is the data sheet shows that the common mode includes ground only when the IC is powered by 12V. When I tested mine, I found no problem even down to 5V.

I have "breadboarded" the circuit shown in #129 (only the CV loop), but using a 2n3055 as the series pass transistor, 2n3904s and 2n3906s for the remaining transistors and a lm358 op-amp. (which only has 1 MHz bandwidth, but its common mode range does include ground). I used whatever I had at hand, so no big expectations. Not the best conditions either with intermittent contacts and long wires. I powered the thing from a 14 V supply and used a 9V battery for the 8V rail (voltage was near 8V). A 100K pot served as the CV ref.

With no load appart from the feedback divider, it regulates without problems from 0 to about 12V. Using a 1K resistor as load made it oscillate, see scope screenshot.
Haven't had time for further testing. Also tried out the 6N137 opto couplers that I plan to use to comunicate both chanels of the psu.They seem to work just fine.

Juan

The oscillation could be due to the breadboard introducing capacitance in connections. This is known especially if trying to build buck/boost PSUs.... as well as linear

Sent from my iPhone using Tapatalk

xavier60 · « **Reply #134 on:** December 11, 2018, 10:03:52 pm »

The oscillation could be caused by layout or the slow op-amp or both.
I didn't expect to see the power transistor and main current paths on the breadboard.
I have the components shown in post #126 physically soldered together at the heat sink and separate from the breadboard. The component connections are no longer than their own leads allow. The idea is to keep the physical loop formed by C1, Q3, C2 and R9, fairly tight.
No "load related" current should be allowed to flow through any parts for the control circuit. These unwanted current paths can also be caused by connecting test equipment. I had trouble caused by a HF ground loop between my DSO and the bench supply that I used to power the mockup.

xavier60 · « **Reply #135 on:** December 11, 2018, 10:33:55 pm »

Quote from: JuanGg on December 11, 2018, 03:37:28 pm

I am aiming at 20-25 V at 0.6 A. The thing is my transformer arrangement 10Vac in series with 15 Vac gives me about 38 V wich drops to 36 V whend drawing 1 A. Would that be too much input voltage? I just want to make sure.
Juan

That voltage drop indicates a light load for that arrangement. You need about that voltage for 30V max out of the regulator output.
Need to keep the SOA rating of the transistor in mind. The transistor stress is difficult to determine because temperature and time are also factors.
http://www.hammondmfg.com/pdf/5c007.pdf

xavier60 · « **Reply #136 on:** December 12, 2018, 03:33:40 am »

Dominant pole compensation can be temporally applied by adding 1uF between the op-amp's output and inverting input.
The 47uF output capacitor should be a low ESR type.

JuanGg · « **Reply #137 on:** December 12, 2018, 11:55:26 am »

Quote

. The oscillation could be due to the breadboard introducing capacitance in connections. This is known especially if trying to build buck/boost PSUs.... as well as linear

Quote

. The oscillation could be caused by layout or the slow op-amp or both

Most likely. I will look for a better op-amp and make it in a perfboard.

Quote

. I didn't expect to see the power transistor and main current paths on the breadboard.
I have the components shown in post #126 physically soldered together at the heat sink and separate from the breadboard. The component connections are no longer than their own leads allow. The idea is to keep the physical loop formed by C1, Q3, C2 and R9, fairly tight.
No "load related" current should be allowed to flow through any parts for the control circuit. These unwanted current paths can also be caused by connecting test equipment. I had trouble caused by a HF ground loop between my DSO and the bench supply that I used to power the mockup.

There is no way I am going to draw more than some milliamps from that setup. Just wanted to test if the thing even worked with the components I used. If all looks ok, I'll solder the thing as proposed and socket the op- amp so it can be changed easily.I'll put the power transistor on the heatsink I have already mounted on the case.

JuanGg · « **Reply #138 on:** December 12, 2018, 12:03:37 pm »

Quote

. That voltage drop indicates a light load for that arrangement. You need about that voltage for 30V max out of the regulator output.
Need to keep the SOA rating of the transistor in mind. The transistor stress is difficult to determine because temperature and time are also factors.
http://www.hammondmfg.com/pdf/5c007.pdf

The 10Vac transformer is 2A rated, while the 15 Vac is 1 A.
As per the datasheet, ≈40V c-e and 0.6 A is well within 2n3055's SOA.

Another thing to keep in mind is the current return for the fan, relay, Arduino and displays. That current has to go somewhere.

xavier60 · « **Reply #139 on:** December 12, 2018, 01:23:14 pm »

Quote from: JuanGg on December 12, 2018, 12:03:37 pm

Quote
. That voltage drop indicates a light load for that arrangement. You need about that voltage for 30V max out of the regulator output.
Need to keep the SOA rating of the transistor in mind. The transistor stress is difficult to determine because temperature and time are also factors.
http://www.hammondmfg.com/pdf/5c007.pdf

The 10Vac transformer is 2A rated, while the 15 Vac is 1 A.
As per the datasheet, ≈40V c-e and 0.6 A is well within 2n3055's SOA.

Another thing to keep in mind is the current return for the fan, relay, Arduino and displays. That current has to go somewhere.

How are you deriving the control rail?
For simple analog control, there will be only a few milliamps of unwanted current through the shunt.
I'm also adding a 10ma load to the output which will return back to the left side of the shunt, so no problem.
Fan and relay current can be returned to the left side of the shunt also. But a micro-controller needs to be referenced to the right side.
A separate control supply will be needed.

Extra: High side sensing will solve some problems if it can be made to work properly.

xavier60 · « **Reply #140 on:** December 13, 2018, 02:01:35 pm »

I have made a PCB for the regulator circuit from post #129.
Only the CV loop is working for now. It's performing the same as it did on the breadboard, very well.
I found some LM358 op-amps here, so I fitted one in place of the TLC072. It worked just fine.
I have the compensation set so that load transient recovery is 5us.

JuanGg · « **Reply #141 on:** December 13, 2018, 04:18:10 pm »

Quote from: xavier60 on December 12, 2018, 01:23:14 pm

Quote from: JuanGg on December 12, 2018, 12:03:37 pm
Quote
. That voltage drop indicates a light load for that arrangement. You need about that voltage for 30V max out of the regulator output.
Need to keep the SOA rating of the transistor in mind. The transistor stress is difficult to determine because temperature and time are also factors.
http://www.hammondmfg.com/pdf/5c007.pdf

The 10Vac transformer is 2A rated, while the 15 Vac is 1 A.
As per the datasheet, ≈40V c-e and 0.6 A is well within 2n3055's SOA.

Another thing to keep in mind is the current return for the fan, relay, Arduino and displays. That current has to go somewhere.
How are you deriving the control rail?
For simple analog control, there will be only a few milliamps of unwanted current through the shunt.
I'm also adding a 10ma load to the output which will return back to the left side of the shunt, so no problem.
Fan and relay current can be returned to the left side of the shunt also. But a micro-controller needs to be referenced to the right side.
A separate control supply will be needed.

Extra: High side sensing will solve some problems if it can be made to work properly.

As we need an 8 V rail, it makes sense to derive it from the 10 Vac transformer, doing so from the full 25 Vac will be wasting power. The 10 Vac transformer is 2A rated as well, so there is more current available from that rail without limiting the power supply output current. I thought of doing it as attached. There is probably a simpler way, but that's what I came up with. In addition, taps can be switched depending on the output voltage to reduce power disipation. The relay can be controlled from the micro as that does not have to be particularly quick. I was thinking of using just an lm317 for the control rail. All current will be returned to the left of the shunt but from the arduino, which can take tens of mA, not drawing current from its pins. High side sensing would be definitely better, do I go ahead and buy some INA196 ICs? Any other solution?

Quote from: xavier60 on December 13, 2018, 02:01:35 pm

I have made a PCB for the regulator circuit from post #129.
Only the CV loop is working for now. It's performing the same as it did on the breadboard, very well.
I found some LM358 op-amps here, so I fitted one in place of the TLC072. It worked just fine.
I have the compensation set so that load transient recovery is 5us.

I can't thank you enough for taking the time. Nice to know that the LM358 does the job.

Juan

perieanuo · « **Reply #142 on:** December 13, 2018, 04:22:53 pm »

Put tap switching before bridge

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JuanGg · « **Reply #143 on:** December 13, 2018, 04:54:55 pm »

Quote from: perieanuo on December 13, 2018, 04:22:53 pm

Put tap switching before bridge

How would I get the 12V rail then?

perieanuo · « **Reply #144 on:** December 13, 2018, 04:57:55 pm »

Simply.
You series the 2 secondaries, 1 bridge, and in bridge insert one end and the middle or the other end of your series.
You have 10 or 25V on your bridge then
You want 2 loads?
If you want 12V and 35 V from same gnd you need 1 bridge only.and the from your 12 or 35 get whatever voltages you want, fixed or variable.
I don't get why 2 bridges help and the current return with 35v is a bit strange

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JuanGg · « **Reply #145 on:** December 13, 2018, 05:56:57 pm »

Quote from: perieanuo on December 13, 2018, 04:57:55 pm

Simply.
You series the 2 secondaries, 1 bridge, and in bridge insert one end and the middle or the other end of your series.
You have 10 or 25V on your bridge then
You want 2 loads?
If you want 12V and 35 V from same gnd you need 1 bridge only.and the from your 12 or 35 get whatever voltages you want, fixed or variable.
I don't get why 2 bridges help and the current return with 35v is a bit strange

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I don't know if I understood you correctly. I don't want to have fans, relay and displays drawing current from the 15V transformer at any time. Plus, 35V (theoretically, in reality it goes up to 38-9V) is too close or even over max input voltage of 780x series regulators or similar.

perieanuo · « **Reply #146 on:** December 13, 2018, 06:17:39 pm »

Quote from: JuanGg on December 13, 2018, 05:56:57 pm

Quote from: perieanuo on December 13, 2018, 04:57:55 pm
Simply.
You series the 2 secondaries, 1 bridge, and in bridge insert one end and the middle or the other end of your series.
You have 10 or 25V on your bridge then
You want 2 loads?
If you want 12V and 35 V from same gnd you need 1 bridge only.and the from your 12 or 35 get whatever voltages you want, fixed or variable.
I don't get why 2 bridges help and the current return with 35v is a bit strange

Envoyé de mon iPad en utilisant Tapatalk

I don't know if I understood you correctly. I don't want to have fans, relay and displays drawing current from the 15V transformer at any time. Plus, 35V (theoretically, in reality it goes up to 38-9V) is too close or even over max input voltage of 780x series regulators or similar.

You're free to choose what you like.
You find regulators for 40 V, I never saw one burned at this voltage.the 4 diodes for 2-nd unnecessary bridge, use them to lower voltage or any other mean to do this.
Put only one diode on 35v and you got lower V for your 7812 for ventilator...

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JuanGg · « **Reply #147 on:** December 13, 2018, 06:37:09 pm »

Quote from: perieanuo on December 13, 2018, 06:17:39 pm

Quote from: JuanGg on December 13, 2018, 05:56:57 pm
Quote from: perieanuo on December 13, 2018, 04:57:55 pm
Simply.
You series the 2 secondaries, 1 bridge, and in bridge insert one end and the middle or the other end of your series.
You have 10 or 25V on your bridge then
You want 2 loads?
If you want 12V and 35 V from same gnd you need 1 bridge only.and the from your 12 or 35 get whatever voltages you want, fixed or variable.
I don't get why 2 bridges help and the current return with 35v is a bit strange

Envoyé de mon iPad en utilisant Tapatalk

I don't know if I understood you correctly. I don't want to have fans, relay and displays drawing current from the 15V transformer at any time. Plus, 35V (theoretically, in reality it goes up to 38-9V) is too close or even over max input voltage of 780x series regulators or similar.
You're free to choose what you like.
You find regulators for 40 V, I never saw one burned at this voltage.the 4 diodes for 2-nd unnecessary bridge, use them to lower voltage or any other mean to do this.
Put only one diode on 35v and you got lower V for your 7812 for ventilator...

Envoyé de mon iPad en utilisant Tapatalk

I wouldn't mind doing it that way, the thing is:
80 mA (fan) + 2*40 mA (led displays) + 40 mA (Arduino) + 30 mA (relay) makes for a 230 mA load, plus regulating circuitry. (It'll be lower on average, specially the fan, just doing a worst case calculation here).
The 15 Vac transformer is 1A rated, resulting in practical ≈620 mA max current as per the chart in #135. I would be limited to ≈ 390 mA output current, and disipating ≈5W on the regulator. Just a thought. Thanks for the input.
Juan

perieanuo · « **Reply #148 on:** December 14, 2018, 06:48:20 am »

Quote

I wouldn't mind doing it that way, the thing is:
80 mA (fan) + 2*40 mA (led displays) + 40 mA (Arduino) + 30 mA (relay) makes for a 230 mA load, plus regulating circuitry. (It'll be lower on average, specially the fan, just doing a worst case calculation here).
The 15 Vac transformer is 1A rated, resulting in practical ≈620 mA max current as per the chart in #135. I would be limited to ≈ 390 mA output current, and disipating ≈5W on the regulator. Just a thought. Thanks for the input.
Juan

hi,
in real life you don't need tap switching.I don't do it even for 5 amps/0-27Vdc (limits of my 2'nd power supply).So I review my own theory, just cut off your budget and pain and series the secondaries, do the regulation and you're done.don't forget measuring current on low side will put problems for a beginner more than adding 4 $ with high side sensing, the right way to do it in your case.
If you do tap switch for future developement, that's another thing.
Anyway with your current/voltage specs you are well within SOA, you can do tap switch only for echology reasons, but buying relay+diodes+... isn't echological no more, it's just like the big lie buying electrical car will do good to the ozone

I just tried to simplify your design and get you focused on important stuff like keep it simple and get the voltage and current measurement simply and the PS stable.tap switch implies further decisional voltage comparator, watching what happens when commuting if you forgot to remove load you'll have to think what you do with the spike when you switch.imho too complicate for almost zero result.
tap switching method is for amps not for mAmps.But what the hell, I'm talking too much, as I said it before, do as you like
pierre

xavier60 · « **Reply #149 on:** December 14, 2018, 07:16:35 am »

The power transistor is mounted in direct contact with the heat sink bracket to reduce thermal resistance.
Although this power supply circuit responds to output short circuits much faster than most others, the LM358 takes 20us to take control of output current which peaks to 20 amps. The TLC072 responds in 2us.
The 3 BD137 transistors from left to right are 8V series pass, Darlington 1st transistor and constant current load.
Ill post an updated schematic after I do more testing. At this stage I have made only minor changes.

coromonadalix · « **Reply #150 on:** December 14, 2018, 08:28:42 am »

@Xavier60 Your montage is bad

You use an aluminum plate for your montage and put it back on an other "big" heatsink with an huge thermal pad, and you say you dont put an thermal pad or any micas on the power transistor ??

You want to avoid thermal loss and you're creating it the other way loll

not1xor1 · « **Reply #151 on:** December 14, 2018, 09:38:14 am »

Quote from: coromonadalix on December 14, 2018, 08:28:42 am

@Xavier60 Your montage is bad

You use an aluminum plate for your montage and put it back on an other "big" heatsink with an huge thermal pad, and you say you dont put an thermal pad or any micas on the power transistor ??

You want to avoid thermal loss and you're creating it the other way loll

it is anyway better than a transistor directly mounted to the heatsink via thermal pad as in this case the thermal pad is much larger.
I would rather object regarding the single screw for the power transistor since that usually do not ensure a proper contact with the heatsink.

JuanGg · « **Reply #152 on:** December 14, 2018, 03:14:25 pm »

Quote

hi,
in real life you don't need tap switching.I don't do it even for 5 amps/0-27Vdc (limits of my 2'nd power supply).So I review my own theory, just cut off your budget and pain and series the secondaries, do the regulation and you're done.don't forget measuring current on low side will put problems for a beginner more than adding 4 $ with high side sensing, the right way to do it in your case.
If you do tap switch for future developement, that's another thing.
Anyway with your current/voltage specs you are well within SOA, you can do tap switch only for echology reasons, but buying relay+diodes+... isn't echological no more, it's just like the big lie buying electrical car will do good to the ozone
I just tried to simplify your design and get you focused on important stuff like keep it simple and get the voltage and current measurement simply and the PS stable.tap switch implies further decisional voltage comparator, watching what happens when commuting if you forgot to remove load you'll have to think what you do with the spike when you switch.imho too complicate for almost zero result.
tap switching method is for amps not for mAmps.But what the hell, I'm talking too much, as I said it before, do as you like
pierre

You are right, I'll do away with tap switching, just wanted to try to as I had two transformers and some relays laying around, but as you said it's not worth it. Better keep it simple. But I'll have to look for a way of not getting control and fan current going through the 15V transistor, so I can have at least 500 mA output current.

Quote

The power transistor is mounted in direct contact with the heat sink bracket to reduce thermal resistance.
Although this power supply circuit responds to output short circuits much faster than most others, the LM358 takes 20us to take control of output current which peaks to 20 amps. The TLC072 responds in 2us.
The 3 BD137 transistors from left to right are 8V series pass, Darlington 1st transistor and constant current load.
Ill post an updated schematic after I do more testing. At this stage I have made only minor changes.

Not many people are willing to do what you are doing. Thank you. I am looking foward to seeing the revised schematic and how you are getting the 8V rail.

Juan

xavier60 · « **Reply #153 on:** December 15, 2018, 03:11:23 am »

The VCC1 8V regulator has a lot of components, it needs to have good performance, mainly a small dropout voltage which is 0.8V and tolerance to high input voltage.
Regulation will understandably be lost if the unregulated supply drops below 8.8V.
The control circuit using the TLC072 also works properly with VCC1 set to 7V.
R23 is just to allow the current to range to include zero in case the CC op-amps input offset happens to be a particular polarity.
If D3 is replaced with a silicon diode, there will be an increase in controlled overshoot current when the output terminals are short circuited.
If the preload circuit is replaced with a resistor, transient response will suffer in situations where B-E of Q5 is not forward biased.
In its present state, I estimate the circuit is safe to work up to 3A and 30V with an adequate heatsink and tap changing.
The next step will be to check the performance with paralleled power transistors. I have also ordered some Darlingtons for testing.

Any power supply design can become dysfunctional if not laid out properly.

* C8 changed to 330pF

xavier60 · « **Reply #154 on:** December 15, 2018, 05:35:02 am »

The purpose of Q4 is to allow the CC op-amp to operate open loop until it takes control of the power supply's output. At this point Q4 turns on, switching the compensating capacitor into circuit.
The waveform shows the shunt's voltage drop at the point the output is shorted and the current limit set to 2 amps.
The large initial spike is because of the 2us it takes the op-amp to respond. After that is an intentionally controlled overshoot.
If Q4 is jumpered out, the current will peak to about 20 amps and take hundreds of microseconds to settle.

JuanGg · « **Reply #155 on:** December 15, 2018, 07:34:23 pm »

Quote

The VCC1 8V regulator has a lot of components, it needs to have good performance, mainly a small dropout voltage which is 0.8V and tolerance to high input voltage.
Regulation will understandably be lost if the unregulated supply drops below 8.8V.
The control circuit using the TLC072 also works properly with VCC1 set to 7V. ...

Ok. Thank you. I will make a bill of materials and order the exact same parts, including the op-amp, so I can have a go myself. Any part number for the voltage reference? I'll order some ina196s just in case we want to try out high side sensing. Any other component suggestions that may come in handy are welcome.
I suppose adjusting the current range is a matter of resistor values.
I was thinking of mounting the perfboard as attached (not many options given the space I have to work with) Rectifiers and filter caps will go between the transformers, no tap switching.
Will a well-laid out perfboard do? If not I can make a pcb.
Also, will the 8 V rail be able to handle a couple hundred milliamps, or would it be better to have a separate regulator for the fan, displays and arduino?

Thanks again, Juan

Kleinstein · « **Reply #156 on:** December 15, 2018, 09:43:06 pm »

The reference could be something like TL431, at least the circuit looks like it is made for this reference. Changing the reference part is likely one of the easier parts. Another interesting (low noise) reference would be the LM329.

It may be tricky to use the VCC1 also use for other uses, as the current would also flow through the shunt. So about the only part one might consider running from here is something like an DAC if needed. One may even consider a different supply (negative side) for the OP.

Alone from the noise and interference, I would use a separate regulator for the µC and display.

Perf board should be OK, if one takes some care with star ground and similar. But this is also important for a board.
If the filter caps are separate, it may take an extra small capacitor on the perf board.

The shunt should of cause higher value for the lower current at some 500 mA.

JuanGg · « **Reply #157 on:** December 15, 2018, 10:55:43 pm »

Quote from: Kleinstein on December 15, 2018, 09:43:06 pm

The reference could be something like TL431, at least the circuit looks like it is made for this reference. Changing the reference part is likely one of the easier parts. Another interesting (low noise) reference would be the LM329.

It may be tricky to use the VCC1 also use for other uses, as the current would also flow through the shunt. So about the only part one might consider running from here is something like an DAC if needed. One may even consider a different supply (negative side) for the OP.

Alone from the noise and interference, I would use a separate regulator for the µC and display.

Perf board should be OK, if one takes some care with star ground and similar. But this is also important for a board.
If the filter caps are separate, it may take an extra small capacitor on the perf board.

The shunt should of cause higher value for the lower current at some 500 mA.

Thank you. That's what I was thinking, a PWM driven fan and multiplexed 7-segments would be no good for the control rail. I'll add a separate regulator, before the shunt. The micro still has to be referenced after the shunt I'm afraid. Displays take 5V, clk and data, I don't know if they can be driven with the micro referenced to the other side of the shunt...will have to test it.
I have some 0R1 resistors around, I can parallel more or less to get different shunt values. Still looking at high side current sensing, which would certainly make things simpler.

Juan

nick_d · « **Reply #158 on:** December 15, 2018, 11:57:23 pm »

Regarding obsolete parts available on Ebay, many are fakes. Do not trust them. If you have a WORKING circuit and can drop in the chip then fine, it will work or not (but may prove unreliable over time). On the other hand if you are developing then you can end up implementing elaborate workarounds for non-problems due to weird characteristics of the fake chips and wasting enormous development time. Don't do it.
cheers, Nick

xavier60 · « **Reply #159 on:** December 16, 2018, 01:14:27 am »

Quote from: JuanGg on December 15, 2018, 10:55:43 pm

Quote from: Kleinstein on December 15, 2018, 09:43:06 pm
The reference could be something like TL431, at least the circuit looks like it is made for this reference. Changing the reference part is likely one of the easier parts. Another interesting (low noise) reference would be the LM329.

It may be tricky to use the VCC1 also use for other uses, as the current would also flow through the shunt. So about the only part one might consider running from here is something like an DAC if needed. One may even consider a different supply (negative side) for the OP.

Alone from the noise and interference, I would use a separate regulator for the µC and display.

Perf board should be OK, if one takes some care with star ground and similar. But this is also important for a board.
If the filter caps are separate, it may take an extra small capacitor on the perf board.

The shunt should of cause higher value for the lower current at some 500 mA.

Thank you. That's what I was thinking, a PWM driven fan and multiplexed 7-segments would be no good for the control rail. I'll add a separate regulator, before the shunt. The micro still has to be referenced after the shunt I'm afraid. Displays take 5V, clk and data, I don't know if they can be driven with the micro referenced to the other side of the shunt...will have to test it.
I have some 0R1 resistors around, I can parallel more or less to get different shunt values. Still looking at high side current sensing, which would certainly make things simpler.

Juan

Having circuitry on both sides of the shunt could be made to work with a lot of thought because the shunt voltage can be kept low enough to not affect even TTL, but transients should be expected that would need to be filtered out which means using LC filtering. It gets messy.
An independent supply from a separate winding or transformer is best.
I have tested the design with a 0R1 shunt if needed, no problem. I would prefer 0R05 or less.
Perf board will be fine if the guideline in post #126 is followed. The most important detail is the single connection between the - output and the ground of the control circuit. I have represented this in the schematic also.
There is an advantage to using the same type parts in the schematic, mainly the transistors because it has all been tested.
The voltage divider and current set resistor,R12, can be changed to suit.

xavier60 · « **Reply #160 on:** December 16, 2018, 01:29:18 am »

The position of the shunt resistors on my PCB is a compromise. Ideally they should be in the middle of the PCB, but they would have obscured access to the transistors mounted on the heat sink bracket.
Yes, U2 is a TL431CLP. Ill update the schematic in post #154.

JuanGg · « **Reply #161 on:** December 16, 2018, 04:29:13 pm »

Quote

Having circuitry on both sides of the shunt could be made to work with a lot of thought because the shunt voltage can be kept low enough to not affect even TTL, but transients should be expected that would need to be filtered out which means using LC filtering. It gets messy.
An independent supply from a separate winding or transformer is best.
I have tested the design with a 0R1 shunt if needed, no problem. I would prefer 0R05 or less.
Perf board will be fine if the guideline in post #126 is followed. The most important detail is the single connection between the - output and the ground of the control circuit. I have represented this in the schematic also.
There is an advantage to using the same type parts in the schematic, mainly the transistors because it has all been tested.
The voltage divider and current set resistor,R12, can be changed to suit.

Ok, I'll try to look for more transformers to power the micro, fans and displays.
I have attached a bill of materials for your chematic, I hope I haven't missed anything. Couldn't find some of the transistors, added some similar ones: BD135 instead of BD137 and BD559 instead of BD558, I've gone through the datasheets and the characteristics are almost the same. I'll order this tonight, as well as some smd to dip adapters and misc stuff. It's going to take at least a week or two, I hope I can get something done this christmas.
I'll prototype the thing on a perfboard, but at the prices PCB services are running at, it may be an option to have some made later on (as I'm planing on having two channels... we'll see)
Juan

xavier60 · « **Reply #162 on:** December 16, 2018, 09:49:17 pm »

Quote from: JuanGg on December 16, 2018, 04:29:13 pm

Ok, I'll try to look for more transformers to power the micro, fans and displays.
I have attached a bill of materials for your chematic, I hope I haven't missed anything. Couldn't find some of the transistors, added some similar ones: BD135 instead of BD137 and BD559 instead of BD558, I've gone through the datasheets and the characteristics are almost the same. I'll order this tonight, as well as some smd to dip adapters and misc stuff. It's going to take at least a week or two, I hope I can get something done this christmas.
I'll prototype the thing on a perfboard, but at the prices PCB services are running at, it may be an option to have some made later on (as I'm planing on having two channels... we'll see)
Juan

Although the BD135 will be fine, try to keep all transistors above 60V where necessary, Is BD139 available instead?
There should be no BD558, only BC558, maybe a misprint. Although I haven't used the transistor types consistently, use BC546, BC556 where the voltage is high and BC548, Bc558 for low voltage positions or just about any BC series at hand.
The BC546, BC556, BC547, BC557, BC548, BC558 are very commonly used general purpose signal transistors. You should take the time to get familiar with the data sheets. Take note of the hFE or Current Gain grading. If you get the ungraded versions, they should be tested but usually have plenty of current gain.
I have ordered more TIP35C transistors on Aliexpress, very cheap. Ill be checking to see if they are fakes.
I tested with an actual mains transformer last night. There was a few millivolts of ripple at the output. I have changed C8 to 330pF

xavier60 · « **Reply #163 on:** December 17, 2018, 12:19:21 pm »

I have attached a bill of materials for your chematic, I hope I haven't missed anything. Couldn't find some of the transistors,
[/quote]
The XLsX file won't open for me, trying 3 different viewers.
Can you upload a text file?

JuanGg · « **Reply #164 on:** December 17, 2018, 03:44:21 pm »

Quote from: xavier60 on December 16, 2018, 09:49:17 pm

Quote from: JuanGg on December 16, 2018, 04:29:13 pm

Ok, I'll try to look for more transformers to power the micro, fans and displays.
I have attached a bill of materials for your chematic, I hope I haven't missed anything. Couldn't find some of the transistors, added some similar ones: BD135 instead of BD137 and BD559 instead of BD558, I've gone through the datasheets and the characteristics are almost the same. I'll order this tonight, as well as some smd to dip adapters and misc stuff. It's going to take at least a week or two, I hope I can get something done this christmas.
I'll prototype the thing on a perfboard, but at the prices PCB services are running at, it may be an option to have some made later on (as I'm planing on having two channels... we'll see)
Juan
Although the BD135 will be fine, try to keep all transistors above 60V where necessary, Is BD139 available instead?
There should be no BD558, only BC558, maybe a misprint. Although I haven't used the transistor types consistently, use BC546, BC556 where the voltage is high and BC548, Bc558 for low voltage positions or just about any BC series at hand.
The BC546, BC556, BC547, BC557, BC548, BC558 are very commonly used general purpose signal transistors. You should take the time to get familiar with the data sheets. Take note of the hFE or Current Gain grading. If you get the ungraded versions, they should be tested but usually have plenty of current gain.
I have ordered more TIP35C transistors on Aliexpress, very cheap. Ill be checking to see if they are fakes.
I tested with an actual mains transformer last night. There was a few millivolts of ripple at the output. I have changed C8 to 330pF

I ordered already, from "LCSC", let's see how it works out. Yes, BD139 was available

. Oh well. And yes, BD558 was a typing error.
I have some BC547 and BC557 transistors around. Anyway, I got ten signal transistors of each kind (minimum order quantity) and four TIP3455C so I should have plenty to play with. I'll eventually end up using them I suppose. While they arrive (at least a week or two), I'll see if I can finish the transistor labs on "Learning the art of electronics" and go through the datasheets so I can get a better understanding.

Quote from: xavier60 on December 17, 2018, 12:19:21 pm

Quote
I have attached a bill of materials for your chematic, I hope I haven't missed anything. Couldn't find some of the transistors,
The XLsX file won't open for me, trying 3 different viewers.
Can you upload a text file?

Uploaded it to the original post.

I should have thought of that.
Juan

xavier60 · « **Reply #165 on:** December 17, 2018, 05:40:17 pm »

I get no search results for " TIP3455C "

JuanGg · « **Reply #166 on:** December 17, 2018, 06:27:00 pm »

Quote from: xavier60 on December 17, 2018, 05:40:17 pm

I get no search results for " TIP3455C "

This thing manages to mess everything up. Here is a screenshot and a pdf. Sorry for the inconvenience, I should be the one dealing with this stuff.

Juan

xavier60 · « **Reply #167 on:** December 18, 2018, 04:15:58 am »

The top trace represents a 0 to 6 amp ramped load on the regulator's output.
The bottom trace was at the Base of Q2. The transconductance is very linear.
The gm of 40 is a bit higher than I would have preferred.
I did notice a problem, some voltage fluctuation that is present all the way up to the Base of the TIP35C, but not at the Emitter.
It could mean that the transistor is failing. It is a spare that was left over from a repair job.
I bought them on Aliexpress and are ST branded. I hope the ones on order are ok.

JuanGg · « **Reply #168 on:** December 18, 2018, 07:14:15 am »

Quote from: xavier60 on December 18, 2018, 04:15:58 am

The top trace represents a 0 to 6 amp ramped load on the regulator's output.
The bottom trace was at the Base of Q2. The transconductance is very linear.
The gm of 40 is a bit higher than I would have preferred.
I did notice a problem, some voltage fluctuation that is present all the way up to the Base of the TIP35C, but not at the Emitter.
It could mean that the transistor is failing. It is a spare that was left over from a repair job.
I bought them on Aliexpress and are ST branded. I hope the ones on order are ok.

Are you putting 6A through this thing?
The ones I ordered are ST as well.
Juan

nick_d · « **Reply #169 on:** December 18, 2018, 07:21:09 am »

No way the AliXpress ones are actual ST. They will be fakes definitely. That does not mean they are no good. Likely they will have similar characteristics to what they are marked as, but not always. See my earlier post and do not waste valuable development time tracking down possible weird characteristics of the fakes. If it works great -- if it doesn't, swap in the Digikey or other genuine part before proceeding.
cheers, Nick
PS I know this from bitter experience... Hello fake LM386...

xavier60 · « **Reply #170 on:** December 18, 2018, 09:19:35 am »

The TIP35C transistors I have ordered are supposed to be the newer TO-247 types. It would be possible to test the transistor's spec somehow, but how does one check for poor quality bonding?
The control circuit can easily drive the power transistor to high currents, but the transistor needs to be kept cool and in its Safe Operating Area.
I have increased the current carrying capacity of the PCB by placing 0.8mm solid core copper wire along the ground track.
The BD137, TIP35C Darlington has a current gain of over 10,000. There is only about 0.7ma through R4.

xavier60 · « **Reply #171 on:** December 18, 2018, 10:21:56 am »

I should have been more suspicious when I had to file the tab flat on the ST marked fake.
This is what I have on order. I wonder what brand they are?

Extra: The description says "Usongshine" . I can't find anything about it.

https://www.aliexpress.com/item/10PCS-LOT-TIP35C-TIP35-35C-35-TO3P-transistor-new-original/32810647217.html?spm=2114.search0104.3.1.bd184394dD2rIC&ws_ab_test=searchweb0_0,searchweb201602_3_10065_10068_10130_10547_319_317_10548_10696_453_10084_454_10083_10618_10307_538_537_536_10131_10132_10133_10059_10884_10887_100031_321_322_10103,searchweb201603_51,ppcSwitch_0&algo_expid=32f8061b-7b70-4998-af35-5105fcc333de-0&algo_pvid=32f8061b-7b70-4998-af35-5105fcc333de

xavier60 · « **Reply #172 on:** December 20, 2018, 09:29:27 am »

Quote from: JuanGg on December 18, 2018, 07:14:15 am

Quote from: xavier60 on December 18, 2018, 04:15:58 am
The top trace represents a 0 to 6 amp ramped load on the regulator's output.
The bottom trace was at the Base of Q2. The transconductance is very linear.
The gm of 40 is a bit higher than I would have preferred.
I did notice a problem, some voltage fluctuation that is present all the way up to the Base of the TIP35C, but not at the Emitter.
It could mean that the transistor is failing. It is a spare that was left over from a repair job.
I bought them on Aliexpress and are ST branded. I hope the ones on order are ok.

Are you putting 6A through this thing?
The ones I ordered are ST as well.
Juan

Let me know when you have received the parts, I will update the schematic.
I have made a few minor changes. The gm has been reduced to 20.
There are reasons for the current overshoot being allowed to occur when the output is shorted.
I have added a method for easily setting the overshoot.
I have not introduced any new part values.

JuanGg · « **Reply #173 on:** December 20, 2018, 03:13:52 pm »

Quote from: xavier60 on December 20, 2018, 09:29:27 am

Quote from: JuanGg on December 18, 2018, 07:14:15 am
Quote from: xavier60 on December 18, 2018, 04:15:58 am
The top trace represents a 0 to 6 amp ramped load on the regulator's output.
The bottom trace was at the Base of Q2. The transconductance is very linear.
The gm of 40 is a bit higher than I would have preferred.
I did notice a problem, some voltage fluctuation that is present all the way up to the Base of the TIP35C, but not at the Emitter.
It could mean that the transistor is failing. It is a spare that was left over from a repair job.
I bought them on Aliexpress and are ST branded. I hope the ones on order are ok.

Are you putting 6A through this thing?
The ones I ordered are ST as well.
Juan
Let me know when you have received the parts, I will update the schematic.
I have made a few minor changes. The gm has been reduced to 20.
There are reasons for the current overshoot being allowed to occur when the output is shorted.
I have added a method for easily setting the overshoot.
I have not introduced any new part values.

I should receive them within a week from now. I'll let you know. Thanks again. I bought a few of each just in case.

I'll start winter break tomorrow, so I'll have more time. I can start planning the perfboard layout or continue working on the case. I'll also look for transformers for the micro and displays.
Juan

ignilux · « **Reply #174 on:** December 20, 2018, 07:54:17 pm »

Gents-

Following along diligently, but finding it a little difficult to keep up. I've attached an annotated schematic highlighting my main questions, but I'll attempt to summarize below:

Does PS2 represent a rectified mains transformer input? If so, what is the voltage? I saw a number of transformers discussed earlier on
Does the uppermost line leading off to the right represent the positive output terminal?
Does the lowermost line represent the negative output terminal?
You've discussed using a microcontroller. Do the CV/CC Ref's correspond to control inputs, or are the related to the TL431 reference?
VCC1 just represents the regulated control voltage created by the TL431 circuit, right? It is NOT an isolated supply?

I've built a few '317 and '338 style power supplies, blew 'em up, and I'm working on finding a suitable topology for my next project. I've been simulating various circuits involving throwing a current control loop around various voltage regulator ICs, but have so far been frustrated trying to compensate the darn things. Anyway, just wanted to say that this is a neat project, and I'm keeping an eye on it. Thanks to everyone who has contributed thus far!

xavier60 · « **Reply #175 on:** December 21, 2018, 12:56:30 am »

Quote from: ignilux on December 20, 2018, 07:54:17 pm

Gents-

Following along diligently, but finding it a little difficult to keep up. I've attached an annotated schematic highlighting my main questions, but I'll attempt to summarize below:

Does PS2 represent a rectified mains transformer input? If so, what is the voltage? I saw a number of transformers discussed earlier on
Does the uppermost line leading off to the right represent the positive output terminal?
Does the lowermost line represent the negative output terminal?
You've discussed using a microcontroller. Do the CV/CC Ref's correspond to control inputs, or are the related to the TL431 reference?
VCC1 just represents the regulated control voltage created by the TL431 circuit, right? It is NOT an isolated supply?

I've built a few '317 and '338 style power supplies, blew 'em up, and I'm working on finding a suitable topology for my next project. I've been simulating various circuits involving throwing a current control loop around various voltage regulator ICs, but have so far been frustrated trying to compensate the darn things. Anyway, just wanted to say that this is a neat project, and I'm keeping an eye on it. Thanks to everyone who has contributed thus far!

Yes, the schematic follows conventional layout. Unregulated input on the left and regulated output on the right.
To reduce clutter, I have omitted the obviously needed bits, such as the usual transformer, rectifiers and large capacitors, or just about any DC supply such as a SMPS.
The references can be from a micro-controller via Digital Pots, DAC or filtered PWM. Or simply from mechanical Pots powered from VCC1 8V.
When Pots are used, make certain that the circuit is fail safe to the possibility of loss of wiper contact which is a real probability with cheap 10 turn Pots. Oiling(PAO) has been fixing them. Read my posts for more details.
The recommended absolute input voltage is limited by the transistor spec, 60V.
The real world limits are determined by difficult to measure and calculate interaction of factors such as operating current, power transistor Safe Operating Area rating and the heat sink's thermal resistance to ambient. Transformer tap switching helps a lot.
VCC1 may need to be from an isolated supply if more is to be powered than currently shown.
Q8's dissipation will be rather high at higher loads because of its large voltage drop. And also, this current will be measured by the shunt resistor.

I have been testing the regulator circuit a lot with good results.
It's yet to be seen if the results can be reproduced.
Transistors can have a large difference in current gain from one device to another of the same part type.
Well designed circuits tolerate this variation.

xavier60 · « **Reply #176 on:** December 22, 2018, 05:24:03 pm »

I have done just about enough tweaking for now.
I have deleted a transistor from the preload circuit.
I used the method in the linked PDF to do Phase Margin tests.
My measurements indicate a PM of 90° at the 0db crossing of 60Khz.
The regulator remains stable with no output capacitor.
The PM is 90° again at the much higher frequency of 300Khz.

http://www.ti.com/lit/an/snva364a/snva364a.pdf

Kleinstein · « **Reply #177 on:** December 22, 2018, 05:45:32 pm »

The phase margin test is good for the given load the output. The case without the capacitor at the output is not really relevant, as the difficult case is normally a load with low ESR capacitance combined with a kind of current sink. This would be the hard case to judge stability. Under these difficult condition the phase margin will usually go down quite a bit - that is OK unless the phase margin turns negative.
A good phase margin with an easy load not always guaranties stability with a difficult load.

Looking at the circuit, I am not sure the regulator will be stable for all reasonable loads. Something like 100 µF with 10 mOhms ESR and some current sink could cause instability.

The output impedance can help to judge stability at different loads.

xavier60 · « **Reply #178 on:** December 22, 2018, 10:05:33 pm »

Quote from: Kleinstein on December 22, 2018, 05:45:32 pm

The phase margin test is good for the given load the output. The case without the capacitor at the output is not really relevant, as the difficult case is normally a load with low ESR capacitance combined with a kind of current sink. This would be the hard case to judge stability. Under these difficult condition the phase margin will usually go down quite a bit - that is OK unless the phase margin turns negative.
A good phase margin with an easy load not always guaranties stability with a difficult load.

Looking at the circuit, I am not sure the regulator will be stable for all reasonable loads. Something like 100 µF with 10 mOhms ESR and some current sink could cause instability.

The output impedance can help to judge stability at different loads.

The capacitor(NCC KME 47-100) on the output that I thought was low ESR, isn't. I measured about 500mΩ ESR.
When I replaced it with a 100µF MLCC, the worst case PM dropped to 15°. It also causes the load transient response to have a 50% overshoot.
I guess this is why I see regulator designs that have multiple series RC's across the output.
Testing with a constant current load didn't seem to cause any problem.

Kleinstein · « **Reply #179 on:** December 22, 2018, 10:29:05 pm »

The capacitors at the output have different purpose. A low ESR capacitor at the output can help to buffer fast transients, where the active regulator can no longer react. However it also is a difficult load to the regulator and make regulation more tricky with more overshoot. A capacitor with defined ESR (e.g. in the 1 Ohms range) does not help much to buffer the output and deliver much power. The purpose is mainly to improve stability. Still it makes regulation at other frequencies slower. So ideally the capacitors should be relatively small, just enough to get good response.

50 % overshoot for a tricky load like the 100 µF capacitor is quite good. The 100 µF are more like not a good idea to have at the output though. The 100 µF with 0.5 Ohms ESR might be suitable for the output, to provide some damping. Chances are one could use a smaller capacitor.

xavier60 · « **Reply #180 on:** December 22, 2018, 11:29:46 pm »

I temporarily fitted the 100µF MLCC just for testing the Phase Margin.
I can eliminate the overshoot by peaking Q1, but the regulator becomes unstable with no output capacitor although this does not have to be a requirement.
If this poor tolerance to low ESR capacitors is generally expected of regulators, the design that I have posted should not be regarded as a failure.
Because it's quite happy with a standard 47µF electrolytic across the output.
I want this design to be successful in having good performance without being overly complicated.
The transient response is currently 15µs.

Kleinstein · « **Reply #181 on:** December 23, 2018, 10:18:25 am »

Having a low ESR capacitor at the output is a difficult situation for any voltage regulator. So a low phase margin for this case is kind of normal - not so good designs have a chance to oscillate under such conditions. I would consider only 50 % overshoot as still good for the difficult conditions. It can actually get better with the 47 µF electrolytic in parallel.

For this type of regulator with a relative high impedance output stage it is normal to require some kind of output capacitor.

A 47 µF electrolytic cap with some ESR sounds like a good solution. It is still relatively small to what other lab supplies have.

JuanGg · « **Reply #182 on:** December 23, 2018, 11:18:52 pm »

I am getting a bit lost with all this capacitors and phase margins. I'll have a careful read.
Anyway, i have done some work on the case, mainly on the front panel, see photos and schematic below. I think it's rather neat as it allows the arduino to drive the indicator leds and read the pushbuttons from the same pin, alternating it as an input our output. 7 segment displays are just modules from e-bay.

I have gotten hold of some more transformers, so this is all I have (see last photo).
From left to right,
- two 12 Vac 500 mA transformers (to power the arduino, fans and displays),
- 22 Vac 2.2 A,
- 2 taps @ 33 Vac 2A each . (Measured with no load, 2 A because wire gauge is the same as in other 2 A transformer)
- and on the right what I had before, 2 taps@ 10 Vac 2A each, two 15 Vac 1 A.

Here is the options I see, all using the 12 Vac transformers for the arduino and displays:
-Use what I have currently installed, 10 Vac tap + 15 Vac transformer per channel.
-Use two 10Vac taps in series (so 20 Vac) for one channel and the 22 Vac transformer for the other.
-Use one 33 Vac tap of the big transformer per channel. This will result on a rather high ≈ 45 Vdc input voltage, perhaps too close to the
max 50 V of the filter caps.

That's it. Sorry for the nonsense. Thank you.

xavier60 · « **Reply #183 on:** December 24, 2018, 01:48:41 am »

A lot of options there. I think that he 33Vac transformer is just too high for most uses. The power supply would have to be derated to 500ma or less to keep the power transistors safe.
You could use the 22Vac transformer to make one medium voltage channel at about 1.5 amps.
And the 2x10Vac+2X15Vac transformer for a switchable low voltage high current / high voltage low current channel.
The safe minimum output voltage for each range at full current will be determined by the heat sinks.
Keep in mind that the thermal washers will add about 0.8° per watt of thermal resistance. You could easily add over temperature protection anyway.

xavier60 · « **Reply #184 on:** December 24, 2018, 05:54:31 am »

If I decide to go further with this project, this transformer looks nice.
https://www.jaycar.com.au/12v-30v-100va-6a-multi-tapped-dual-type-2170-transformer/p/MM2015
Switches or relays could be used to select the 12/15 taps as well as series or parallel for higher current on the low voltage ranges.
Relays would allow auto switching to be added which would greatly lower heat dissipation if the output is shorted on the high voltage ranges.

JuanGg · « **Reply #185 on:** December 24, 2018, 09:28:15 am »

I think I'll leave it as-is for simplicity's shake. I can make changes later on.
Juan

xavier60 · « **Reply #186 on:** December 24, 2018, 09:40:42 am »

Quote from: JuanGg on December 24, 2018, 09:28:15 am

I think I'll leave it as-is for simplicity's shake. I can make changes later on.
Juan

Yes, because you are going for lower output current, the secondary switching isn't really needed.

JuanGg · « **Reply #187 on:** December 24, 2018, 12:57:18 pm »

All right. I'll make a perfboard with the main rectifier and filter cap, plus the auxiliary PSU: another small rectifier taking 12 Vac from the small transformers and regulating it to 12 V with a LM7812 regulator (for the fan) and to 5 V with an LM317 (arduino and displays).
Or maybe I should put the auxiliary PSU on the main board?(second photo, on the right, mounted vertically).
Regulators will be disipatting 0.5 W and 1.5 W respectively. As per the datasheet, LM7812 has a Juction to Ambient thermal resistance of 20ºC/W, so it should heat up to 35º assuming an ambient temperature of 25ºC (fine withought heatsink). LM317 has a Junction to Ambient thermal resistance of 38ºC/W, so it would heat up to 80ºC, I suppose it'll be fine without heatsink, but I can bend it and solder the tab to the perfboard just in case.

Juan

xavier60 · « **Reply #188 on:** December 24, 2018, 01:22:48 pm »

I also prefer having a separate board for the AUX rails along with the main rectifier. Keep ground return current paths for the 12V and 5V rails separate. They should meet only at a common point on the regulator board.
IF you power the op-amp with the 12V rail, don't run a ground wire from the AUX supply board to the regulator board. The only ground connection to the regulator board should be the 5V ground from the control board.
The fan might impose some ripple onto the 12V rail.

Extra: Im not certain how the LEDs should be powered. What voltage do the displays need?

xavier60 · « **Reply #189 on:** December 24, 2018, 01:36:00 pm »

I see it's 5V for the displays. I need to think about that.
Does the micro-controller have its own accessible internal reference?

JuanGg · « **Reply #190 on:** December 24, 2018, 01:39:04 pm »

5 V would be for the micro and displays. I don't know if the displays would make the 5 V rail noisy and if that would impact performance. The arduino itself has a 5 V reg, but its an smd package and I don't think that it can handle 1 W. It can be used for control circuitry though.

What are the op-amps powered from? I thought they would be from the 8V rail. Fans, being PWM driven, would definitely add some ripple to the 12 V rail.

The micro has an internal 1.1 V reference for the ADC's that can be enabled or disbled.

Juan

xavier60 · « **Reply #191 on:** December 24, 2018, 01:47:55 pm »

Quote from: JuanGg on December 24, 2018, 01:39:04 pm

5 V would be for the micro and displays. I don't know if the displays would make the 5 V rail noisy and if that would impact performance. The arduino itself has a 5 V reg, but its an smd package and I don't think that it can handle 1 W. It can be used for control circuitry though.

What are the op-amps powered from? I thought they would be from the 8V rail. Fans, being PWM driven, would definitely add some ripple to the 12 rail.

The micro has an internal 1.1 V reference for the ADC's that can be enabled or disbled.

Juan

You can stay with the 8V regulator as in my schematic to power the op-amps. Using the 12V instead would have eliminated many parts but might cause complications.
If the internal reference is used, there sould be no need to worry about fluctuations on the 5V

JuanGg · « **Reply #192 on:** December 24, 2018, 01:51:38 pm »

I have like ten LM7812 regulators, so I could use one for the fans and another one for the regulating circuitry. I have ordered all parts, so any option is feasible. I also have some LM317's.
Juan

xavier60 · « **Reply #193 on:** December 24, 2018, 02:00:19 pm »

Quote from: JuanGg on December 24, 2018, 01:51:38 pm

I have like ten LM7812 regulators, so I could use one for the fans and another one for the regulating circuitry. I have ordered all parts, so any option is feasible. I also have some LM317's.
Juan

Just occurred to me that increasing the supply voltage to the op-amps will affect the response to overloads as well as some other component value changes. You would need to use an LM317 set to 8V.

JuanGg · « **Reply #194 on:** December 24, 2018, 03:02:48 pm »

Quote

Just occurred to me that increasing the supply voltage to the op-amps will affect the response to overloads as well as some other component value changes. You would need to use an LM317 set to 8V.

Ok. So I would have a 7812 regulator for the fan, an LM317 for the 8 V control rail, and another LM317 / 7805 for the displays. The Arduino can be powered from either 12 or 8 V and it'll generate it's own 5 V.
Would it be adequate to daysy-chain all regulators: Unreg. V > 12 V > 8 V > 5 V?
Juan

xavier60 · « **Reply #195 on:** December 24, 2018, 03:51:19 pm »

Quote from: JuanGg on December 24, 2018, 03:02:48 pm

Quote
Just occurred to me that increasing the supply voltage to the op-amps will affect the response to overloads as well as some other component value changes. You would need to use an LM317 set to 8V.

Ok. So I would have a 7812 regulator for the fan, an LM317 for the 8 V control rail, and another LM317 / 7805 for the displays. The Arduino can be powered from either 12 or 8 V and it'll generate it's own 5 V.
Would it be adequate to daysy-chain all regulators: Unreg. V > 12 V > 8 V > 5 V?
Juan

I would use the 12V for the fans only and use the 8V for the Arduino as well as for the regulator control rail. At the moment, I can't see any big problem with daysy-chaining so long as it helps to share the dissipation.

soldar · « **Reply #196 on:** December 24, 2018, 03:54:35 pm »

Quote from: JuanGg on December 24, 2018, 03:02:48 pm

Would it be adequate to daysy-chain all regulators: Unreg. V > 12 V > 8 V > 5 V?

I would rather have them be independent as it makes troubleshooting and repair easier. OTOH it could be a case where staggering them would balance power dissipation better. Your call.

xavier60 · « **Reply #197 on:** December 24, 2018, 04:00:16 pm »

Also, the regulator mentioned earlier that is expected to reach 80° bothers me. It's difficult to know what temperatures components really reach after the case is closed unless there is some continuous air flow.

JuanGg · « **Reply #198 on:** December 24, 2018, 05:37:05 pm »

Quote

Also, the regulator mentioned earlier that is expected to reach 80° bothers me. It's difficult to know what temperatures components really reach after the case is closed unless there is some continuous air flow.

I'll build the thing up on a headboard, test it and measure temperatures just in case. I am thinking of deriving both the 8V and the 5 V rail from the 12 V one. Another thing to keep in mind is return current paths. Fan and displays wouy go before the shunt. I think the displays will be fine being driven from the other side of the shunt. I'll just put in a couple resistors in series with the clk and data lines, just in case.
Juan

xavier60 · « **Reply #199 on:** December 24, 2018, 10:05:21 pm »

Quote from: JuanGg on December 24, 2018, 05:37:05 pm

Quote
Also, the regulator mentioned earlier that is expected to reach 80° bothers me. It's difficult to know what temperatures components really reach after the case is closed unless there is some continuous air flow.
I'll build the thing up on a headboard, test it and measure temperatures just in case. I am thinking of deriving both the 8V and the 5 V rail from the 12 V one. Another thing to keep in mind is return current paths. Fan and displays wouy go before the shunt. I think the displays will be fine being driven from the other side of the shunt. I'll just put in a couple resistors in series with the clk and data lines, just in case.
Juan

Because AUX current will be from an independent supply rather than from the main unregulated rail, you should be able to reference everything to the top side of the shunt. Do a sketch to confirm.