Linear lab power supply

[JuanGg]

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]

A middle ground option could be digital pots.

[W2DML]

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]

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]

... 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]

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]

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

A middle ground option could be digital pots.

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

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.

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.

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.

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.

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!

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.

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.

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.

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]

Excellent replies: you have a good solid engineering approach. 

[Mr. Scram]

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]

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]

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]

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]

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

[spec]

+ 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]

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]

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]

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]

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]

-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]

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]

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.

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]

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]

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]

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]

+ 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).