Question on Power Supply Diodes

[MK14]

so no metal case or any case.... so I guess I just have to be careful

I assumed you were going to case it up and do it all safely.

In that case, rather than messing with a 'live' exposed transformer, please consider the fully pre-cased version of apparently the same rated transformer, from the same supplier.

Here:
https://www.jameco.com/z/GPU572402000WA00-Jameco-Reliapro-24VAC-2A-AC-to-AC-Wall-Adapter-Power-Supply_2197548.html

[floobydust]

A 24VAC transformer could be too much for the board and push the op-amps too far.
At no or light loads, a transformer will have a higher output voltage. 24VAC is the absolute max. and measuring that AC with a multimeter doesn't always work because it's a distorted sine-wave when feeding a rectifier.

With say +35VDC and -5.1VDC that is over 40V on TL081 which is spec'd 36V or 40V typical, 42V max. TL081H
Lots of threads about this problem, so keep an eye out for what you get across the big cap.

[Electro Fan]

A 24VAC transformer could be too much for the board and push the op-amps too far.
At no or light loads, a transformer will have a higher output voltage. 24VAC is the absolute max. and measuring that AC with a multimeter doesn't always work because it's a distorted sine-wave when feeding a rectifier.

With say +35VDC and -5.1VDC that is over 40V on TL081 which is spec'd 36V or 40V typical, 42V max. TL081H
Lots of threads about this problem, so keep an eye out for what you get across the big cap.

Would this make you more comfortable?
https://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=149957&CID=EPIPRODUCTLINK

[Electro Fan]

so no metal case or any case.... so I guess I just have to be careful

I assumed you were going to case it up and do it all safely.

In that case, rather than messing with a 'live' exposed transformer, please consider the fully pre-cased version of apparently the same rated transformer, from the same supplier.

Here:
https://www.jameco.com/z/GPU572402000WA00-Jameco-Reliapro-24VAC-2A-AC-to-AC-Wall-Adapter-Power-Supply_2197548.html

Just use a barrel to barrel to wires connection to the same PCB terminal block (or just cut the barrel off)..... seems easy enough and likely much safer.  What do you give up when it gets shrunk down to fit in the case?

I'm guessing there might be a 20V version that might address floobydust's concern too.

[MK14]

Just use a barrel to barrel to wires connection to the same PCB terminal block (or just cut the barrel off)..... seems easy enough and likely much safer.  What do you give up when it gets shrunk down to fit in the case?

I'm guessing there might be a 20V version that might address floobydust's concern too.

They do, an 18 volt version, of what I linked to. But, another one, has caught my eye, which seems to be even better, overall, cheaper and more suitable for experiments.

Here:
https://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=2230635&CID=EPIPRODUCTLINK

It is 16.5 V (Don't forget that you will get around +50% that voltage in practice, minus some losses, after it is turned into DC (Root 2, to be exact, x1.414 etc, minus diode drops).

But, it has a higher current rating, of 2.4 Amps (Don't forget that you have to divide that AC current by root 2, to get the DC current rating, then derate it even further, because the output is capacitive, rather than purely resistivity, which it is rated for).

So, probably around a rated 1.5 Amps DC, (from 2.4 Amps AC).

But, in addition, it actually has on its fully enclosed (safe) case, it has screw terminal output connectors. Making it even easier to experiment with, and possibly more useful in the future. Without needing to get the right barrel connectors, or cut off connectors, or solder etc etc.

Although 18V at 2 Amps (AC), sounds bigger, the higher current one, will tend to NOT drop its voltage so much, as the output current increases. Also, it will keep the output transistor cooler (less dissipation), which will allow higher output currents, for the same waste heat in that transistor.

[MK14]

I'm guessing there might be a 20V version that might address floobydust's concern too.

Consider the 16.5 V, I linked to above.

Unloaded, it is probably 20 V or more, because transformers tend to increase their output voltage, when they have little/no output current (Transformer's Load Regulation).

(Rough rule of thumb, before voltage losses, such as diodes), gives approaching 30 V, but the circuit also has 5.1 V (see original schematic) on the other rail, giving around 35 V, minus diode drops.

But the op-amps (depending on what you have exactly), may only be specified to 36 V.

So, maybe 16.5 V (AC), is around the practical limit, for this exaggerated spec power supply kit.

(Including diode drops, could increase that to 18 V AC, but on the other hand, mains voltage, can fluctuate, so best to have a safety margin).

tl;dr
Even 20 V AC, could be too high, when considering transformer load regulation, and possible mains voltage fluctuations.

On the other hand, you could be very lucky, and have the 44 V rated op-amps, people talk about, but it is not clear, what you have.

EDIT:
I can believe/agree, that if you carefully checked the components you have in the kit (especially what op-amps they really are), accurately calculated all the voltages (including diode drops etc), that some AC figure of around 20 V (AC), would be ok (with the possible exception of mains voltage fluctuations).
But, I feel more comfortable, with a bit of safety margin, hence the lower transformer V AC recommendations, plus your supplier doesn't seem to do ones of 20 V AC, that I could see (pre-cased ones).

[floobydust]

For that 18V 2A transformer, its design is 230VAC but you have say 240VAC almost 5% higher mains. I estimate a 36VA part has 12% regulation so 21VAC at no load to give around 27VDC.
Using the same math, a 24VAC transformer would give almost 37VDC. I usually get about 35VDC with them.
A 20VAC transformer would give about 30VDC, for total 35V to the op-amps. These are just rough math values.

It's hard to know the quality of the imported transformer, it might be worse (than 12%) for regulation. The datasheet says "Voltage mobility rate: below 10%, Voltage deviation: within 5%" so I'm not sure what to expect.

[Electro Fan]

Just to confirm, I'm on US AC (~120V) in case this effects any of the derating calcs.

[george.b]

With say +35VDC and -5.1VDC that is over 40V on TL081 which is spec'd 36V or 40V typical, 42V max. TL081H

I wonder if a good old 741 could be used here instead. It is rated up to 44V after all.

[Electro Fan]

fwiw, the OpAmps look to be TI TL081CP's.

https://www.ti.com/store/ti/en/p/product/?p=TL081CP

I've mounted them in sockets so if they needed to be changed out it's easy to do.

Attached are photos of the partially assembled PCB and a few parts waiting installation.

The total investment in this has not been huge so if I need to experiment a little with some parts that don't make it or need to be upgraded it's doable.  I'm kind of looking forward to plugging it in to see what happens but I can hold off if something looks like it's a no-op or if something would clearly benefit from some re-doing.

[MK14]

fwiw, the OpAmps look to be TI TL081CP's.

The datasheet seems to specify a recommended maximum supply voltage of 30 V (where it is guaranteed to work).
Also, the Absolute maximum supply voltage is 36 V, above which, the device could be damaged.

Really, the power supply should have been properly designed in the first place, so that the transformer that they include and/or specify for it. DOESN'T violate or break, any of its components, datasheet specifications.

tl;dr
AKA Cheap Chinese Electronic Kits, don't always meet the advertised specifications.

[floobydust]

It's not a chinese design, they just copied it, switched op-amps and added hype and a bad pcb trace to the sense resistor.
The history of this 0-30V 2mA-3A power supply goes back to Practical Electronics 1978 October pg. 1070 and that original used three LM741's (44V max.) with transformer spec 20 to 25V, 2A.
There is mention of an old "Greek kit" and other modern revisits of the design. Czech version added 33V regulator.

Lots of discussion on this kit:
https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/
https://www.eevblog.com/forum/projects/chinky-0-30v-3a-psu-issues/

[george.b]

It's not a chinese design, they just copied it, switched op-amps and added hype and a bad pcb trace to the sense resistor.
The history of this 0-30V 2mA-3A power supply goes back to Practical Electronics 1978 October pg. 1070 and that original used three LM741's (44V max.) with transformer spec 20 to 25V, 2A.
There is mention of an old "Greek kit" and other modern revisits of the design. Czech version added 33V regulator.

Lots of discussion on this kit:
https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/
https://www.eevblog.com/forum/projects/chinky-0-30v-3a-psu-issues/

Ha, that answers my question, then. The original project used a 741. Makes sense.

[MK14]

It's not a chinese design, they just copied it, switched op-amps and added hype and a bad pcb trace to the sense resistor.
The history of this 0-30V 2mA-3A power supply goes back to Practical Electronics 1978 October pg. 1070 and that original used three LM741's (44V max.) with transformer spec 20 to 25V, 2A.
There is mention of an old "Greek kit" and other modern revisits of the design. Czech version added 33V regulator.

Lots of discussion on this kit:
https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/
https://www.eevblog.com/forum/projects/chinky-0-30v-3a-psu-issues/

I hadn't read those threads (to save time), now that I partly have, I better understand the situation. Thanks for letting me know.

So the original 1978 electronics magazine design was reasonable. By the time we arrive at the Chinese kit, they have introduced mistakes, and used inferior parts (especially voltage ratings). So, that it is no long anywhere near a 30 V, 3 Amp, power supply.
In fact, it seems it was originally, a 2 Amp, 30 V design.

Sadly some purchasers of the kit, buy it without realizing, and can end up attempting to get too much voltage and/or current out of it.

[Electro Fan]

Ok, good to understand the history.  I'm guessing that the current kit iteration is in the ballpark but is probably not spot on, and that it could be just a threshold here or there away from an oops.  No worries - I'm looking at this as a learning project rather than expecting to get a useful power supply out of it. 

Once the power supply is assembled my plan is to start at relatively low voltage and current levels and use the ps as a test bed to measure and better think through the circuity.  I think this will help to glue ( solder ) together the physical and the logical.

There are a bunch of things that I hope to learn more about including the role of the op-amps, but even before I get to that this has caused me to realize I need a better understanding of the the AC transformer specs as they lead to rectification and the DC specs.  The kernels of this are now sprinkled through this thread including some formulas, calculations, and margin estimates but I think the next step for me is to figure out how the AC to AC and AC to DC math should work so I can do it in addition to reading about it.  I'm sure I can find lots of info on this but if anyone has any articles or videos that you think are especially good with respect to the power calcs please post them.

After I better figure out what the AC will/should enable the DC to do I'll settle on a transformer.  After that I'll try to work forward toward the rest of the circuit to see where the potential weak links might be.
 

[MK14]

if anyone has any articles

I've had a look on your behalf, and I like the following. Because it is brief, to the point, and seems to give reasonably accurate formulas.

https://www.sowter.co.uk/rectifier-transformer-calculation.php

BRIDGE

Vac = Vdc x 0.71

Iac = Idc x 1.61

Pac = Pdc x 1.14

To give even more accurate results, you would need to adjust for the 0.7 V to 1 V (load current dependent), voltage drop (figure needs doubling for a bridge), from each rectifier (bridge) diode. Which is around 1.5 V to 2 V, for your full bridge rectifier (4 diodes), in the kit.

[MK14]

Ok, good to understand the history.

The original circuit/magazine, seems to be here:
https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1978-10.pdf

Towards the middle of it. It has a fairly extensive explanation of how it works (if I remember correctly).

EDIT: I tried to upload it (not sure about ownership rights, either), but it was apparently too big.

[WattsThat]

One comment, based on your build photos. I believe you have the pass regulator rotated 90 degrees, the transistor leads should exit parallel with the heatsink fins. In that way, the mounting holes of the heatsink will line up with the holes in the pcb.

As for improvements, here is a link to a site that provides some fixes, the nasty turn off spike and some better op amps, for starters.

http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

[MK14]

One comment, based on your build photos. I believe you have the pass regulator rotated 90 degrees, the transistor leads should exit parallel with the heatsink fins. In that way, the mounting holes of the heatsink will line up with the holes in the pcb.

As for improvements, here is a link to a site that provides some fixes, the nasty turn off spike and some better op amps, for starters.

http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

I've had a quick look. The combination of reducing the negative 5 V supply, down to only 1.3 V, and using the 44 V op-amps in the article, allows it to use the 24 V AC transformer, and hence get healthier maximum output voltages, and yet be safe from giving the op-amps too much voltage, which is another good improvement.

[Electro Fan]

One comment, based on your build photos. I believe you have the pass regulator rotated 90 degrees, the transistor leads should exit parallel with the heatsink fins. In that way, the mounting holes of the heatsink will line up with the holes in the pcb.

As for improvements, here is a link to a site that provides some fixes, the nasty turn off spike and some better op amps, for starters.

http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

Thanks, but I'm not sure what could be rotated 90 degrees....but apparently I'm missing it....  if you can, plz mark up the photo pointing to where the 90 degree rotate should be.  Thx again

Edit:  I think I know what you might be referring to; I've attached two photos showing similar but different PCB layouts - the top photo is from my partially built board and the other is from somewhere else.  One of the transistors is rotated 90 degrees.  It's a little optically confusing depending on which view of the board you have but it's further confusing because that particular transistor has not yet been installed on my PCB (because I was waiting to maybe find a heat sink that would fit before soldering the transistor to the board).  Somewhere between 1978 and 2020, probably fairly recently, someone did the board layout a little different for that transistor.  I'm pretty sure this explains the situation but thanks again for the QC/QA support!

PS, I just noticed the other photo with the transistor rotated 90 degrees is also using sockets for the op-amps.  My kit didn't include the sockets but when I read the various concerns earlier about stuff blowing up I figured sockets might be good; apparently whoever built the board in the other photo came to a similar conclusion.

I added another photo at the bottom which is the specific item I purchased.  It was a little extra ($10.88 total) because it included the radiator heat sink and the fan.

https://www.amazon.com/gp/product/B01KNSCDNU/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

In the photo you can see it didn't have the op-amp sockets.

For $8.99 you get everything but the heat sink and fan:

https://www.amazon.com/dp/B01NBE97WH/ref=dp_prsubs_1

[Electro Fan]

Ok, good to understand the history.

The original circuit/magazine, seems to be here:
https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1978-10.pdf

Towards the middle of it. It has a fairly extensive explanation of how it works (if I remember correctly).

EDIT: I tried to upload it (not sure about ownership rights, either), but it was apparently too big.

Wow, MK14, Amazing Find!  1978 article - you are a very good finder.  I'll read this and add it to the learning process, for sure.  Thx!

[Electro Fan]

This article suggests using a variac to bring up a newly built power supply. 

https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1978-10.pdf

Any reason not to do this, or any suggestions regarding using a variac for this purpose?  Maybe ramp to about 10V, monitor/check the components, continue on and stop at about 16V to begun operating/testing the power supply?  If all goes well then shut down and replace the variac with a ~16V transformer?

[MK14]

This article suggests using a variac to bring up a newly built power supply. 

https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1978-10.pdf

Any reason not to do this, or any suggestions regarding using a variac for this purpose?  Maybe ramp to about 10V, monitor/check the components, continue on and stop at about 16V to begun operating/testing the power supply?  If all goes well then shut down and replace the variac with a ~16V transformer?

The Variac doesn't replace the transformer at all. It 'replaces' the mains input connection, to your power supply.
So, instead of connecting to the mains, which would always be around 120 V AC, or whatever your voltage is. Instead, you would plug the Variac into the mains, and the Variacs output (it may have a mains socket on it), is used to power the DUT (Device Under Test = Your newly built prototype power supply).

The Variac, has a dial, which allows its output (socket), to vary between a rather low voltage, and approximately the full mains voltage (maybe a slight percentage more).

I'd suggest doing exactly what it says in the article, which is to use a resistor, of the type/value & current meter, when first testing your PSU out.

[ledtester]

One reason to ramp it with a variac is to check for shorts or other wiring mistakes by monitoring the input current as you slowly increase the voltage. You should first estimate what the current consumption of the circuit should be (say with no load.)

Often power supply circuits are tested by using another current limiting power supply for the input power. In this case, however, an AC input source is needed to create the negative voltage rail (as  voltlog discovered). And, of course, this runs into the chicken-and-egg problem if you're building this to get a current limiting power supply in the first place.

[Electro Fan]

One reason to ramp it with a variac is to check for shorts or other wiring mistakes by monitoring the input current as you slowly increase the voltage. You should first estimate what the current consumption of the circuit should be (say with no load.)

Often power supply circuits are tested by using another current limiting power supply for the input power. In this case, however, an AC input source is needed to create the negative voltage rail (as  voltlog discovered). And, of course, this runs into the chicken-and-egg problem if you're building this to get a current limiting power supply in the first place.

Thanks for posting the VoltLog link - that was very relevant and helpful for this project.  Lots of good learning lessons so far from building this kit.  If I manage to get it turned on and working I am going to recommend the kit highly but even if it doesn’t work the process of building it and discussing it in this thread has been pretty worthwhile.  Thanks again for the link - I’m going to look through the comments on the video to see what other nuggets might be in there.