What to do with a pile of ATX power supplies

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The Lord Flashheart

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Joined 2018
#1
Hi gents,

I have been reading around the subject of SMPS and after a quick chat with the guys at the local computer repair shop have been given three identical examples of two different power supplies to experiment/use/abuse/learn from.

I have read about the difficulties of isolating the grounds for the purposes of connecting them in series for more voltage, I would like to have a crack at modifying them for this if at all possible.

I'm also considering paralleling them to produce a 12v high current supply to mess about with an old car amplifier for home use.

I would be grateful for suggestions as to learning projects to build with these supplies, they don't have to be goal focused apart from helping me work through the rather dry reading material that is: https://www.onsemi.com/pub/Collateral/SMPSRM-D.PDF

Any ideas?
 
#5
Exactly, what better way to learn than by experimenting.

It turned out that the three smaller PSU's were scrapped because they folded under load.

That leaves three identical HP 250W PSU that can support at least 150w each on the 12v rail ( tested with three 50w 12 v spots borrowed from the living room lights).

With a bit of metal work I'm sure they could be fashioned into a single ~35A supply.

I have opened them up and had a look, they seem to use these ICs to control the output voltages

http://www.ti.com/lit/ds/symlink/lm339.pdf

https://www.onsemi.com/pub/Collateral/TL494-D.PDF

It seems from the application notes that the voltage on the TL494 can be adjusted compared to the 5v reference, perhaps I can tweak the 12 v rail high enough to get ~13-15v.

If I can do that then I can power a small in car class D amplifier quite handily for at least a couple of hundred watts RMS.

If that works then I shall have a dig around for a single larger PSU to but am a little worried about the ATX current limits on each single rail, presumably there is no problem with parallel connection of multiple rails from the same PSU?
 
#6
Not really the answer your after, but they are a good source of coloured wire/cable (often rated at 300v or over), mosfets, IEC sockets, fans, 78** regulators, heatsinks, X type caps, bridge rectifiers, etc, etc. A rich source of parts.

Perhaps better to scavenge the parts and build a PSU from scratch.

Off topic which is your favourite G.M.F book?

Andy.
 
#7
I would agree with the last post. Easiest thing is to just scavenge from them.

The 12V output is quite modest in ability. Only real option is to get in there, do major surgery, and turn the 5V output into something else.
 
#9
Diabolical Artificer said:
Not really the answer your after, but they are a good source of coloured wire/cable (often rated at 300v or over), mosfets, IEC sockets, fans, 78** regulators, heatsinks, X type caps, bridge rectifiers, etc, etc. A rich source of parts.

Perhaps better to scavenge the parts and build a PSU from scratch.

Off topic which is your favourite G.M.F book?

Andy.
Click to expand...

In terms of book I think it has to be ...In the Great Game but I confess that in terms of GMF's opus and the greatest effect it had on my adolescent mind his work on Red Sonja is more quotably etched in my memory...😱 😀

You make a good point about the parts, I was going to strip the non-working ones for just those bits.

The PSu boxes are quite handy, I built a PID controller for a lead melter that fitted very nicely into a oddly shaped PSU enclosure from the same computer repair shop and it solved my enclosure, safe grounding and switching issues in one fell swoop.

P.s. Love your username.😎
 
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#10
leadbelly said:
The 12V output is quite modest in ability.
Click to expand...

Can you elaborate a little on this please? I understood that the rails could provide a decent amount of current, usually about the same as on the 5v rail, but with the more useful higher voltage?

leadbelly said:
Only real option is to get in there, do major surgery, and turn the 5V output into something else.
Click to expand...


You have my attention, doing this would let me doctor a 1200w PSU into something very useful for another, non-audio, project.
 
#13
😀
jackinnj said:
For years, the ARRL handbook had a "how-to" of converting an ATX supply into a high voltage transceiver supply. If you noodle around on the web, there are copies of the article.
Click to expand...

There's a Welsh enclave in NJ?! 😱😀

Thank you for the tip, I've found various PDFs, some very long but nothing on quick search with such a useful guide.

Might I trouble you for a link or more to go on please?
 
#14
The 12 volt output may only be good for 10 to maybe 20 amps. Not near as much total wattage as the 5 or 3.3 volt outputs. Rewinding the transformer or replacing it with another to get the voltage you really want is the best option for re-purposing. It will take some study of what you do have - it may have auxiliary windings (for regulation feedback, running the Control chip and FET/IGBT drivers, etc) that you will need to duplicate. If it is some sort of resonant mode supply, that will further complicate things as the output filtering needs to be specifically designed for it and if the values are off it will run hot and fry. If it uses synchronous rectification it is not easily adjusted to just any voltage, but it could be done away with if going from 5V where it is really needed to 48v where you could live without it. Old school hard switched supplies with opto feedback were easily converted to higher voltage outputs, but modern ones may take too much work to be worthwhile.
 
#17
Mine would be Flashman and the Redskins. His memoir "Quartered safe out here" is worth a read. As you say he did write a lot for the screen

Forgot about the cases, yep, useful too.

AFAIK, most ATX PSU's drop the mains voltage, then each rail is regulated separately, so maybe all you need to do is get rid of the 3 and 5v etc rails, then modify the 12v for more current OP, how you'd do this am not sure, am more familiar with linear regulators.

Andy.
 
#18
Diabolical Artificer said:
AFAIK, most ATX PSU's drop the mains voltage, then each rail is regulated separately, so maybe all you need to do is get rid of the 3 and 5v etc rails, then modify the 12v for more current OP, how you'd do this am not sure, am more familiar with linear regulators.
Andy.
Click to expand...

Not the traditional ATX - they rectify the mains voltage and all rails are converted in one transformer and regulated together, not separately; there is a big common mode choke which passes all rails to improve cross-regulation; TL494 is the main regulator and gets at least the 5v and 12v of the rail voltages via a common voltage devider; the 3v is post regulated with a MagAmp (magnetic regulator making use of saturable inductor); this can be dropped.
There is also usually under- and over- voltage detection for the main outputs (via the LM393 comparator, shuts down when +/- 5% is exceeded) as well as peak current limiter in the mains section.
So both the devider networks for the regulator as well as the comparator will have to be tweaked; also mind the electrolytics and rectifiers have not much margins e.g. 16v for the 12v rail.
Forget about rewinding / modifying the existing ferrite core transformer, it is potted ...
Concerning the defective units which drop voltage under load, they may have worn electrolytics; if they exhibit a bulge on their tops, they are gone; replacing them often restores full functionality. This may also be the case on the "good ones" ...
Example schematics of many old ATX PS can be found here: AT and ATX PC computer supplies schematics
 
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