OMAPS - One More ATX Power Supply conversion

Picture of OMAPS - One More ATX Power Supply conversion
Computer power supplies cost around US$15,but lab power supplies can run you $100 or more! By converting the cheap (free) ATX power supplies that can be found in any discarded computer, you can get a phenomenal lab power supply with huge current outputs, short circuit protection, and very tight voltage regulation.

In this instructable I will show you how to quickly convert one of those many computer power supplies into something that you can use to power your electronics projects, for electroplating, for electroetching, for heating wires for foam cutting, etc.

The voltages that can be output by this unit are 24v (+12, -12), 17v (+5, -12), 12v (+12, 0), 10v (+5, -5), 7v (+12, +5), 5v (+5, 0) which should be sufficient for most electrical testing. Many ATX power supplies with a 24-pin connector for motherboards will not supply the -5V lead. Look for ATX power supplies with a 20-pin connector, a 20+4-pin connector, or an AT power supply if you need -5V.


These instructions were originally posted by me on

I finally had to package the PSU in a nicer box. The wooden book was from a local craft chain called Michaels which I lined with foil and then packaged the electronics in. I also added back the 3.3V terminal as this was useful and I missed not having that in my previous version.
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russ_hensel2 months ago

Just a note to let you know I have added this instructable to the collection:
Encyclopedia of ATX to Bench Power Supply Conversion
Take a look at about 70 different approaches to this project.

BrenBren1 year ago
Thanks Abizar! Although I'm using my supply for a specific application rather than benchtop, it was nice to have a solid reference to speed things up. Very well written!
weatherjack4 years ago
I have been thinking about building one of these. I am an amateur electronic hobbiest. I still cannot figure out he +/- o equal certain voltages eg. +5v and -12v makes 17v, guess I am thinking +5-12=7? Anyone can shine some light? I feel pretty silly for asking but.. Knowledge is King!
Voltage is the electric potential difference between two points. So if you have the points +5v and +12v the potential difference between them is 12-5=7v. If you have the points +5v and -12v the potential difference is 5-(-12)=5+12=17v.
zacker Synck1 year ago
so when you have two plus's you subtract the smaller from the larger to get the volts? and if you have one plus and one minus, you add both together to get the voltage? what about two minus's like -12v and -5v? also, if i wanted say, 17v, i use one lead in the +5 and the other lead in the -12 jacks? does it matter what lead? red or black? what about if Im connecting them to something? would it matter what voltage went to neg and what went to pos? also, no ground in those scenarios?
abizar (author)  zacker1 year ago
Zacker, what are you trying to power? In general stuff like light bulbs, LEDs, etc. a ground is not required as long as the polarity of the device is respected. The + terminal of device needs to be connected to the more positive (or less negative) voltage. So, if I was using the -12V and the -5V terminals. I would attach the -5V to the positive terminal of the stuff I was powering and the -12V to the negative terminal. The device would think it is being powered by +7V! If I was using the +12V and +5V, then I would connect the +12V to the positive terminal (as it is more positive than +5V) and the +5V to the negative terminal. The device would still think it is being powered by +7V.
This will not work if the device really needs 0V (ground). For example, if it is connected to something else which is at 0V from another power supply. Hope this is not too confusing!
zacker abizar1 year ago
Its a little confusing, ill just have to read it a couple times to get it to sink thanks for explaining it.
I am looking for something to power an electrolysis tank for rust removal. I basically need like, 12V with about 2 Amps. A lot of people will try and go with as many Amps as possible but the more seasoned guys say 2Amps works well, it just takes a bit longer. I used me 12v car battery charger set to the 30 Amp "Quick" Charge setting but after 3 Hrs. it shut its self down. Most people leave these going for 8 hrs. or more depending on size of items being cleaned or amount of rust... in the three hrs. I ran mine on the 30 amp setting, its got almost all the rust off and old vise. So Im thinking if I can run this at 12v with anywhere in the 2 amps and above, it would be good. But then Im seeing all these PSU's being turned into bench power supply's and now, not only do I want to make this for the Rust Removal, but I may as well use it for a bench power supply too.. lol its just confusing, all these Volts and numbers and - / + and stuff... Like the pin outs on this PSU, some wires are +12v or +12VC DC or +12 VB DC or VA DC or worse, +12 VD DC / SE... lol
abizar (author)  zacker1 year ago
Hi Zacker. I do use the powersupply for electrolysis too, like you, mostly for converting rust back to iron. For electrolysis or for electroplating I typically use the 0V to +5V as it has the most current capacity. If you use 12V your powersupply may shut down if the current draw is too high. The current draws are high if the salt in your tank is too high.
zacker abizar1 year ago
Hmmm.... What are the Amps on the +5 side? Ill be doing mostly small parts, the Vice I did was an 8" Cardinal Milling Machine vice and probably the biggest thing id have to de rust. Mostly it would be smaller stuff. I like to buy old tools and clean them up to look as good as new (or as close to it as I can get... lol) I recently re did a 1950 Craftsman Drill Press and some of the Accessories that were available for it at the time. Well, I gotta get this puppy wired up if it works well, I have a Dell 750W PSU to use next. But that ones a whole new set of issues, its got both a 24 and a 20 pin Molex on it...
Machine2 years ago
The wood box looks fancy!
Jeffrey G C2 years ago
in step 5 you said, "most power supplies have a brown or mauve wire." My PSU doesn't fit in there because I have neither of these wires, I assume I need some kind of current consumption device but I'm not sure where else to put it. There aren't any wires doubled in the 20 pin connector and the PSU was manufactured 8/01 by Allied, model AL-A300ATX
Never mind, I didn't read carefully enough

I read mauve and it says somewhere else purple too, I got things mixed around
RadGlobe2 years ago
thanks Abizar!!! I have favorited 2 of your 3 instructables that I plan to do myself...great work!!!
samumar2 years ago
Hello and nice instructable!
I liv e in Europe and using 220V AC input.
Do I have to change the 10 ohm 10 W resistor with some different resistor or is it good using the same?
I need 6 amp out in the 12V line!

Thanks a lot!

abizar (author)  samumar2 years ago
10 ohm should be good.
Most of the PSU are compatible with 100-240V input so you could use it in most countries.
nixons2 years ago
Really nice instructable!
I have a question. My PSU has +12V 8A and -12V 0.5A. If i use 24V, how much current (A) i can get? tnahks
abizar (author)  nixons2 years ago
You would get about 0.5 A only.
jrd11212 years ago
what is the max current this could give at 12 volts
abizar (author)  jrd11212 years ago
Depends on the specific power supply but typically you get more than 10 amperes from the +12 V terminal.
OmegaRa2 years ago
I have a question. I have a power supply that doesn't have the -5V but does have a +12V2...there are the usual +12V yellow wires (+12V1) and then there are two yellow/black wires that say +12V2...should I just group them with the regular yellow ones?
juggleboy2 years ago
Thanks for posting this. It was super helpful and clear.
Power supply.jpg
sleepyjz2 years ago
I hope this doesn't qualify for a TLDR, but it's quite frustrating. I apologize if there is an answer to this somewhere above, though I don't see it. I have the whole set-up ready, grey wire to black wire via switch, an LED + grey wire, and my own LED + 5v standby (lights up if the unit is on in general). Both are current limited and work nicely. Ive tried many combinations to get the thing running and finally got it to work by connecting the 3.3v line to the brown sense wire AND my 2, 5WR10 resistor dummy load on this line as well - adding the dummy load to the 5v did nothing. Every binding post works nicely in terms of tolerance when powering things, except the ATX shuts down with any load on the +12v line. I couldn't even run a small fan off of it without shutting down instantly. I reduced the dummy load to a measly 5w2R resistor to experiment, which got hot obviously, but I got a small fan to work with the 12v rail, but any other device even remotely more power consuming shuts the ATX down as well. Dummy load on the +12v in addition to the 3.3v does nothing - shuts down. Same case with 12v dummy + 5v dummy. Any help would be more than appreciated. Thanks,
- sleepyjz
Raf723 years ago
Hi all, i am new here so nice to meet everyone. i already did this conversion and now i am going to do a second one but i noticed some doing the 1st and i checked that on the new one... i see that all v got a fuses protection and that is ok but i am wondering this, the ground goes on the psu case by the screws that set the pcb in the case so if for some "reasons" we accidentally short the v+ with the case..inste a good thing..., maybe we shouold isolate the case, am i wrong? infact the test on the new psu conversion, that i tested with the pcb out of the case uses only the gnd wire. so what about isolating the pcb by the case?...also if i see that the case id grounded with the wall AC so i dont know if it will resolve. any ideas or i am pretending too much by a simple and cheap conversione?.thanx a lot
anyway this project it's very nice and got the chance to reuse many psw that i got at home...cause of my job and passion
Kinnishian3 years ago
If you're having an issue with the current being limited on the 12v rail, make sure you have tried the resistor upgrade on the 5v line. That is, put appropriate resistors on some 5v wires to draw 300-3000ma. 

Otherwise, your question is not quite clear...

[This is a little old, maybe you have long given up].
Ok so i have the resistor on the 5v wire but I only get 11.71 - 11.89 volts. It it settles at 11.88v. How do i get more the 12v? Like 12.6 or 12.8v. Thanks for the help!
Just to make sure, what kind of resistor do you have on the 5v line? If you have too high of a resistor, you might not have enough of a load to stabilize the 12v line. Many sources say 100-300ma is enough (like, I think this instructable), but other sources I've found say 500-2000ma is optimally necessary.

In my case, with a 10ohm resistor (pulling 0.5amp) + a small lightbulb (pulling 0.3amp) I'm getting 12.35V. That might be the upper limit of this power supply, but I will check sometime with a bigger load (like 1ohm) to double check. But my 12.3V stays stable under load.

From what I've read,
You might also have a problem if your voltage sense wire isn't done properly. In my case there was an brown wire that went into an orange cable in the biggest connector of the original cables (the 24pin output). I had to make sure that brown wire was again shorted to the orange cables. Whether the brown cable exists an is shorted to the 5v or 3.3v (or in some cases even 12v) line depends on a number of things, so I just had to go through the trash to find the 24pin connection that I cut off and see what the case is.

When you say "it settles at 11.88v" I'm wondering what you mean by settles. Do you mean no-load voltage is 11.88v? The more likely problem tends to be that no-load voltage is "12.xx" V, but under load the line drops to 11v or something like that. That is primarily what the resistor is supposed to help with.
ok so i have a 10ohm 10watt resistor on the 5 v line. with nothing on the supply the voltage is 11.88v
It's possible that for your supply you won't get much more than 11.88v. What are you using the supply for?

I would also try and see if you can add another 10ohm resistor in parallel, or a small incandescent lightbulb, just to see if increasing the load helps. That will double the amount you pull from the 5v line (from 500ma to 1amp).

So 1) i would try adding another resistor (in parallel, not series)
2) Is there anyway you can work around your 12.6 or 12.4v requirement?
pro52003 years ago
I have a PSU from my old computer pentium II, I have connected the red and black wires to the power resistor, and green wire to black wire I connected to the switch, my PSU have 5 brown cable & just 1 brown cable connected it to orange cable, but the psu still would not start, can you help me?
Exiumind3 years ago
I'm trying to use a computer psu to power up a rgb led strip(12v, 3amp)..
The 12v rail is giving 12.4v (no load), the outputs of these psu's arn't regulated right?
If i use a high current voltage regulator i'l get a fairly large voltage drop, and that wont give me the full 12v i need.. the only v reg i have right now is a sharp pq12rd11, since it has low Vd, the output i get is around 11,9v.
What do you think, should i use this one and some transistors to handle more current?

Need some tips guys =)

abizar (author)  Exiumind3 years ago
The RGB strip will work when connected to the 12 V end. More than 3 amps should be available on most PSUs. If you do want to drop voltage an easy way is to put a diode in series. A silicon diode will drop about 0.6-0.7V. Make sure that the diode is rated to handle the current.
Exiumind abizar3 years ago
the point is that i want to use the computer psu that puts out 12,4v or so, and with this regulator the problem is that even having a low Vd it only supplies 1 amp max..

anyway, ill powerup the stirp using just the regular 12v output
If you're having an issue with the current being limited on the 12v rail, make sure you have tried the resistor upgrade on the 5v line. That is, put appropriate resistors on some 5v wires to draw 300-3000ma. 

Otherwise, your question is not quite clear...
[This is a little old, maybe you have long given up].
Gantman4eva3 years ago
how would you go about adding a USB connection. Rocketman221 added one in his power supply, but wasnt quite sure how he wired the unit up. Any help would be appreciated. He threw this pic up but wasnt sure what wires you would use for port 2 and 3. Thanks!!
As far as i know you would just ignore the rx/tx on the USB. You only need to wire 5v+ and GND.
brusho1503 years ago
Brilliant instructable I am new in electronic engineering and wanted a lab power supply will you please tell me that how Can we add a potetiometer(voltage regulator),rheostat, voltmeter and ammeter to calculate and vary voltage and current?
abizar (author)  brusho1503 years ago
Wondering what your application is? What voltages are you looking for and at what current?
You could do the potentiometer (variable resistor) but would need a beefy one if it isfor high current.
I actually constructed a little box (about 1x1x2 inches) with a knob on it to control voltage. This was based on an LM317 chip (check on google). Two input wires from this cube were connected to any voltage source and two output wires went out to whatever I was powering. The knob varied the voltage. Only limitation was that the maximum current needs to be less than 1 ampere and the maximum voltage was input voltage minus about 1.3 V.
So if you connected it to the +5V of the lab power supply you would get any voltage from 1.2 V to 3.6V. If you connected it to +12 V you would get a variable voltage from 1.2 V to about 10V.
Let me know if you are interested and I can post an instructable about this little cube.
Thanx alot bro I will try to construct it now.
rhobbies3 years ago
Hey Guys, I'm hoping you can help me out. I've got the mod done per the instructions above and triple checked my connections.

I get the desired voltages in the designated ports according to my multi-meter. I can do a small load with an LED light and small computer fan and the power supply operates as it should.

But anything larger (rc motor, motor dyno) the power supply immediately shuts off. I cannot find a reason for this at all. Any ideas?
abizar (author)  rhobbies3 years ago
Are you exceeding the current available? I would test your supply with a 12 V bulb to see if that lights up. If it does then your motor might be requiring more current. The other option is that yr motor may be shortcircuited or it has a big capacitor that sucks up a lot of current at start up. Do you have a horsepower rating on the motor? We can calculate the current from that.
Current (in Amps) = (Horsepower x 746) / Voltage.
A 1/4th hp motor will require 15 Amps from a 12 V supply. Or, 37 Amps from the 5V supply.
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