ATX to Lab Bench Power Supply Conversion

ATX to Lab Bench Power Supply Conversion
In my sophomore year of college at the University of Minnesota, I started into my main electronics classes, and needed a good power supply for working on lab projects at home. My roommate Adam told me about somebody online who had converted an ATX computer power supply into a lab bench power supply, so I decided to do the same thing. You can also check out this link for a very similar guide by their user Abizarl. I have also documented this project on my website at http://www.mbeckler.org/powersupply/ if you are interested.

Warning! There are several large capacitors in ATX power supplies, that will store a dangerous charge for a long time. Please let your power supply discharge, completely unplugged from the wall outlet, for a few days before opening it up. You can probably be seriously hurt, so please be very careful. I am not a lawyer, but I hereby release myself from as much liability as I can, for any sort of injury you sustain, or any trouble you get into.
 
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Step 1Background

First, a bit of background on a typical ATX power supply:

Computer power supplies are Switch Mode Power Supplies (SMPS), which use high-frequency switching circuit elements to provide a high-quality output voltage, with good energy efficiency. One side effect of this technology is the minimum load requirement that each power supply has. In order to function properly, the power supply needs at least a very small electrical load connected to it. In other words, ATX power supplies will only work if you have something connected to it. We will be using a power resistor to provide this minimum load.

Also, modern power supplies do not simply have an OFF/ON switch, they have what is known as a "soft" power switch. This normally makes no difference to the user, as the computer behaves the same, but when you shutdown your computer, the motherboard can turn off the power supply when it has finished shutting down. This requires us to add our own power switch to the power supply chassis.

To protect our circuit from accidental (and careless!) short circuits, we will install some fuse-holders and fuses, which will disconnect the circuit supply lines if too much current flows. The size of the fuses are up to you, but a 1 amp fuse will work just fine for most circuits. You really should put fuses on all supply lines.

Update: While the diagrams show fuses on all voltage rails and no fuse on the ground line, when I actually built my power supply, I was young and foolish and only put a fuse on the ground wire. It's much safer and a better idea to put fuses on all signal lines and not the ground line. Thanks to many emails and messages on Instructables about this oversight.
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341 comments
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Jan 18, 2012. 1:45 AMmilkywayman says:
Hi all, just wanted to give you my 2 cents :)

I used a 350W ATX power supply to power a CD car radio and an automated aerial over 12V and here's the results:
- the 3.3V line had to be loaded with almost 2 Ohms of resistance!
The reason is this: the 12V are supplied from the same transformer as the 3.3V and are not regulated as opposed to the 3.3V line. So when I cranked up the volume the PS would shut down since the load on the 12V caused the 3.3V to dip. But since the 3.3V line had no current going out the PS decided to shut down. So in order to get any good power out of the 12V line the 3.3V line should be loaded with at least 1 Amp.
Now I can draw almost 3 Amps on the 12V line without a problem.

PS: your mileage might wary with the load resistor :)
Jan 2, 2012. 10:34 AMpcpowerstereosystem says:
and this is a high end stereo system, my power supplys are 30a at 12v and 36a at 5, it will be drawing around 35-40amps, im just powering a car audio amp with these, headunit is on a different smaller one
Jan 2, 2012. 10:32 AMpcpowerstereosystem says:
i have two 12v 650w power supplys and im looking to power a stereo system, is there anyway i can run them both at like 15v?i have thought about using a red for postive white -5v to the red wire of second, then ground from the sencond, would this give me 15v?
Dec 27, 2011. 6:49 AMbbeckwith says:
Awesome instructable BTW. So i have built one of these with success about 3 days ago. tested the PSU before "moding" it and it powered on by jumping the "PS_ON" to ground. opened it up, performed the mod exactly as yours minus fuses, and a couple of minor wire color differences. Unit works like a champ. i noticed that this unit has an internal "sandbar" type resistor built into it. it is currently being used to power a fan on my transmission cooler im using to cool my mineral oil submersed PC.
So my brother-in-law is an R/C plane hobbiest, saw my little project last night, and really wanted one of these to charge his batteries with. so with another PSU i had laying around, i first tested it by jumping "PS_ON" and ground, and it worked like a champ.
after all internal mods were done, i went to test the PSU before giving it to my brother-in-law and nothing......my +5vSB LED comes on but thats it. i did notice that this PSU had a +3.3vS line so i connected it to the +3.3v wires in the PSU, no worky worky. it puzzles me that this PSU would turn on before taking the PSU "apart" but not now. im going to take the switch out of the loop tonight and solder the "PS_ON" to ground, but other than that any ideas? maybe i have to connect the 3.3vS wire to the +5.5v line?
just a little background on my self, 9.5 year aircraft avionics tech with basic Electronic Principals knowledge. I can use an "O" scope and DMM like nobody business, and know OHM's Law very well. i appologize for not doing anymore troubleshooting before posting this, but im at work and would like as many ideas as possible to go home with to troubleshoot. thank you everyone in advance!
Jan 1, 2012. 10:35 AMbbeckwith says:
added a pic for everyone of my set-up. the PSU conversion powers a 12v Transmission cooler fan from the 5v rail. it is supper loud if i have it connected to the 12v. the cooler cools the mineral oil for the PC :)
IMAG0059.jpg
Jan 1, 2012. 10:24 AMbbeckwith says:
come to find out it was a bad switch from Radioshack. the first thing i did when i got home was short the PS_ON to Gnd and like a champ the PSU turned on. I did however find that even though the PSU doesn't require a load on the 5v rail, the power that comes off the 12v rail is "cleaner" (less ripple) when a load is applied. so i did in fact add a 10ohm 10w resistor on the 5v rail. thanks for the reply and Happy New Year!


P.S. do you have any experience with server PSU's? im about to buy one i found online for $15 that has a 32amp rating on the 12v rail. for R/C hobbyists that charge batteries with these its perfect. wish me luck :)
Jan 2, 2012. 8:41 AMbbeckwith says:
let me save you some money. this also has a 32a rating on the 12v rails for $24 shipped.


http://www.newegg.com/Product/Product.aspx?Item=N82E16817101053

Dec 21, 2011. 7:29 AMn_fernandes says:
good afternoon
recently been made to maintain the equipment that has ATX supplies, maintenance consists of cleaning the equipment with dry air blower.
These devices are connected 24 hours during the last 3 years.

When you reconnect the equipment one source gave no sign of life, and broke in two others the next day.

What is the reason for this situation?
Jul 13, 2009. 10:45 PMkhr1stian says:
Does anyone know of a way to disable the short circuit protection? I am using mine to power a jacob's ladder / plasma speaker, and it really is quite annoying having my psu randomly shut off on me.
Dec 5, 2011. 2:47 PMChrisInVT says:
You ever find a resolution to disabling the short circuit protection? I've been googling for a while now, and this is all I've been able to find.
Dec 18, 2009. 4:22 PMshadowdude77 says:
Ugh, that's a shame, I was gonna use mine to power a flyback for a plasma speaker as well. How often does it shut off? There must be a way to disable short circuit protection, and I can't imagine it's a complicated procedure.
Jun 27, 2010. 2:02 PMhitachi8 says:
yeah , you can put something like a fake load , on the PSU ... like a Resistor , a little light ...
Nov 5, 2011. 7:55 PMtinkerist says:
i'm very sad. i spent the whole day working on this project, and was rather proud (as it's the prettiest electronics project i've put together, to date). but i don't have power to anything except the LED on my rocker switch (green wire) which i used in place of the toggle on yours. the pcb in the atx power supply did have a "3.3vS" wire, which i took to mean that it would need a 3.3v load as well as a 5v, so i added that to the circuit, but it's otherwise identical. any ideas?
Nov 6, 2011. 6:27 PMtinkerist says:
it's the same 10 ohm resister as is on the 5vS line and then to ground. does it need to go to the 3.3v+ instead? should i put a lower resistance sandbar on it? and the ground wire is on the switch.
Nov 17, 2011. 3:44 PMtinkerist says:
ok, so here's what i've got. i've got 5v from the 5vSB and PS ON (purple and green perspectively). no combination of grounding or loading these makes the grey wire (PG) go live. i've found that by grounding both of them i get fractional voltages (less than .02v) from some of the supply wires and the grey PG wire. i'm at a loss. i tried banging my head against it, that didn't work. any ideas?
you're working on a PhD in electrical engineering. if your not an expert, who is? :)
Nov 18, 2011. 8:00 PMtinkerist says:
thank you! i'll try all that and get back to you. i had the 5v (red) and the 3.3vS running through 10Ω sandbars. I also tried running 3.3v and the 3.3vS wire together and then through the sandbar. i'm not sure i understand how the 3.3vS wire would get a signal from a 3.3v wire that has no voltage. but i understand practically nothing about electronics, this is part of my learning process. the atx did power the computer the last time i used it, but that was like 3 years ago. thanks again for the advice, i'll let you know how it turns out.
Nov 9, 2011. 1:15 PMbrusho150 says:
HI!
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?
Thanks
Nov 10, 2011. 5:43 PMbrusho150 says:
Thank you mam.
Oct 25, 2011. 12:18 PMMr.crispi says:
Hi, im in the process of building this and i accidentally crossed my data wire with (i think 3.3v) now the unit stays on when i plug in the cord. Making the switch (data to ground) useless.. Is there any way i can fix the data wire? Or can i put a switch somewhere else besides at the 110vac input? Thanks in advance
Oct 26, 2011. 6:41 PMMr.crispi says:
Hey,
Yeah by data wire i mean the green wire that needs to be grounded for the psu to turn on. And i accidentally touched it to the 3.3 wire for maybe a second or two and now the data wire is useless for a switch to turn it on/off... There is not a rocker switch in the back sadly. Lol so the problem i have now is that when i plug this in it turn right on even with the data wire not on ground, do you think there is a way to fix the green wire so i can use it and ground for the swittch shown in your schematic or should i find another place to put a switch?

Thanks for taking the time to give me advice, i appreciate it!
Oct 27, 2011. 12:23 PMMr.crispi says:
You indeed are making sense, but im using this as a lab top power supply basically for testing my daily projects. So i'll just get a spst switch rated for 120vac and put itinline with the input to controll on and off. Thanks :)
Oct 26, 2011. 3:38 PMEagleGrip says:
Well the other day i went to radio shack and purchased the only sandbars they had thinking that they were 10 ohm 10 watts, but to my surprise i have found out that they are only 1 ohm, 10 watt. is there a way i can make this work with such a difference? that is the only radio shack in my near area so my only other option would be to order them on eBay, but if i can make these work that would be awesome! please help!
Oct 18, 2011. 6:28 PMcoolsciencetech says:
Mine is running fine without any load on it except for any fans inside the case. It runs cool, and the voltages are good for the 5, 3.3, and 12v rails, and slightly off on the -5, and -12v rails.

Everything seems fine but should I still have the power resistor? I just want to make sure nothing is going wrong :P
Oct 15, 2011. 8:33 PMcrf250rider14 says:
Hey matthew its me again, I was wondering if I would be able to connect a switch to a rail of the power supply. Say the 12v rail. Could this be done without the power supply turning off thinking it is shorting out? If it can be done how would I connect it and what kind of switch should I use. I have been trying to figure this out and have not found a solution yet.
If you get a chance please respond.
Thanks again.
-Nick
Oct 7, 2011. 7:35 PMdlumpkins says:
yo, the bubbles coming off the cathode are turning on and off, pulsing like, and the fan is in sync, i would assume because it turns on when power is used. why is it pulsing?
Oct 10, 2011. 9:13 AMdlumpkins says:
i am using it as a chlorate cell, and i was using the .5A setting instead of the 30A setting, so i think i was drawing about 5A from the .5A setting and therefor crashing it, and it would then start up again, and crash again. i switched it to the 30A setting and it works great now. also i got the PC from a dumpster at a college, sooooooooo the PSU could have been the reason it was thrown away.
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Author:matthewbeckler
A PhD student in Electrical and Computer Engineering at Carnegie Mellon University in Pittsburgh, PA.