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 http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

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.

Step 1: Harvesting the ATX Power Supply

1) Unplug the power cord from the back of the computer. "Harvest" a power supply from a computer by opening up the case of the computer, locating the gray box that is the power supply unit, tracing the wires from the power supply to the boards and devices and disconnecting all the cables by unplugging them.
2) Remove the screws (typically 4) that attach the power supply to the computer case and remove the power supply.
3) Discharge the power supply by either letting it sit unconnected for a few days, or by attaching a 10 ohm resistor between a black and red wire (from the power cables on the output side). Using a resistor will only take a few seconds to fully discharge the power supply.
4) Gather the parts you need: binding posts (terminals), a LED with a current-limiting resistor, a switch (optional), a power resistor (10 ohm, 10W or greater wattage, see Tips), and heat shrink tubing.

Step 2: Preparing the ATX Power Supply

5) Cut off the connectors (leave a few inches of wire on the connectors so that you can use them later on for other projects).
6) Open up the power supply unit by removing the screws connecting the top and the bottom of the PSU case.
7) Bundle wires of the same colors together. The color code for the wires is: Red = +5V, Black = Ground (0V), White = -5V, Yellow = +12V, Blue = -12V, Orange = +3.3V, Purple = +5V Standby, Gray = power is on (output), and Green = Turn DC on (input).

Step 3: Drill Holes for Attaching Binding Posts

8) Drill holes in a free area of the power supply case by marking the center of the holes with a nail and a tap from the hammer. Use a Dremel to drill the starting holes followed by a hand reamer to enlarge the holes until they are the right size by test fitting the binding posts. Also, drill holes for the power ON LED and a Power switch.

Step 4: Attach Binding Posts and Start Wiring

9) Screw the binding posts into their corresponding holes and attach the nut on the back.

Step 5: Connecting the Wires

10) Connect one of the red wires to the power resistor.
11) All the remaining red wires to the red binding posts.
12) Connect one of the black wires to the other end of the power resistor.
13) One black wire to a resistor (330 ohm) attached to the anode of the LED(see the next image)
14) One black wire to the DC-On switch
15) All the remaining black wires to the black binding post.
16) Connect the white to the -5V binding post, yellow to the +12V binding post, the blue to the -12V binding post, the gray to the cathode of the LED.
  • Note that most power supplies have either a mauve or brown wire to represent "power good"/"power ok". Check the ATX plug (the plug with many connections) to see if there is a small mauve or bown wire plugged into the same hole as an orange wire (+3.3V) or a red wire (+5V). If the small wire is connected to the orange in the ATX plug then do the same, hook these two together. If it is connected to the red, then hook it to the red wire. This wire must be connected to either an orange wire (+3.3V) or a red wire (+5V) for the power supply to function. When in doubt, try the lower voltage first (+3.3V).

Step 6: Wrapping Up the Connections

17) Connect the green wire to the other terminal on the switch.
18) Make sure that the soldered ends are insulated in heatshrink tubing.
19) Organize the wires with a electrical tape or zip-ties.
20) Check for loose connections by gently tugging on the wires.
21) Inspect for bare wire, and cover it to prevent a short circuit.
22) Put a drop of super-glue to stick the LED to its hole.
23) Put the power supply cover back on.

Step 7: Testing the Power Supply

24) Plug the power cord into the back and into an AC socket.
25) Flip the main switch on the PSU (on the back). The fan will come on.
26) Check to see if the LED light comes on. If it has not, then power up by flipping the switch you placed on the front.
27) Plug in a 12V bulb into the different sockets to see if the PSU works, also check with a digital voltmeter. It should look good and work like a charm!

Updated the packaging a bit so it looks sexier and wife acceptance factor increases ... used a ready made unfinished wooden box from Michaels.

Step 8: Repackaging the Power Supply in Nicer Enclosure

I still use that supply quite a lot but Wife Acceptance Factor was low as it looked - well -ugly. So I used one of the cool looking wooden boxes we had bought from Michaels as a new skin for this power supply (above). Essentially installed the circuit board into the box (after lining the insides of the box with foil and then with insulating tape), reconnected all the wires to the terminals, installed a smaller fan, and added an old car amp meter that I had bought from Harbor Freight and viola a new sexier looking power supply.

Stained the box, carved some cool lettering with a Dremel ... still have to add labels for the different terminals ... but it now looks good when placed in the bookshelf with no hint of what it is till you flip it around!

Step 9: Troubleshooting

# If you are not sure of the power supply, test it in the computer before you harvest. Does the computer power on? Does the PSU fan come on? You can place your voltmeter leads into an extra plug (for disk drives). It should read close to 5V (between red and black wires). A supply that you have pulled may look dead because it does not have a load on its outputs and the enable output may not be grounded (green wire).
# If the LED light does not come on, check to see if the fan has come on. If the fan in the power supply is on, then the LED may have been wired wrong (the positive and negative leads of the LED may have been switched). Open the power supply case and flip the purple or gray wires on the LED around (make sure that you do not bypass the LED resistor).
# ATX power supplies are "switch-mode supplies"; they must always have some load to operate properly. The power resistor is there to "waste" energy, which will give off heat; therefore it should be mounted on the metal wall for proper cooling (you can also pick up a heatsink to mount on your resistor, just make sure the heatsink dosn't short anything out). If you will always have something connected to the supply when it is on, you may leave out the power resistor.
# You can add a 3.3 volt output to the supply by hooking the orange wires to a post (making sure the brown wire remains connected to an orange wire) but beware that they share the same power output as the 5 volt, and thus you must not exceed the total power output of these two outputs.
# If you don't feel like soldering nine wires together to a binding post (as is the case with the ground wires) you can snip them at the PCB. 1-3 wires should be fine. This includes cutting any wires that you don't ever plan on using.
# The +5VSB line is +5V standby (so the motherboard's power buttons, Wake on LAN, etc. work). This typically provides 500-1000 mA of current, even when the main DC outputs are "off". It might be useful to drive an LED from this as an indication that the mains are on.

Options: You don't need the switch in the front, just connect the green and a black wire together. The PSU will be controlled by the rear switch, if there is one. You also don't need the LED, just ignore the gray wire. Cut it short and insulate it from the rest.

Step 10: Almost 10 Years Later ...!

Am still using the original Power Supply though now it is packaged in a more professional case. I also added an ammeter in the 0 V line so that I could measure the Amps being delivered. I also added a USB socket to charge USB devices. The last diagram shows a simplified wiring diagram of all the parts together.

The fact that this PSU is short circuit proof has turned out to be a big advantage. It has withstood a lot of abuse.

<p>Thank you for this great and clear explanation. I'll try it. Here are 4 old PSU around. I want to provide a digital camera and flash units with it.<br>I do not understand that with the ammeter and the connection to 0V? </p><p>If you are 12V with 0V and 5V with 0V connects simultaneously, the total current is then displayed as?<br>Maybe you can post a circuit of your update version, would by nice.<br>Could I install a voltmeter and a switch, to switch to the different outputs?<br>Sorry, my English is just as bad as my knowledge in electronics.<br>Here's perhaps someone who has experience or ideas with the PSU power supply of a digital camera and one or two flash units, each Note and advice would be great.<br>Thanks again, your update looks just as good</p>
<p>thank you for your answer, it helps a lot. I like your projects</p>
Forgot to add to my previous reply. If you connect your device between +12V and -12V then the current will not flow through the 0V line and in such cases you will see no reading in the ammeter. The ammeter will only work when current is flowing into or out of the 0V line.
Hi HD DigitalArtJ, have added a circuit diagram showing where the ammeter was placed. (The circuit diagram does not show the power leads for the ammeter but most will require connection to +12V and 0V). As you know current flows from + voltage to 0V. If we place the ammeter in the zero volt line, all the current returning through the zero volt line will be measured. For example, if you connect a bulb between the 0V and 12V, the current passing through the bulb will be displayed. If at the same time you add another bulb to the 5V line and the 0V line, this new current will be added to the reading. That is, let us say the 2 A for the bulb connected to the 12-0V and 1 A for the 5-0V would result in a reading of 2+1 = 3A on the Ammeter.
<p>An update. Have further modified this power supply by putting it in a better case and adding an ammeter. The digital ammeter (ebay) was added in the 0 V (Gnd line) so any current returning through ground can be measured. Also added a USB power port that connects to the +5 V and 0 V.</p>
<p>Just a note to let you know I have added this instructable to the collection: <br>Encyclopedia of ATX to Bench Power Supply Conversion <br>&gt;&gt; <a href="https://www.instructables.com/id/Encyclopedia-of-ATX-to-Bench-Power-Supply-Conversi/" rel="nofollow">https://www.instructables.com/id/Encyclopedia-of-ATX-to-Bench-Power-Supply-Conversi/</a><br>Take a look at about 70 different approaches to this project.</p>
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!
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.
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?
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. <br>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!
Its a little confusing, ill just have to read it a couple times to get it to sink in...lol thanks for explaining it. <br>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 &quot;Quick&quot; 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
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.
Hmmm.... What are the Amps on the +5 side? Ill be doing mostly small parts, the Vice I did was an 8&quot; 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...
The wood box looks fancy!
in step 5 you said, &quot;most power supplies have a brown or mauve wire.&quot; 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 <br> <br>I read mauve and it says somewhere else purple too, I got things mixed around
thanks Abizar!!! I have favorited 2 of your 3 instructables that I plan to do myself...great work!!!
Hello and nice instructable! <br>I liv e in Europe and using 220V AC input. <br>Do I have to change the 10 ohm 10 W resistor with some different resistor or is it good using the same? <br>I need 6 amp out in the 12V line! <br> <br>Thanks a lot! <br> <br> <br>Sam
10 ohm should be good. <br>Most of the PSU are compatible with 100-240V input so you could use it in most countries.
Really nice instructable! <br>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
You would get about 0.5 A only.
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?
Thanks for posting this. It was super helpful and clear. <br>
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, <br>- sleepyjz
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 &quot;reasons&quot; 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 <br>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
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.&nbsp;<br><br>Otherwise, your question is not quite clear...<br><br>[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. <br><br>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. <br><br>From what I've read,<br>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. <br><br><br>When you say &quot;it settles at 11.88v&quot; 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 &quot;12.xx&quot; 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?<br><br><br>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). <br><br>So 1) i would try adding another resistor (in parallel, not series)<br>2) Is there anyway you can work around your 12.6 or 12.4v requirement?
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 &amp; just 1 brown cable connected it to orange cable, but the psu still would not start, can you help me?
I'm trying to use a computer psu to power up a rgb led strip(12v, 3amp)..<br>The 12v rail is giving 12.4v (no load), the outputs of these psu's arn't regulated right?<br>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.<br>What do you think, should i use this one and some transistors to handle more current?<br><br>Need some tips guys =)<br><br>
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.
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..<br><br>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.&nbsp;<br> <br> Otherwise, your question is not quite clear...<br> [This is a little old, maybe you have long given up].
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.
HI!<br>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?<br>Thanks
Wondering what your application is? What voltages are you looking for and at what current? <br>You could do the potentiometer (variable resistor) but would need a beefy one if it isfor high current.<br>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.<br>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.<br>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.<br>
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.<br><br>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.<br><br>But anything larger (rc motor, motor dyno) the power supply immediately shuts off. I cannot find a reason for this at all. Any ideas?
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.<br>Current (in Amps) = (Horsepower x 746) / Voltage. <br>A 1/4th hp motor will require 15 Amps from a 12 V supply. Or, 37 Amps from the 5V supply.
PSUs shut down if they are overloaded
would a 20w 2ohm resistor work<br>
P (watts) = Voltage (5V) x Voltage (5V)/R (2 ohm)<br>=12.5 W.<br>So you would burn away about 12.5 W in the resistor which it should be able to handle, though you will waste more power.
hi i try making this and in my atx 20 pin power supply i have a gray, green, brown, and purple wires... i did exactly what the guide said but for some reason when i turn it on the fan would spin for like a split second and would shut down. i tried everything and is still not working... the only power i saw was connecting the purple wire to the led and that works when i flip the switch at the back of the power supply not the switch i made with the green and black wire... also i tried taking out the switch and connecting the green and black together so i know the connection is right.. but i get notting...it seems like it is shutting down but i dont know why... i have a load on the 5v line.. tried the car light buld (did not work).. i tried the 10amp &gt;10watt resistor (did not work).. please help cuz i have alot of project that i need for my car and dont want to keep bringing my car battery inside the house.. thank you
try to put the green wire with the red wire that mite help
put 2 10ohm 10watt resistor in parallel
I need some help. I am trying to make one of these and the yellow wire is +12v and my meter reads +9.98V. And the blue wire -12v reads -7v. And the fan is barely spinning. Any help would be appreciated. Thanks in advance.

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