Introduction: Convert a Computer Power Supply to a Variable Bench Top Lab Power Supply

Picture of Convert a Computer Power Supply to a Variable Bench Top Lab Power Supply

Prices Today for a lab power supply well exceed $180. But it turns out a obsolete computer power supply is perfect for the job instead. With these costing you only $25 and having short circuit protection, thermal protection, Overload protection and varied output voltages of 3v, 5v and 12v but we will me modifying it to give out 1.5v to 24v. They are perfect for general electronics.

This is my first Instructable for what I think is a brilliant idea, I'm only 14 and i can build it

WARNING: This will void warranty's and can shock you if you don't have your wits about you

NOTE: This Tutorial is littered with bad grammar and spelling mistakes. English Teachers may want to look away now

Your going to need:
Screw Driver
Computer PSU (I recommend 250W+)
PSU Cable
Wire Snaps
Soldering Iron
A 10ohm, 10W or greater power resistor (Some new power supply's don't work properly without some load so this can provide that)

2 LEDs of any colour (Red and Green is the best)
If your using the leds you need a 1 or 2 330 OEM Resistor(s)
Heat Shrink Tubing
External Enclosure (Some people cram it all inside the Power supply case or you can put it in a external enclosure.)

These Depend on which method you use: (More on that later):
Terminal Blocks
LM317 or LM338K Voltage regulator
100nF Capacitors (ceramic or tantalum)
1uF Capacitors Electrolytic
1N4001 or 1N4002 Power Diode
120 Ohm resistor
5k Ohm variable resistor
Binding Posts
Crocodile Clips

Step 1: Harvesting & Preping the Power Supply

Picture of Harvesting & Preping the Power Supply


Capacitors can bite and if not give a painful shock kill you. Please discharge the power supply by letting it sit unconnected for a few days or connecting a 10ohm resistor between the red and black wires.

If you hear buzzing when you turn on the power supply it means there is a short or another serious problem. If you hear buzzing (that not coming from the soldering iron) when soldiering it mean your power supply is on. There is still power flowing through the PSU if it plugged in but not switched on

OK lets get straight into it remove the computer case and take out the screws (usually 4) at the back of the computer to release the power supply. Now take out the 4 screws on top of the case and take the wires out of the hole then group wires of the same colour together and snip off the ends.

Just To tell you , you've just void your warranty

Step 2: Wiring It All Up!

Picture of Wiring It All Up!

Now comes the tricky part, this is were we really get into it and add LED's and switches and other such objects. There are alot of each type of wire so I recommend using 2-4 of each type. Some people cram everything inside the box i used another external enclosure but it depends which method you use in the next step.

If you want to add a Standby or a Mains On LED then you will need a LED (Reds recommended but not a necessity) and a 330 Ohm Resistor. Solder a black wire to one end of the resistor and the short leg of the LED to the other. The resistor will reduce the voltage down to stop it damaging the LED. Before you soldier the other one on optionally slip on a little bit of heat shrink tubing to stop shorts. Solder the purple wire to the longer leg and when you plug it in but don't turn it on, it should light.

You can also have another LED (Green Works Best) to light when you turn the PSU on. Some say to use the grey wire for the power for the LED but you need another 330 Ohm resistor. I just connected it to the orange 3.3v wire.

If you are using the Grey wire:
Before you solder it on slip another bit of heat shrink tubing over it to stop shorts. Solder the grey wire to one end of the resistor and the other end of the resistor to the longer leg of the LED and a black wire the the shorter leg.

If using the Orange 3.3v Wire:
Before you solder it on slip yet another bit of heat shrink tubing over it to stop shorts. Solder the orange wire to the longer leg of the LED and a black wire the the shorter leg.

Now for the switch, if you have one on the back of your PSU i suppose you don't really need this but i think you should still use it regardless. Connect the Green wire to one contact on the switch and a black to the other. If your really against using a switch then just tape together the green and black wires.

You can also use a 1 amp fuse. All you do it get the clump of black wires you'll be using and cut them somewhere along the wire and then bridge them with a fuse in a fuse holder.

Some Power supply's need a load to work properly. To provide this load solder a red wire to one end of a 10 ohm, 10 watt resistor and a black wire to the other. This will trick the power supply into thinking its powering something.

If this is all confusing there is a diagram attached to help. The diagram shows the binding post method to connect the wires. I will explain more about these in the next step. It also connects the grey wire to the Power on LED but you can also use the orange wire and it also shows the wiring for the high wattage resistor.

Step 3: Presenting the Power

Picture of Presenting the Power

OK from all the other tutorials I've read there are a lot of different methods of connectors for connecting your devices to the power, Ill start with the best one and work my way down to the worst.

Some tutorials will tell you to stuff it all inside the one case but that is dangerous and will make it very warm and crushed. I recommend using a external enclosure.

1.Adding a Variable Resistor:
I personally think this is the best method as this can provide any voltage between 1.5 to 24 volts. The reason that its 22v and not 12 is because it uses the Blue wire which is -12 volts not the common earth (black wire). You will need:

LM317 or LM338K Voltage regulator
100nF Capacitors (ceramic or tantalum)
1uF Capacitors Electrolytic
1N4001 or 1N4002 Power Diode
120 Ohm resistor
1x 5k Ohm variable resistor

First build the circuit from the main picture and connect your +12 and -12 volt lines. Now drill holes in either the power supply or an external case to fit the variable resistor, All the other circuitry should be kept inside. I suggest now adding Two terminal blocks so you can wire devices directly in. You could also connect some alligator clips in to the terminal blocks aswell. When you turn the variable resistor the voltage should range between 1.5 and 24 volts. NOTE:There is a typo in the main picture it should read +24v variable instead of 22v. If you had an old volt meter you could wire it in to the output so it can tell you what voltage you are at.

2. Binding Posts
2nd is using binding posts to connect equipment. First drill hole for the binding posts (make sure to wrap the circuit board up in plastic as metal shards can short circuit it) then check they are the right size by inserting the posts and tightening the bolt behind them. You chose what voltage to hook up to what post and how many posts to put in. The colour Codes for all the wires are:

Red: +5v
Yellow: +12v
Orange: +3.3v
Black: Earth/Ground
White: -5v

There is a image below using the binding post method.

3.Basic Crocodile Clips
If you don't have that much experience or don't have the above parts and for some reason can't buy them you can just hook up whatever voltages you want to Crocodile clips. If you do chose this option I would suggest a sleeve over the Crocodile clips to prevent short circuits.

Tips and Troubleshooting:

- Dont be a bit afraid to spice the box up a bit, you could add leds, stickers or anything!

-Make sure you are using a ATX Power Supply. If it is a AT or older power supply it will most likely have a different colour scheme for the wires. Unless you have some data on the wiring dont attempt this as you could get caught on the wrong end of a wire and get your head blown off.

- PSU means Power Supply Unit

-If the LED on the front doesn't come on chances are you have the leg wired up the wrong way around just switch the wires on the legs and it should light.

-Some modern Power supply's will have a "sense wire" this has to be connected to power for the Power supply to function. If the wire is grey connect it to an orange wire, if it is pink connect it to a red wire.

-The High wattage power resistor can become quite hot; you could use a heatsink to cool it down but make sure it doesn't short anything out.

-If you insist on putting everything inside, you can put the fan on the outside rather then the inside.

-The PSU fan can be noisy , it is powered by 12v. Since it isn't power computers anymore and doesn't heat up as much you can snip the red wire of the fan and connect the orange 3.3v wire. Keep an eye on your circuit after you do this, if it produces too much heat connect the fan back up to the red wire.

CONGRADS You have successfully finished your Power supply!

Thanks to other tutorials on Wikihow and Instructables because I used some of there pictures.

This instructable was published quite a while ago and unfortunately I am no longer in a position to provide support for it. But there's lots of good stuff in the comments below. Thanks for all the likes, shares and follows!


ChrisB764 (author)2017-07-15

Just wanted to say thanks for your instructable it is very helpful despite the above mentioned bad grammer, (not an english teacher by the way!). I'm using a switching power supply, and it reads out correctly in the voltages yellow = 12v red = 5.5v and orange = 3.3v roughly. When I switch the meter to amps however the fan on the box stops working ( I have to unclip the green wire and reconnect it to turn it back on. Do you have any advice?

velix94 (author)ChrisB7642017-11-07

You can't simply switching to amp cause you'll short-cut the circuit. Probably the power supply have a protection against it. To measure the current you must have a load and put the amperometer (or multimeter) between the load and the power supply, in series.

suncoaster2 made it! (author)2017-06-15

Thanks for the nice connection diagrams. Turned out well. Simple but functional.

zahilslmn (author)2017-05-05

He isnt joking when he says make sure you've left it unplugged. I just wanted to find out if my green wire was a ground cord. Learn from my mistake. I believe I shorted the negative and the ground, but all I know for sure is that I got a giant spark. It was scary. Don't think you're too good for safety precautions. Electrical fires are very dangerous.

dranil made it! (author)2015-10-22

I made it, but made some modifications. Thank you

ShawnM53 (author)dranil2017-03-18

Looks nice, my dude!

bkeskin (author)2016-12-11

Hi, I really wonder what -12V is? If I want to get 12V, I get wires connected to +12 and ground. In which circumstances should i use -12? Thanks in advance

ivang176 (author)bkeskin2017-02-22

If you connect your positive wire to +12V and your negative wire to -12V, you will get 24V. It is like AC mains power: it doesn't alternate between +230V and 0V 50 times a second (Europe) but it alternates between +115V and -115V 50 times a second. The final voltage will be your positive voltage minus your negative voltage: if you have 12V on one wire and 0V on the other, then 12-0=12V and if you have +6V on one wire and -6V on the other, then +6-(-6)=6+6=12.

Major Breakdown (author)ivang1762017-03-05

Just for safety's sake your AC maths need to be corrected. 230 V 50 Hz AC means that the voltage is alternating between the peaks of +325 V and -325 V.

The negative DC voltages are useful for op-amps and other applications where you need a -12 - 0 - 12 V supply. The power capacity of the -12 V output does not make it very useful for creating a general purpose 24 V. Also you would potentially enter into trouble with separate earth levels if you use that kind of 24 V in the same circuit as for example 5 V from the same supply.

bkeskin (author)ivang1762017-02-22

Thank for enlighting me Ivang. Have a great day.

ivang176 (author)ivang1762017-02-22

By the way, as other people have said, don't try to connect big loads when using the -12V cable, as it supports very small currents (around 500mA or less, depending on the power supply).

SuwandyS (author)2017-02-27

hi there can i use to power 2 peltier module for aquarium chiller??

ivang176 (author)2017-02-22

What can I add to be able to regulate current as well?

supertin (author)2017-01-08

-12v line on modern PSUs is VERY low output. First random PSU I just
looked at was 800mA. Second was only 500mA. And these are older PSUs.
Since -12v is rarely used now, the outputs have been dropping off.
And remember, when you start putting a load close to the max rated output, the voltage may drop somewhat.

Do not rely on the -12v line for power! It's OK for small loads, but even a small fan may overload it.

aldolo (author)2016-12-04

avoid to connect the regulator to the psu metal body

ArijitB11 (author)2016-08-11

Hello, I need 12v 5 amp current from a pc smps. What I need to do?

BertH11 (author)ArijitB112016-11-09

Nothing just use 3 or more yellow(+12V) and 3 or more black(GND) wires

don't forget the resistor 10 ohm 10W between black(GND) and red(+5V)

Electrowan (author)2016-06-07

i just built it and all my multimeter is showing is a variability from 0.3 volts to 1.5 volts what did i do wrong

M-core (author)2016-03-25

Hi, I made this project. I also added a 2 port usb i hacked from the same old computer. I simply connected green (power on) to GND (black) to make the original switch work. This case I don't have the above mentioned 5V when plugged in but not switched on. I consider working on that to i order to power the usb port.

My observations:

The PSU itself gives only 23V. After adding the variable circuit with the LM317T it dropped another 1V, so the schematics states correctly (in my case), that output is 22V.

Problems to clarify:

I observed a .1 to .3 variation on the voltage output. I have a suspicion that it could come from my digital multimeter, so I made the following steps to verify:

1. I wired a 1K load on the output.

Result: around .5 volt dropped but still varying .1 to .3.

2. I measured with an analogue display multimeter.

Result: stable.

3. I wired a 24V 50W halogen bulb to it. (This was at hand).

Result: visually stable. Ampmeter showed steady 1.5A.

4. I took measurement on 5V (red) and GND (black) on the PSU itself.

Result: variation.

I'm still concerned with this variation. I will test it with other multimeters when I have the chance, to determine whether it is a measurement error or not.

TorgnyN (author)2016-03-09

I think the -12 +12 can cause some issues with sensible electronics. I connected a device that works perfectly fine with every other connection 3.3, 5 and 12V but not with this. Also it refused to show the right measurements with my multimeter. Just didn't work well at all with many things. Eventually it broke so the next time I will use a regular 24v transformer for this job. Takes a lot less space and gives off a lot of more amperage as well.

BobyL (author)2016-02-29

u mixed -12V & +12V regardless current sourcing, -12V is usually so small sourcing current!

ilovefood213 made it! (author)2016-02-17

need help with this? can you help?

JesusG33k (author)2016-02-09

I flipped my breaker!

AleksandarI4 (author)2016-01-27

Hello All,

I have created this about 2 days ago and all went great. It was working fine and I was very happy with my project. But then I was testing some circuit with led diode and I have increased the voltage by mistake while the circuit was still connected. LED Diode of course blow up :) but also my power supply is somehow damaged. I will transfer the symptoms to you and hopefully someone will provide me with possible fix:

When I turn the switch ON it doesn't run (only the fan will try to run and at the same moment will go off. Please note that I do have 9W47ohm resistor as a fake load between GND and 5V (black and red wires). I know it was suggested to use 10ohms but I couldn't find one and this was perfectly fine. Just to exclude the resistor as a possible reason. Also please note that the issue started to happen after the incident)

If I connect some DC motor that operate with let's say 18V, and then turn ON the switch, it starts working but the minimum voltage (on the regulated outputs) is 12V and not 1.2 as before and it only goes up to about 14.6. Basically that is it, just I am not sure how to fix it. I did one mistake I didn't used PCB because I considered it to be fairly simple project, but now fixing will be real pain. I will now create PCB so if there is a need for future fixes it will be easier and faster.

I would be really thankful if someone suggest what could be damaged and how to check/test because although I do have some general knowledge, I am far from being expert in electronics.

Thank you in advance.

MikkoK7 (author)2016-01-27

Hi.codegen 350W. I have the resistor on 5v line and about pc. 12V yellow 12V wires connected. I only need 12V for my lipo charger. Psu gives out about 2amps now. On the side label ot states that 12V out is 15A.
any tips to get hihher current output ?.

zacker (author)2013-05-26

hi, this is just what im looking for except one thing. I am using a PSU from a DELL XPS and its a 750W and it has both a 20 wire plug AND a 24 wire plug besides all the other little connectors with 6 or 4 wires.
Do I use ALL the wires? take every black wire and put them together, every red, every orange, blue (well, blue with a white stripe)... ? also, do i use a 10w, 10 ohm resistor or do i need a bigger one? thanks!

LowerT1 (author)zacker2016-01-20

i would suggest you use maybe 3 wires, at most, of each color. any more than that and it is a waste. just cut out all the lead wires, and leave only 3 of each color and you'll be good to go.

zacker (author)LowerT12016-01-21

Thanks for the update!

nitewing76 (author)zacker2014-07-30

The more wires of the same color you tie together...only increases the amount of amps you can pull without melting any wires.

10ohm 10W should be fine for most applications.

FikretÇ made it! (author)2015-04-28

Thank you for the instructable,

I have used LM317 because I do not need high amperage. I have replaced 120 Ohm resistor with 240 Ohm and added extra diode (1N4002) to the right side of C2 (1uF capacitor) .

It works well, I get variable voltage between 1,1 v - 23 v


LowerT1 (author)FikretÇ2016-01-20


mikail.garcia (author)2015-01-08

Hi, i tried building the variable voltage circuit according to what has been mentioned, however, strange thing is that, when I turn the potentiometer towards the lower end, it starts to burn the potentiometer ( i though new year just passed :-) ). Where could I be wrong? Btw, I use LM338K. Any suggestions? Thanks

You could be wiring it wrong somewhere. This happened to me too when I first tried to assemble this circuit. Make sure you follow not only the circuit diagram but the LM317's pinout. From the front, pin 1 is the adjust, pin 2 is Vout and pin 3 is Vin. Try assembling it on a breadboard first to make sure that everything is properly wired and working before you solder it.

and use a pot with a high enough wattage rating so as not to burn the windings

zitt (author)2015-06-06

Don't do the variable circuit this way. It'll fry your PSU. Look at the schematic; he's tying -12V to Ground. No way that will work right.

LowerT1 (author)zitt2016-01-20

looks okay to me. the -12v isn't going to ground, it is floating, and acting as ground only for the purpose of his schematic. no problem

Ziaius (author)zitt2015-07-26

Are you referring to a green line on the bottom? :)

rosimo (author)2015-06-24

Hi, I've a Dell N875EF-00 PSU (825W) that I scavenged. I don't really understand the sticker, it says 18A / 12V. Does it mean I'll have 90A if I connect all the 12V together ???

LowerT1 (author)rosimo2016-01-20

no, you will not have 90A.

if you look closely at the circuit board, you'll notice that all the 12v lines are already connected together.

have fun.


JesusG33k (author)rosimo2016-01-09

Each 12v line is usually derived from the same transformer, but I'm unaware of the internal circuitry beyond that. I don't recommend tying them together.

FikretÇ (author)2015-05-29

One month ago I have made this. Everything was fine until today. I have used it in different voltage settings. But, today while I was working on project with 18.9 Volts, it suddenly blowed up.

I made no short circuit on the project. Didn't even touched the breadboard for nearly 10 mins.

I checked later on, the fuse soldered on the main board (5A L240V) has blown.

Can you please give me any idea what caused this ?

TorgnyN (author)FikretÇ2016-01-17

I think I know why and think everyone who follow this guide should too! If you read +12V on your PSU, it says about 14A, atleast on mine. But if you on the other hand read on the -12V, not the GND, then the maximum current is 1A. That means that you can pull lots of amps out of the 3.3V, 5V and 12V. But on the variable output you can never pull more than 1A from the PSU, atleast not for any extended period of time. I am puttin a 1,25A fuse onto my circuit as I write this.

JesusG33k (author)FikretÇ2016-01-09

Anything could have. Try replacing the fuse with an equivalent type.

Unmesh55 (author)2015-02-20

Hey dude please reply fast..
I am using 10k pot instead of 5k
Lm317t for lm338k
And 1N4007 instead of 1N4002
Suggest me the changes in circuit and also let me know why my pot gets red when it is in lowest positin

TorgnyN (author)Unmesh552016-01-12

I use a LM317T as well and a 1N4007. I used a 4,5k potentiometer and it got hot and started smoking at lowest too. Then I tried a bigger one and then I could only switch between 11 and 12 volts. Now the old one doesn't work either and I don't seem to be able to regulate at all. If I disconnect the potentiometer (connected it through a terminal block) and it sends out 12V. Is it supposed to do that?

Did you figure out your problem?

Unmesh55 (author)TorgnyN2016-01-12

Sorry bro..i didnt find any alternatives and nor i had continued on project but i will be working on it soon. Till then u can google and sort out, if u could get the solution plz tell me @ I will also reply you as soon as possible. Till then good luck!!!!!???

TorgnyN (author)Unmesh552016-01-13

If I couldn't figure it out, I got the advice to try this instead: Might be of interest to you as well.

Unmesh55 (author)TorgnyN2016-01-13

Thanks also trying to find any issue in circuit..

TorgnyN (author)2016-01-11

I am trying to make this out of 100% salvaged material from my garbage room. The only thing lacking right now is the diode and a potentiometer. The latter I know I will find in a couple of days but for the 1N4001 or 1N4002 it doesn't look as promising. I've found loads and loads of 1N4007 though which seems a lot more common in new electronics. From my very much layman based research I've found out it would be just as good for the purpose, just that it's made for higher voltage. My question is, can I use a 1N4007 instead?

PrasadA (author)2015-03-30

Please tell me,

Does power supply leak current from its case when it turn on.

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