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
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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.

If you have any questions email me at prod_lad@hotmail.co.uk

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Coolinst5 years ago
 My PSU has no green wire. What should I do?
most Dell supplies, are wired slightly different on the ATX connector.. Sometimes a Grey, sometimes a totally different color.. Once it gets this confusing, the next step is to unplug the supply from the wall, and let it sit overnight.. then break-out the screwdriver, and open the box.. Sometimes, if you're lucky, they have the wires labeled on the PBC of the supply.. I'm looking at a 350W supply from Dell, that I pulled from a Optiplex G570 someone killed. So-far, the Pwr-On wire is blue, and I haven't fried anything yet. (still remembering my 1st Dell supply going up in smoke..)

One thing you might do when re-using an old computer PSU , would be to pay particular attention to what the wires were connected to in the old computer , in case the wire colors are not standard . Also , the wires that feed power to LED's will most likely be a smaller sized wire . For instance :


Cheers , take care , and have a good day !!

I have the same problem how you overcome it. Please tell..
prodlad (author)  Coolinst5 years ago
its not atx standard. it could say on the side or search for wiring schemes on google
zitt1 month ago

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.

Ziaius zitt3 days ago

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

rosimo1 month ago

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 ???

mikail.garcia6 months ago

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.

nvasilev15 months ago

Hi thanks for the instructable

I wander what Amperage the Adjustable Voltage component can pump out and what do I need to do to utilize the full capacity of the power supply if it is let say 550w.


The LM317 has a max current output of 1.5 amps so you wouldn't be able to pull that much power from the VARIABLE part of the supply. If you want to maximize the amount of current you get from each supply output (3.3V, 5V, 12V) then you can wire as many of the corresponding wires together as each individual wire has a maximum current rating so by wiring them together you increase this maximum current you can get from each output voltage.

FikretÇ2 months ago

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 ?

Cliff40042 months ago

hi I made this last week and all worked ok. Today I was testing some LEDs on the variable side using a breadboard at about 3v when suddenly the voltmeter dropped to zero. I have tested the fixed posts and all are ok at 3.3,5 & 12v but the variable side is showing -12v on both posts. I am a newbie at electronics and could do with some advice on identifying the problem.

RijeeshP4 months ago

please help me
i have 200w SMPS Model No GPS-200BB C A00
Power Good and Stand by voltage are coming properly, but fan not rotating continusly
its getting stop with in 2 second
no other voltages are coming

PaulG14 RijeeshP2 months ago

Sound like you have no load. ATX power supplyes need to have a constent load on them, somthing like a 10 ohms 10watt power resistor. This in normaly placed between the 5V rail and ground.

FikretÇ made it!3 months ago

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


variable psu.jpg
PrasadA4 months ago

Please tell me,

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

npimpfellow4 months ago

Ok not sure what I'm doing wrong here...

Connected: grey wire to red wire - power good sensor

Connected: green wire to black wire - power on switch

Connected: blue wire to yellow wire and then to RED connector on volt meter- 24v

Connected: 1 black wire to BLACK connector on volt meter

Powered the power supply on and.... nothing... no fan... no reading.... nothing...

I need to generate 24v to test a circuit board I have that isn't working...

any advice? what am I doing wrong???

hi you have a couple of misplaced wires.
first the grey wire(5v) is going to led then to black (not red).
second the yellow(+12v) goes to red on meter.
white(-12v) goes to black on meter. this then creates 24v.
sorry white(-12v) should read blue(-12v) oops!
daniellong24 months ago

please, could you tell me the pinout of the lm338k in your diagram? I didn't found any datasheet that includes "line voltage, common and vreg" in this model

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
russ_hensel6 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
>> http://www.instructables.com/id/Encyclopedia-of-ATX-to-Bench-Power-Supply-Conversi/
Take a look at about 70 different approaches to this project.

dent2443 years ago
where did you learn about electronics so well? i am 16 and i have been trying to teach myself, i have done enough to understand the basics but i get lost when i try to go further, like i have some gaps in my learning? did you have a formal education?
TerryE1 dent2448 months ago

well I am glad to see young people interested in electronics. I started myself at 5 yrs old when my dad and I built a crystal set and I was hooked. that was 1954 and I have never looked back I got my MSEE from UCLA in 1977 and the life in electronics has been very good to me. all I can say is pick up some used text books and read, experiment, read everything you can. I also became a ham at age 12 that was 53 years ago and I too love electronics, it's never boring.

the best of luck

LuísF19 months ago

I tested the circuit on step 3 and when you rotate the pot from 0% to 60%, the output voltage barely changes (around 22v as expected), and from then it starts to drop dramatically until you reached 100%. However, if you change the value of R1 to 300 ohm, you get a much more smooth variation.

nitewing761 year ago

Depending on the LED, not using a resistor (even at 3.3V) will shorten the life of the LED and cause it to be too bright.

If you only group 2-4 wires, then you cannot safely utilize the full amperage output of the PSU. If you are unsure about anything, then get an "ATX Breakout Board".

A switch should not be an an option, but a requirement. But, the switch of a surge protector is a suitable substitute (as long as it is within your immediate reach).

Leave the fan inside the case and connected where it is or else connected to the yellow wire (not the red). If your case is so cramped that you feel the need to move the fan to the outside, then chances are whatever is cramping that space will affect/reduce the airflow over the heatsinks and not a good idea. Just shorten the wires and put the LM338 circuit on the outside. If the fan is too loud, then upgrade it. Just ensure you compare the CFM (cubic feet per minute) in the datasheet of the current fan with the one you plan to purchase. The datasheets (or manufactures website) will also tell you how loud the fans are.

Also, electrical tape is NOT a suitable substitute for heatshrink as the heat can cause it to come loose. Unless you hotglue the end of the electrical tape.

Have twice as much heatshrink, hot glue, and zip-ties than you anticipate on using. Home Depot sells a very nice heatshrink variety pack. I would also recommend using the smaller size hot glue gun to reduce the mess.

DIYTech2 years ago
Can i use all black Ground wires instead of -12 wire ? i don't need 22+ volts output just need 1v to 12v variable power with good amp.. And can i use LM-350 instead of LM317 or LM338K ?
Thank u

You can use the LM350, but you can only get about 3 amps. The LM338 will offer you 5 amps. But, you will not get the full 12V...some will be lost in the circuit.

If you do not need 22+V, then clip the -12V line and put a large blob of hot glue on the end of it and just use the black GND wires.

zacker2 years ago
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!

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.

emcelhannon2 years ago
I used a lm350 and a 10k pot. I only get a .2 volt range, (full power to -.2). Is this indicative of a bad pot, regulator, caps or component choices?

I would use an LM338, as it will offer a higher amperage output. Did you referece the datasheet when building the circuit?

wes13712 years ago
instead of the 10w 10Ohm resistor you can also rewire the PSU's standard fan in place of the resistor as a "load"

Not a good idea. Doing so will introduce more "electrical noise" into the system and cause unreliable output voltages. Stick with a power resistor.

Dave__1 year ago

Thank you for this great tutorial. It has worked for me...

But today, I used it for half an hour. Suddenly, it has stopped working. I've tried to switch it on and off, but nothing. Just a little buzzing when I used the switch. I've opened the case and looked for a short, but everything has seemed to be OK.

Just to know: I was using the -12V and +12V for an electrolysis.

What could be the problem?

Probably...just use the +12V and GND.

arush21 year ago

hi friend

Currently I'm working with some dc motors and i need negative voltage with high current, but when I connect the GND of the "power supply 1" to the 12V of the "power supply 2", to use the GND of "power supply 2" as -12V, i have a short circuit, and therefore the "power supply 2" is switched off. Is there some way to get -12V with high current?


Are you trying to daisy chain two PSU's? Or does your PSUs have a 12V1 and 12V2?

You cannot get a high current from the -12V rail. You're best option is to use an H-Bridge and a flyback diode. There's plenty on info on those here and on YouTube.

There is no need to isolate anything.

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