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


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!

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

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!

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152 Discussions


6 months ago

My version of this build. +12V, + 5V and variable with an additional two usb power supply port.


3 years ago

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.

2 replies

Reply 10 months ago

The voltage drop is from your extra components. When building a circuit of any kind, any components you add to your circuit cause a drop of 0.6volts.


Reply 9 months ago

This was the most useful piece of equipment since i've had it done. Thanks mekert for your answer. This was my first project in electronics three years ago. I have learnt much since.


Tip 1 year ago

Problémom je, že vetva - 12V je iba na malý prúd cca 0,5 - 0,8 A. Takže zdroj do 24 V je iba na zaťaženie týmto malým prúdom.


1 year ago

As an EE who has worked extensively with these power supplies in the industry, I would like to add or reinforce some cautionary statements here. Please beware:

- use an external fire enclosure made of metal or some other material with a minimum fire rating of V-1 (ref. UL94).

- make sure all metal parts of the enclosure are adequately connected to Protective earth.

- Make sure enclosure is adequately ventillated for cooling. The mains transformer should not exceed 60C rise in temp.

- not all power supplies of this type share a common return between +/- 12vdc outputs, also they may not have the same current rating between the two outputs. Make sure you know before building the 0 to 22v output circuit. Fire/energy hazard. Recommend a fuse be used.

- some or all of secondary dc outputs maybe above energy hazard limit of 240VA. May cause probes, connections etc. to vaporize if shorted with explosion. Inherent current protection of power supply does not prevent this danger. May cause blindness. These outputs are not intended to be accessible.

2 replies

Reply 1 year ago

very informative !
guess there are many reasons people use expensive SMPS


Reply 1 year ago

yes but unfortunately cost has very little to do with it.


2 years ago

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?

1 reply

Reply 1 year ago

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.


2 years ago

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.


2 years ago

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

4 replies

Reply 2 years ago

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 Breakdownivang176

Reply 2 years ago

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.


Reply 2 years ago

Thank for enlighting me Ivang. Have a great day.


Reply 2 years ago

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).


2 years ago

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


2 years ago

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


2 years ago

-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.


2 years ago

avoid to connect the regulator to the psu metal body