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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dent2442 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 dent24413 days 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

LuísF11 month 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.

nitewing763 months 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.

DIYTech1 year 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.

zacker1 year 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!
nitewing76 zacker3 months ago

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.

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?

wes13711 year 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__7 months 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?

nitewing76 Dave__3 months ago

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

arush25 months 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?


nitewing76 arush23 months ago

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.

pffm12 arush24 months ago

You'll probably need to isolate the ground of your wall plug from the power supplies plugs. That way you won't get a shortcircuit through your house wiring.

anubisankh made it!4 months ago

I put mine in a Subwoofer case. Great instructions! I blogged about mine here:

en_rov7 months ago

To reduce the speed the fan spins, why not hooking it to the 5v line instead of the 3.3v?

zacker1 year ago
I am a total newb and am trying to follow this but I have a couple questions...
-5v, -12v or +5v, +12v? what are those? and whats with the plus, minus? i thought 12v was 12 v?
How would these read on a tester? I want to use this mainly to power an Electrolyte Rust Removal tank (13 gallon tub) and it really only needs to be like, 12v dc with about 2 amps... But for Amps, the more I can safely get, the better I guess. With 2A I'd most likly be leaving it running for up to 8 hrs at a clip. Thoughts? I have a Delll, 305W PSU its like a P/N: N305N-00 or something like that. please see my next comment...
agr00m zacker9 months ago

Voltage is a potential difference between two points, where one is typically 'ground'. A negative voltage means that it has a negative voltage potential when referenced to ground. Voltage then flows from a point with a higher potential to one with a lower one.

Think of it kind of like elevations with ground being sea level. Anything higher than sea level has a + elevation, and anything below sea level is a - elevation. But ground can also be just any arbitrary reference point. For example, our above "sea level" could be changed to say the elevation of the first floor in your house. Anything above it has a + elevation and below a - elevation. Yet in reference to sea level everything has a + elevation.

The reason there are +/- voltages in electrical applications is because you sometimes need voltage differentials that are lower than ground. Audio is a common application.

As all that applies to this project though, a basic reason for having +/- voltages is to allow for a wider range of voltage outputs than just the 3.3, 5 and 12. For example if connected a device between -3.3v and +5v, you would have 8.3v. Between +12v and +5v you get +7v. Between -3.3v and -12v you get 8.7v. That is if you connect + to the higher voltage output and - to the lesser output.

Jeebiss zacker1 year ago
If you follow common electrical convention, you can have a positive AND a negative voltage reading. I dont fully understand the specifics, but essentially everything is relative to your ground source.

I was recently researching the subject, and the most to the point explanation I found was this -
zacker zacker1 year ago
THis is the Pin Out directly off the Dell Site for my PSU....
Pin Number, Voltage / Name, Color of Wire.

1 +3.3 VDC* Orange
2 +3.3 VDC* Orange
3 COM Black
4 +5VDC Red
5 COM Black
6 +5 VDC Red
7 COM Black
8 P_OK Gray
9 +5 VFP Purple (Whats VFP?)
10 +12 VBDC White (Whats VBDC?)
11 +12 VBDC White
12 +3.3 VDC* Orange
13 +3.3 VDC*/SE* Orange
14 ? 12 VDC Blue
15 COM Black
16 PS-ON Green
17 COM Black
18 COM Black
19 COM Black
20 NA NA
21 +5 VDC Red
22 +5 VDC Red
23 +5 VDC Red
24 COM Black
*The orange +3.3 VDC output wires must be 16 AWG. The +3.3 VDC terminals are high current type (9 A current rating/Molex-HCS type).
*The +3.3VDC/SE is a brown sense wire for +3.3VDC and is optional.
Hey, this instructable is great! i have a question, i have a single pink wire and a single brown wire on my ATX PSU. what are they for?
I searched another how to and I found out that they are 2 sensing wires. Brown goes to orange +3.3v and the pink goes to red +5v. hope that helps others!
jasshopper1 year ago
how can i get(or can you)24v at a minimum of 10A out of it?(the modifications)?
I am trying to convert a new 1200W PSU and have found that in addition to the expected wire colors there are Yellow & Green wires as well. I looked around online and found that they are "Protective Ground" wires.

What do I do with them?

Any help would be great.

how many amps does this put out???
You can not get more Amperes than the -12Volts rail.( probably max. 2 Amperes) LM317 only gives out 1,5Amperes. you can add a 2N3055 for more Amperes.The variable with LM317 is not suitable for a CB radio. Am I wrong?
Depends on the supply.. Most have the amperage listed on the side label, BUT.... here's the catch.. To achieve that maximum amperage, will require ALL wires for that voltage, not just 1 or 2.. I.E. for a 250W, you'll need all 10 red wires for the +5V, with a 10W load resistor, to get 240W (48-Amp MAX!).. and that supply will be maxed out if you pull that total, and shut-down from overload.

I've made a few supplies, +/-12V, +5V, +3.3V (No -5V.) out of the 200W supplies from a bunch of old Optiplex desktops.. usually use a 30-Ohm, 5W resistor across the +5. Been able to run gadget projects, Amateur Radio equipment, (with little to NO noise, surprisingly.) external hard drives, even charge Gel-Cell batteries from them.

It also depends on the condition of the supply... Brand New? Pulled from a working computer? or pulled from a failed computer? If the last, suspect even the supply may be bad, and NOT a good candidate for conversion. (See remark below I made to Coolinst, about the one that went up in smoke.)
Gelfling62 years ago
I've always used the lower-right schematic as my basis for all of the supplies I've converted. Never used the Standby +5 voltage, though, though I imagine it would be usable for an electronically switched (On/Off) supply that you would want to be able to power-down to idle, but bring-up with an external circuit. (shades of a power-up on LAN)..

There are some circuits for ( for example) for manual control, but thermal control would require making a circuit that uses a thermal varister in place of the potentiometer... most of the time, I'd simply leave the noisy fan be, as it's doing its job, and unless noise is a problem (needing quiet for recording, etc.) it's not worth baking the supply, but slowing the fan.

Most supplies have a built-in speed control now, and the last few supplies I've converted, I replaced the straight-wired fan with a thermal circuit & fan (if it fits) from another supply I've scrapped.
Coolinst4 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..)
I have the same problem how you overcome it. Please tell..
prodlad (author)  Coolinst4 years ago
its not atx standard. it could say on the side or search for wiring schemes on google
Sean E2 years ago
hi, for the variable voltage method what voltage capacitors did you use?
lone_viper2 years ago
I am putting one of these together and have come across something that I might be getting wrong....

In mocking up the LM317 circuit in breadboard, I have got the thing metering -11v to +11v??? I am using one of the black ground wires on the black side of the volt meter, but cant figure out why I am not just getting positive voltages??? Could the diode be faulty? Or is this in fact correct?

Any help appreciated...
The way you are measuring, it is correct.

reg set for max: meas +22var(actual +23)-->R1-->R2--> -12-->psu gnd= +11v

reg set for min: meas +22var(actual +1)-->R1-->R2--> -12-->psu gnd= -11v

0V has to become new gnd or (0V adj)
jsorbo2 years ago
Just wanted to say I loved the grammar and spelling disclaimer. Can't wait to build one of these. Cheers!
Will10YO2 years ago
This is sweeeeet! I'm 12yrs old and I built this- well, almost. I want to put in a resettable fuse / circuit breaker but I don't know what I should limit the current to. I want at least 2 amps but the only way to test it's potential could ruin it. What do you think I should limit it to?
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