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:
Tape
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)

Optional:
Switch
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
Drill
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

Warnings:
BEFORE WE CONTINUE MAKE SURE YOUR POWER SUPPLY IS UNPLUGGED.

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

Comments

[emcelhannon]

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?

[jasshopper]

how can i get(or can you)24v at a minimum of 10A out of it?(the modifications)?

[zacker]

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...

[zacker]

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.

[zacker]

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!

[Rahib172]

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

[Shawn112270]

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.

Thanks,
E

[somebody12345]

how many amps does this put out???

[31mimo]

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?

[Gelfling6]

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.)

[Gelfling6]

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 ( http://img.photobucket.com/albums/v472/n1np/local/manual_fan_controller.jpg 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.

[Coolinst]

 My PSU has no green wire. What should I do?

[Gelfling6]

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..)

[brusho150]

I have the same problem how you overcome it. Please tell..

[prodlad]

its not atx standard. it could say on the side or search for wiring schemes on google

[Sean E]

hi, for the variable voltage method what voltage capacitors did you use?

[lone_viper]

Hi,
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...

[nhorn]

lone_viper,
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)

[jsorbo]

Just wanted to say I loved the grammar and spelling disclaimer. Can't wait to build one of these. Cheers!

[Will10YO]

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?

[M0HIZ]

Well done! I'm 14 too and also have a keen interest in electronics.

[dent244]

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?

[park47]

Hi Thank You...for careful and detailed instructions. Please keep on doing this , there're new-ones in e-town everyday. It's very useful...:)

[DJ-AS]

It's a nice idea, but the variable +22v just have 1A like -12A, or i'm wrong?

[pcmxa]

Nice tutorial. I am a total newbie at this and I was wondering if anybody had troubleshooting tips. I have wired one up (coolmax nw-650b) but it isn't working. green is connected directly to black I have connected the small orange wire (the 3.3v sense wire) to another orange wire. I have a 10ohm 10 watt resistor on one red wire and one black wire. The standby led lights when the unit is plugged in. But the mains power led connected to gray wire doesn't light when PSU is turned on. The fan doesn't come on either. The resistor doesn't heat up.. There is a very faint whine a few seconds after power-up and power down. The capacitors on the PSU are holding a charge. There is an 8 ohm resistance on the red and the orange wires. Between the yellow and the black I am getting 9.85 kohms. There is no brown wire. The blue wire is not connected to anything. I have two yellow and black striped slightly smaller gauge wires that at first I had connected to nothing and then connected to other yellow wires. No difference. Any thoughts? Thanks

[prodlad]

There probally is a short some where. Make sure the mains on led is the right way around. Try disconnecting everything and just short the green and black wire. Make sure any wires arnt touching any other wires except the green and black. If the capictors dont hold charge your PSU has lost all hope. Il add a troubleshooting section to the tutorial.

[HVahead]

i had the same issue and found that there is a switch used for switching between us voltage and foreign(now used as my power switch).there are two gray wires off of it try shorting one of them to the case while your supply is plugged in and "on"(if it applies to you) and if that doesn't work use the other one... i must do this every time the psu is unplugged if this doesnt work then get a new supply 'cause som'ns fried

[pcmxa]

Thanks, a troubleshooting section would be great. Where I am at now: The fuse on the circuit board is good, the capictors hold a charge (When I plug in the PSU the standby LED comes on, when I turn on the switch on the PSU casing after a few seconds the standby led goes out. with a slight whine from the capacitors.). I have a circuit (tested using Ohm meter) from the green wire (the one originally connected to the 20 pin device) and the black(ground)wire bundle. There is no circuit between the black wire coming off the PSU casing switch to the board or to the white (hot ) wire coming from the power plug in. I also get a circuit from the second heat dispenser to a ground wire but not from anything before it in the circuit. So my best guess is I somehow shorted something out, or I don't have a sense wire connected properly. I will disconnect everything and see if I redo step by step, but I think something is shot. Thank you for the info.

[HVahead]

i had the same issue and found that there is a switch used for switching between us voltage and foreign(now used as my power switch).there are two gray wires off of it try shorting one of them to the case while your supply is plugged in and "on"(if it applies to you) and if that doesn't work use the other one... i must do this every time the psu is unplugged if this doesnt work then get a new supply 'cause som'ns fried

[rich_moe]

without looking, I would say that the sense line (normally a small, brown wire) needs to be grounded, but through a resistor. something that is rated for 25W at about 2-5 Ohms shoud do it. other PSU's need a load on the +5VDC line in order to regulate properly and have a smooth output. i don't know which yours is, but if your sense line or the +5VDC line has no load on it, the PSU 'sees' this as a short to ground and shuts down. YMMV.

[prodlad]

Tips and troubleshooting added

[brusho150]

HI!
Brilliant instructable I am new in electronic engineering and wanted a lab power supply will you please tell me that how Can we connect a potetiometer(voltage regulator),rheostat, voltmeter and ammeter in atx power supply to calculate and vary voltage and current?
Thanks

[ecabrales]

Yeah also you are limited by the current rating on the -12V side so all you really get as a max ouput current rating is 800mV. Well, depending on the power supply you might end up using.

[BOOJAN]

how many amps can this power supply give at his outputs??(I mean the version with lm317)

[hannobisschoff]

LM317 can max out at 1.5 amps

[prodlad]

depends what voltage lines your using. if your adding the variable resistor i wouldnt draw any more than 3 but the 12v and the earth you could draw up to ten. I suggest fusing the -12v line or the earth. Dont fuse the positive as by the time the fuse blows the damage will be done.

[CrouchingFather]

Ok did some Internet research for, and yes you can run them in parallel and even in series if you isolate the grounds (more complicated), i have learned that older PS had V out adjustments, and if you were to parallel two +12v PS you would want to add diodes on both sides to prevent feed back. If your PS has a manual V adjustment you can compensate for the voltage drop across the diodes and still get +12v out. They do this to run radio equipment and to make chargers for RC cars. This link has some really good info on the subject of getting +15v/25A or higher.
http://www.antennex.com/preview/archive3/powers.htm

[CrouchingFather]

If you really needed a higher current, couldn't you use 2 power supplies? As long as they were grounded together you could combine the +12v's, and then vary that. I know it would be a little clumsy with two plugs, but you could get 12V/12A+/- or higher. Don't know much about the power supplies, but makes sense if you think about the PS as two batteries run in parallel. Don't take my word on this it was just a brain storming solution and has not been tested.

[Randel]

Hey..Just built one of these from a tower I was throwing out. And it works Great! I brought out the 3.3, 5, 12, and -12. And used the Gray wire for a power on LED, except used 220 ohm resistor. The 3.3 required the small orange and brown wire to be tied to it, for feedback. And didn't use the stand by wire...But like I said, It works Great! And I give you, prodlad, all the credit..Thanks for a fun and useful project! I've attached a picture..but still need to label output posts....

[prodlad]

thanks