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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en_rov7 days ago

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

zacker10 months 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 zacker1 month 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 zacker9 months 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 zacker10 months 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.
Scarrmakerz8 months ago
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!
wes13719 months ago
instead of the 10w 10Ohm resistor you can also rewire the PSU's standard fan in place of the resistor as a "load"
emcelhannon10 months 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?
jasshopper10 months ago
how can i get(or can you)24v at a minimum of 10A out of it?(the modifications)?
zacker10 months 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!
Rahib1721 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
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.)
Gelfling61 year 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 E1 year 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?
M0HIZ2 years ago
Well done! I'm 14 too and also have a keen interest in electronics.
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?
park472 years ago
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-AS2 years ago
It's a nice idea, but the variable +22v just have 1A like -12A, or i'm wrong?
pcmxa5 years ago
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 (author)  pcmxa5 years ago
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 prodlad2 years ago
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
pcmxa5 years ago
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 pcmxa2 years ago
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 pcmxa5 years ago
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 (author)  pcmxa5 years ago
Tips and troubleshooting added
brusho1502 years ago
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?
ecabrales2 years ago
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.
BOOJAN5 years ago
how many amps can this power supply give at his outputs??(I mean the version with lm317)
LM317 can max out at 1.5 amps
prodlad (author)  BOOJAN5 years ago
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.
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.
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.
Randel3 years ago
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....
power supply.jpg
prodlad (author)  Randel3 years ago
rfxcasey3 years ago
Something doesn't seem right here. Using a 120 ohm and 5k ohm the math doesn't work out right, I get 53.333 volts. Also you are extremely vague on when you state all you need is a 120 ohm and 5k ohm (which don't seem right anyways) as that really depends on whether you are using a LM317 or a LM338 as the LM317 is going to need something more like a 280 ohm R1 resistor and a 5k ohm R2 resistor to get 23.6V out. And no matter what you do your not going to be able to pull the max amps the supply is capable of on the 12 or 5 volt rails using a LM317 or LM338 as the 317 is only good for about 1.2 amps and the 338 for 5 amps. Still pretty good though for a super cheap PSU with good regulation. And as others have stated you are not going to be able to get 24 volts with considerable amperage as you will be limited by the -12v rails current capability. But you could use say the +12 volt and -5volt as the ground to get +17 volts at the 5 amp limit of the LM338. I'm super rusty on my electronics theory so pardon me if I have said something completely a miss. I do however have an AS degree is electronics I just to practice much.
hitachi83 years ago
i made one , but i put my Fuse on one of the AC wire ( input 110AC )... is it correct ?
what are the colors on the resistor?
senafe4 years ago
(removed by author or community request)
syfire senafe4 years ago
Sorry, but every PSU are isolated from the power line. And your argument "Thereis a reason to have this type of power supply inside a steel case insideanother steel computer case well isolated from the user" don't stand the road because if a steel case touvh  another steel case, is will be just one big steel case not two. Also look inside the PSU, I can see 6 of small transformer, so you ARE isolated.
muttyfutty4 years ago
How many amps output can you get? Nice I'ble Thanks,
Well, most power supply can support up to 400 watts of power so it depend of what voltage you are using. You can know the maximun amperage with the Ohm Law P = V * I  so I = P/V where P is power, V is voltage and I is amerage. You just have to check the maximun output of the PSU.
MaXoR4 years ago
I Love the hacked horse fence charger. I HATE that people are ripping apart old systems, because they think they are "Junk".... What a waste, when you get older... I hope someone chops you up, because you were just old tech, and taking up space.......
adicontakt4 years ago
some times ago i made somethings like that
if wanna wach
i own the same soldering iron
billy1574 years ago
I like it, I'm going to try it. :)
trammanaka4 years ago
Hi everyone! I love this post, i've been woundering of a way to avoid spending 100€ on a 24vdc++ voltage regulated supply source. So as soon as I could I went off bought most of the material required...and only after having my regulated circuit all ready to go with a lm338k indicated in this instructable did I go look at the current supply for each voltage output. And on my ATX for +12v I've got a max current of 8A (which is great) and for my -12v I have a max current of 1A (terrible) unfortunately because most -12v outputs on a PC don't need that much current they have limited the current to a smaller value. This means that when I connect any equipment to my -12v and +12v the max current the power supply will be able to give me is 1A. In conclusion: -the power supply will be limited to the lowest current value. -there's no need to waste more money on lm338k to get a max of 5A if it will only give you 1 amp. -Most equipments powered with 24v will need more current so they may not work correctly. If however there is a great brainiac out there who can figure out what components to switch to achieve higher current values on the -12v circuit, that would be great to here from, although I must recommend everyone else not even to think of it. !!the ATX has a complex, well built structure that when tampered with may cause serious accidents!! keep instructing! o/
prodlad (author)  trammanaka4 years ago
Well if you cant bear with the 1amp max just get the 12v line and put it directly onto the variable resistor bypassing all the other parts. Then you have 8amps of power which can be varied between 0 and 12v
sjecstudent4 years ago
has this been tried and tested? because the -12V terminal can only handle .5A....The 12 V terminal can handle 30A...So would not that damage or overoad the -12V terminal???Please test and let know....
sjecstudent4 years ago
has this been tried and tested? because the -12V terminal can only handle .5A....The 12 V terminal can handle 30A...So would not that damage or overoad the -12V terminal???Please test and let know....
phozfate5 years ago
you know all I did was put the wires together and put some rca cables (the red and white ones) female end<< and i works perfect. so is there something unsafe about not have all those regulators. resistors and all that stuff????
prodlad (author)  phozfate5 years ago
no, thats just if you were going to add the variable resistor.
phozfate5 years ago
here i my mod of the supply the board fit really well eave coments of your oppinoins of what could be changed
prodlad (author)  phozfate5 years ago
looks cool, good idea putting it into a nintendo game console case. Just make sure it has proper ventalation and a fan.
jackshimano5 years ago
using +12 and -12v no good (as shown above diagram)
-can only drag 1-2 amps max out of supply since -12v almost useless now
-used to be used in 286 days-obsolete now most pc's

but if +12v and ground (black) used ,and any lm-305 (5v regulator used)
instead of lm-317,with lots of heatsinking ,of lm-305 regulator can drag up
to 20 amps at poor regulation out of +12v supply
=ok for semiregulated 5v to 12v @10 amps or more
but since no feedback to circuit,regulation will be poor
=but not bad for next to no price
jackshimano5 years ago
both the dual shottkey diodes marked ->i<- _and h.o.t. transistor are usually large to -218 0r to-220 plastic construction (not insulated from bare based backplate ) so must be isolated from common metal heatsink usually aluminium -use the plastic insulator(s) and hardware as mounted in supply and good thermal grease for best results the m.o.v.s in better supplies are good to use in surge bars-up to 2 per atx/at supplies-cheap supplies may not have m.o.v.'s-often blue,sometimes red
jackshimano5 years ago
-inside all at and atx supplies is a very good dual shottky diode-ideal for low loss rectification for up to 24v dc supplies -up to,if heatsinked properly, 6-30 amps with lower rectification losses than regular diodes that drop 2-3 volts -so also ideal for isolation of solar cells and 3-10 v dc circuits like dual battery packs to prevent discharge since they rectify with only 0.3 to 0.4 v drop at high current-most are common cathode so eg-if +15 is applied to anode,+14 v is available at cathode-(if reg diode +15 would only give you ~+12v at cathode at any real current) so diode losses way less and .being shottky construction 100's of times faster than regular diodes for switching on and off for high efficiency 20khz uses -also fast switching (H.O.T.)transistor good in older crt monitors in the high voltage section
nilimili5 years ago
I was wondering if we can bypass the ac area and use old smps circuit to produce regulated DC voltage to charge battery for any solar projects? I mean using old smps as voltage regulator with solar panels!
Mikey735 years ago
AT or ATX SMPS supplies, at first, may seem to the uninitiated to be too good to be true - cheap, clean, powerful and also to offer a variety of voltages that can be easily tapped to use in virtually any home project, etc. However, most SMPS, especially the AT (obsolete) and ATX family are complicated designs having been designed and built by power-electronics engineers - not children. Be careful using these things as bench power supplies especially if you have "modified" ANY part of the circuit! These are not toys and adding the -12V to the +12V rails to obtain (a possibly non-ground referenced!) 24V is plain stupidity. The -12V rail is only capable of supplying a small amount of current whereas the +12V rail supplies tens of Amps. Be very careful taking advice from a 13 year-old when it comes to these types of power supplies (or any other for that matter) - they can be deadly. Sorry to come across as a stick in the mud - just a warning for those who may not know. Happy instructing...
prodlad (author)  Mikey735 years ago
yeah i suppose your right i am only 14, most power supplys are really advanced but hey it isnt that hard to make this all your doing is shorting 2 wires and just making it provide power for a variable voltage circut
I finally got around to making a power supply out of an old unit from a dell optiplex. I used a retro looking cool shell from an old electric fence box, mounted everything inside, and installed a secondary fan on the back side with the power switch. I also made the old electric fence terminal on top 12v for good measure. Thanks for the instructable !!
power supply.gif
prodlad (author)  World_Groove5 years ago
looks great! good idea using volt meter leads
wierd idiot5 years ago
Hmm... Very Nice. Just wanting to know can you change the fixed voltage lines ie the 12 and 24 volt ones?
prodlad (author)  wierd idiot5 years ago
i dont get what you mean. If you mean change the voltage they give out the answer is no but you could if u out it through a variable resistor or ordinary resistor if you could calculate the what ohm you need. Tip: Look up "Ohms Law"
Geosync prodlad5 years ago
I wouldn't recommend using a variable resistor. Also, IMHO, knowing Ohms Law alone won't address the construction issues involved in creating an adjustable output.

Probably the best way to provide an adjustable voltage is to attach a variable voltage regulator and associated parts to one of the fixed outputs.

Check out this link:
prodlad (author)  Geosync5 years ago
I based the variable resistor method on this page!
Geosync5 years ago
I should also say I marked this Instructable as a favorite. Nice job!

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