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
<p>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.</p><p>My observations:</p><p>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. </p><p>Problems to clarify:</p><p>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:</p><p>1. I wired a 1K load on the output. </p><p>Result: around .5 volt dropped but still varying .1 to .3.</p><p>2. I measured with an analogue display multimeter. </p><p>Result: stable.</p><p>3. I wired a 24V 50W halogen bulb to it. (This was at hand). </p><p>Result: visually stable. Ampmeter showed steady 1.5A.</p><p>4. I took measurement on 5V (red) and GND (black) on the PSU itself. </p><p>Result: variation.</p><p>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.</p>
<p>I think the -12 +12 can cause some issues with sensible electronics. I connected a device that works perfectly fine with every other connection 3.3, 5 and 12V but not with this. Also it refused to show the right measurements with my multimeter. Just didn't work well at all with many things. Eventually it broke so the next time I will use a regular 24v transformer for this job. Takes a lot less space and gives off a lot of more amperage as well.</p>
<p>u mixed -12V &amp; +12V regardless current sourcing, -12V is usually so small sourcing current!</p>
<p>need help with this? can you help?</p>
I flipped my breaker!
<p>Hello All,<br><br>I have created this about 2 days ago and all went great. It was working fine and I was very happy with my project. But then I was testing some circuit with led diode and I have increased the voltage by mistake while the circuit was still connected. LED Diode of course blow up :) but also my power supply is somehow damaged. I will transfer the symptoms to you and hopefully someone will provide me with possible fix:</p><p>When I turn the switch ON it doesn't run (only the fan will try to run and at the same moment will go off. Please note that I do have 9W47ohm resistor as a fake load between GND and 5V (black and red wires). I know it was suggested to use 10ohms but I couldn't find one and this was perfectly fine. Just to exclude the resistor as a possible reason. Also please note that the issue started to happen after the incident)</p><p> If I connect some DC motor that operate with let's say 18V, and then turn ON the switch, it starts working but the minimum voltage (on the regulated outputs) is 12V and not 1.2 as before and it only goes up to about 14.6. Basically that is it, just I am not sure how to fix it. I did one mistake I didn't used PCB because I considered it to be fairly simple project, but now fixing will be real pain. I will now create PCB so if there is a need for future fixes it will be easier and faster.<br><br>I would be really thankful if someone suggest what could be damaged and how to check/test because although I do have some general knowledge, I am far from being expert in electronics. </p><p>Thank you in advance.</p>
Hi.codegen 350W. I have the resistor on 5v line and about pc. 12V yellow 12V wires connected. I only need 12V for my lipo charger. Psu gives out about 2amps now. On the side label ot states that 12V out is 15A.<br>any tips to get hihher current output ?.
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. <br>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!
<p>i would suggest you use maybe 3 wires, at most, of each color. any more than that and it is a waste. just cut out all the lead wires, and leave only 3 of each color and you'll be good to go.</p>
Thanks for the update!
<p>The more wires of the same color you tie together...only increases the amount of amps you can pull without melting any wires.</p><p>10ohm 10W should be fine for most applications.</p>
<p>Thank you for the instructable, </p><p>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) . </p><p>It works well, I get variable voltage between 1,1 v - 23 v</p><p>Regards</p>
<p>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 </p>
<p>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.</p>
<p>and use a pot with a high enough wattage rating so as not to burn the windings</p>
<p>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.</p>
<p>looks okay to me. the -12v isn't going to ground, it is floating, and acting as ground only for the purpose of his schematic. no problem</p>
<p>Are you referring to a green line on the bottom? :)</p>
<p>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 ???</p>
<p>no, you will not have 90A.</p><p>if you look closely at the circuit board, you'll notice that all the 12v lines are already connected together.</p><p>have fun.</p><p>aidan</p>
Each 12v line is usually derived from the same transformer, but I'm unaware of the internal circuitry beyond that. I don't recommend tying them together.
<p>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. </p><p>I made no short circuit on the project. Didn't even touched the breadboard for nearly 10 mins. </p><p>I checked later on, the fuse soldered on the main board (5A L240V) has blown.</p><p>Can you please give me any idea what caused this ? </p>
<p>I think I know why and think everyone who follow this guide should too! If you read +12V on your PSU, it says about 14A, atleast on mine. But if you on the other hand read on the -12V, not the GND, then the maximum current is 1A. That means that you can pull lots of amps out of the 3.3V, 5V and 12V. But on the variable output you can never pull more than 1A from the PSU, atleast not for any extended period of time. I am puttin a 1,25A fuse onto my circuit as I write this.</p>
Anything could have. Try replacing the fuse with an equivalent type.
Hey dude please reply fast..<br>I am using 10k pot instead of 5k<br>Lm317t for lm338k<br>And 1N4007 instead of 1N4002<br>Suggest me the changes in circuit and also let me know why my pot gets red when it is in lowest positin
<p>I use a LM317T as well and a 1N4007. I used a 4,5k potentiometer and it got hot and started smoking at lowest too. Then I tried a bigger one and then I could only switch between 11 and 12 volts. Now the old one doesn't work either and I don't seem to be able to regulate at all. If I disconnect the potentiometer (connected it through a terminal block) and it sends out 12V. Is it supposed to do that?<br><br>Did you figure out your problem? </p>
Sorry bro..i didnt find any alternatives and nor i had continued on project but i will be working on it soon. Till then u can google and sort out, if u could get the solution plz tell me @ unmeshsawant55@gmail.com. I will also reply you as soon as possible. Till then good luck!!!!!???
<p>If I couldn't figure it out, I got the advice to try this instead: http://www.eleccircuit.com/wp-content/uploads/2012/09/Circuit-of-my-first-variable-dc-power-supply-1-2v-to-30v-1a-by-lm317.jpg Might be of interest to you as well.</p>
Thanks bro....me also trying to find any issue in circuit..
<p>I am trying to make this out of 100% salvaged material from my garbage room. The only thing lacking right now is the diode and a potentiometer. The latter I know I will find in a couple of days but for the 1N4001 or 1N4002 it doesn't look as promising. I've found loads and loads of 1N4007 though which seems a lot more common in new electronics. From my very much layman based research I've found out it would be just as good for the purpose, just that it's made for higher voltage. My question is, can I use a 1N4007 instead?</p>
<p>Please tell me,</p><p>Does power supply leak current from its case when it turn on.</p>
<p>how you connect the screen to show the voltage also how i can read the AMp</p>
<p>I made it, but made some modifications. Thank you</p>
<p>I've tried to make this but I've found a couple of problems:</p><p>What should I do with the gray wire? It's the only wire left unconnected and the supply only stay in standby mode.</p><p>And I've teste the bornes for short circuits and apparently all of them are connected (5V, 12V, GND...), what have I done wrong?</p><p>Thanks in advance.</p>
<p>Connect the gray wire to gnd (on your own risk)</p>
&nbsp;My PSU has no green wire. What should I do?<br />
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..)
<p>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 : </p><p><a href="http://www.pcguide.com/ref/power/sup/partsDrive-c.html" rel="nofollow">http://www.pcguide.com/ref/power/sup/partsDrive-c....</a></p><p>Cheers , take care , and have a good day !!</p>
I have the same problem how you overcome it. Please tell..
its not atx standard. it could say on the side or search for wiring schemes on google<br />
<p>Hi thanks for the instructable</p><p> 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.</p><p>Regards</p>
<p>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.</p>
<p>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 &amp; 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.</p>
<p>please help me<br>i have 200w SMPS Model No GPS-200BB C A00<br>Power Good and Stand by voltage are coming properly, but fan not rotating continusly <br>its getting stop with in 2 second<br>no other voltages are coming </p>
<p>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. </p>
<p>Ok not sure what I'm doing wrong here...</p><p>Connected: grey wire to red wire - power good sensor</p><p>Connected: green wire to black wire - power on switch</p><p>Connected: blue wire to yellow wire and then to RED connector on volt meter- 24v</p><p>Connected: 1 black wire to BLACK connector on volt meter</p><p>Powered the power supply on and.... nothing... no fan... no reading.... nothing...</p><p>I need to generate 24v to test a circuit board I have that isn't working...</p><p>any advice? what am I doing wrong???</p>
hi you have a couple of misplaced wires.<br>first the grey wire(5v) is going to led then to black (not red).<br>second the yellow(+12v) goes to red on meter. <br> white(-12v) goes to black on meter. this then creates 24v.<br>
sorry white(-12v) should read blue(-12v) oops!
<p>please, could you tell me the pinout of the lm338k in your diagram? I didn't found any datasheet that includes &quot;line voltage, common and vreg&quot; in this model</p>

About This Instructable




More by prodlad:Things to do with a old computerConvert A Computer Power supply to a Variable Bench Top Lab Power Supply
Add instructable to: