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>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>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>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>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>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>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>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>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>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>
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>Just a note to let you know I have added this instructable to the collection: <br>Encyclopedia of ATX to Bench Power Supply Conversion <br>&gt;&gt; <a href="http://www.instructables.com/id/Encyclopedia-of-ATX-to-Bench-Power-Supply-Conversi/" rel="nofollow">http://www.instructables.com/id/Encyclopedia-of-ATX-to-Bench-Power-Supply-Conversi/</a><br>Take a look at about 70 different approaches to this project.<br> </p>
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?
<p>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.</p><p>the best of luck </p>
<p>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. </p>
<p>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.</p><p>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 &quot;ATX Breakout Board&quot;.</p><p>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).</p><p>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.</p><p>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.</p><p>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.</p>
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 ? <br>Thank u
<p>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.</p><p>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.</p>
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>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>
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?
<p>I would use an LM338, as it will offer a higher amperage output. Did you referece the datasheet when building the circuit?</p>
instead of the 10w 10Ohm resistor you can also rewire the PSU's standard fan in place of the resistor as a &quot;load&quot;
<p>Not a good idea. Doing so will introduce more &quot;electrical noise&quot; into the system and cause unreliable output voltages. Stick with a power resistor.</p>
<p>Thank you for this great tutorial. It has worked for me...</p><p>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. </p><p>Just to know: I was using the -12V and +12V for an electrolysis.</p><p>What could be the problem?</p>
<p>Probably...just use the +12V and GND.</p>
<p>hi friend</p><p> Currently I'm working with some dc motors and i need negative voltage with high current, but when I connect the GND of the &quot;power supply 1&quot; to the 12V of the &quot;power supply 2&quot;, to use the GND of &quot;power supply 2&quot; as -12V, i have a short circuit, and therefore the &quot;power supply 2&quot; is switched off. Is there some way to get -12V with high current?</p><p>thanks</p>
<p>Are you trying to daisy chain two PSU's? Or does your PSUs have a 12V1 and 12V2? </p><p>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. </p><p>There is no need to isolate anything.</p>
<p>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.</p>
<p>I put mine in a Subwoofer case. Great instructions! I blogged about mine here:</p><p><a href="http://www.imustbebored.com/2014/06/bench-top-power-supply-mod.html" rel="nofollow">http://www.imustbebored.com/2014/06/bench-top-powe...</a></p>
<p>To reduce the speed the fan spins, why not hooking it to the 5v line instead of the 3.3v?</p>
I am a total newb and am trying to follow this but I have a couple questions... <br>-5v, -12v or +5v, +12v? what are those? and whats with the plus, minus? i thought 12v was 12 v? <br>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...
<p>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.</p><p>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 &quot;sea level&quot; 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.</p><p>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.</p><p>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.</p>
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. <br> <br>I was recently researching the subject, and the most to the point explanation I found was this - http://electronics.stackexchange.com/questions/10322/what-is-negative-voltage

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