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In my sophomore year of college at the University of Minnesota, I started into my main electronics classes, and needed a good power supply for working on lab projects at home. My roommate Adam told me about somebody online who had converted an ATX computer power supply into a lab bench power supply, so I decided to do the same thing. You can also check out this link for a very similar guide by their user Abizarl. I have also documented this project on my website at http://www.mbeckler.org/powersupply/ if you are interested.

Warning! There are several large capacitors in ATX power supplies, that will store a dangerous charge for a long time. Please let your power supply discharge, completely unplugged from the wall outlet, for a few days before opening it up. You can probably be seriously hurt, so please be very careful. I am not a lawyer, but I hereby release myself from as much liability as I can, for any sort of injury you sustain, or any trouble you get into.

Step 1: Background

First, a bit of background on a typical ATX power supply:

Computer power supplies are Switch Mode Power Supplies (SMPS), which use high-frequency switching circuit elements to provide a high-quality output voltage, with good energy efficiency. One side effect of this technology is the minimum load requirement that each power supply has. In order to function properly, the power supply needs at least a very small electrical load connected to it. In other words, ATX power supplies will only work if you have something connected to it. We will be using a power resistor to provide this minimum load.

Also, modern power supplies do not simply have an OFF/ON switch, they have what is known as a "soft" power switch. This normally makes no difference to the user, as the computer behaves the same, but when you shutdown your computer, the motherboard can turn off the power supply when it has finished shutting down. This requires us to add our own power switch to the power supply chassis.

To protect our circuit from accidental (and careless!) short circuits, we will install some fuse-holders and fuses, which will disconnect the circuit supply lines if too much current flows. The size of the fuses are up to you, but a 1 amp fuse will work just fine for most circuits. You really should put fuses on all supply lines.

Update: While the diagrams show fuses on all voltage rails and no fuse on the ground line, when I actually built my power supply, I was young and foolish and only put a fuse on the ground wire. It's much safer and a better idea to put fuses on all signal lines and not the ground line. Thanks to many emails and messages on Instructables about this oversight.
<p>Why the extra fuses? I am sure I read that the reason converting psu's into desktop lab psu's is because of the built in protection they already have. I Am trying to convert a modern 535 watt into a desktop psu. I don't know how many wires I should join together being that my psu is much larger then all the examples I can find. I think if I joined them all together it would be far too much power possibly far to much wattage but my memory on the subject is very cloudly. Can anyone tell me how many 12 v wires I can safely join together?</p>
<p>Also, at least for me, the reason people convert PSUs into a bench PSU is because there are many old and cheap (usually free) PSUs that aren't powerful enough for a new computer, but still work perfectly fine and provide more than enough power for hobbyist needs.</p>
Hello, sorry if this results in double-posting, but my first reply seems to have disappeared. The extra fuses are added to protect your load from over-current situations, and are not intended to protect the power supply itself (the built-in protection is to protect the power supply, not to protect your circuit).<br><br>A power supply like this is known as a &quot;voltage supply&quot;. It creates the rated voltage (5v or 12v for example) and tries to provide as much current as needed by the load circuits. It can provide up to the rated load current (listed on the label on the side of the PSU). There's no danger of providing &quot;too much&quot; current, as the load will only draw as much current as it needs. If you join too-few wires, then you draw a lot of current, the individual wires might start to heat up, so I would connect as many wires as you can. It's sort of like how in your city's water supply, they provide water at a certain pressure, no matter if you use a little bit of water, or use a ton of water.<br><br>One thing that complicates the answer is that modern power supplies seem to have split their 12v rails into two or more &quot;domains&quot;. Presumably this is because it's easier and cheaper to include several smaller 12v regulators than to include one giant 12v regulator. For example, take a look at the attached image, showing this power supply has three 12v regulators each capable of providing 18 amps of current. I'm not sure if the PSU would like having all three 12v domains connected together (they might fight each other). I'm not sure how to tell which domain a certain yellow wire belongs to, but you might be able to look at the circuit board in the PSU, and maybe the yellow wires are grouped together into two or three groups? Of so, I would feel somewhat confident in combining <br><br>Hope that helps, let me know if you still have questions.
<p>Just an idea, but if you need a load to draw a small current on each voltage rail, could you not use an LED and an appropriate resistor to drop the voltage instead of these power resistors? It would act as a status light to prove each voltage rail is powered up. </p><p>Or would it not work due to the current being too low?</p>
A &quot;power good&quot; status LED for each voltage rail is an excellent idea in and of itself, and they might even draw enough current. The whole &quot;small minimum load&quot; stuff is really black magic, nobody really knows anything except &quot;this worked for my power supply&quot;, and really takes a bit of experimentation with your specific power supply in your specific use case. Some people use their PSU as a battery charger, so there is always a large load and no power resistor is needed. Most LEDs only draw a max of a few dozen milliamps, so that might not be be enough current draw to keep the mystery &quot;minimum load&quot; circuits happy :-) Good luck!
<p>Is a 10 Ohm 10W Resistor really needed ? </p><p>And what fuse have u used and why ?</p>
Some power supplies won't run if there is nothing connected, and need a minimum small load to keep it from doing an auto-shutdown. It really depends on your specific power supply, so you must do some experiments to determine what your power supply needs. If you will always have some load connected, then you don't need the power resistor.<br><br>This original design used a single fuse on the ground wire, but that is a really bad idea. The better way is to put no fuse on the ground wire, and put a fuse on each non-ground wire (like 5v and 12v). I would use a fuse that can handle just a bit more current than you expect to use in your project. This way if something goes wrong and your project starts to draw much more current, the fuse will blow and prevent a fire or other dangerous situation.
<p>maybe a stupid question, but wouldn't it be sufficient to use the cooler fan for the required minimum load? </p><p>Nice project though!</p>
Thanks gungajin, that's a good suggestion. There are a million different power supplies out there, and many require a minimum load separate from the case fan, and many don't require an additional load. Your mileage may vary, and these things require a little experimentation to see what it needs. Good luck!
<p>this guide has been really helpful to me as I work on my own PSU. I found this handy wire color and purpose chart, hope it helps. Heres the link to it http://www.instructables.com/id/A-Makers-Guide-to-ATX-Power-Supplies/?ALLSTEPS</p><p></p>
<p>I love this project. I have everything connected but I can only get it running if I turn on and off the switch quickly many times. It is the only way I can get it running. Any Idea why this happens?</p>
<p>Hook the load up and then turn it on. Mine stays on just fine with a load on it, but shuts off after a few minutes if I remove it. </p>
<p>Great idea. I'm building mine now. Going to one thing a bit different though since I also work on computers. I'm leaving the computer wires on the supply intact so I can use it on a motherboard or use the terminals installed on it for running my ham radios, testing things, etc. Be even more useful that way. The one I'm working with puts out 12 volts at about 17 amps. That such power an Icom IC 735 ham rig just great!</p>
<p>Thank u for this =)</p><p>One question...</p><p>Can I add a regulator on the 12v? and connect a voltmeter? So I can change between 1-12v? Please let me know :) And maybe give me a link to a regulator =)</p>
You can definitely use a regulator to convert the 12v down to a lower voltage.<br><br>The LM317 is a popular adjustable voltage regulator, but the max output voltage is about 2 volts less than the input voltage, so you'd be limited to 0-10 volts. There maybe other types of regulators that have a smaller &quot;dropout&quot; voltage. This is a &quot;linear&quot; regulator, meaning it burns excess power as heat, equal to the current * (Vin - Vout), which can waste a lot of power. Maybe you do or do not care about that, but you might need a heatsink.<br><br>There are also switch-mode buck converters that don't waste as much power, since they are more efficient.<br><br>Good luck!
<p>I have to turn it on and of 4 times and quikly to make it run, but hey, it is working!</p>
<p>I connected PSU's green wire with black (ground) wire, but not with the gray wire. Should that make any problems or is it OK?</p>
The gray wire is only used to indicate if the power supply is turned on, so you don't need to do anything to it if you don't want to have an LED indicate when the power supply is turned on.
<p>Sorry, I forgot to write that I have already put LED, resistor and switch between green and black wire. When I pull the switch, the LED indicates that PSU is turned on and the fan in PSU starts spinning. Aparently, the LED indicates that PSU is turned on altough it's not connected with the gray wire.</p><p>BTW This instructable is great! :)</p>
<p>so if i use one wire from each source I can make 11 outlets on a box?? I know I couldn't USE all 11 at once, but I could use three or four as long as it didn't exceed psu amp capacity, yes??</p>
<p>Wire everything in parallel and you could have a thousand outputs on a box, provided that the total current load doesn't exceed the psu specs. You don't necessarily need to use the wires the box comes with. For instance, the molex connectors for the hard drives, floppy, optical drives etc. are daisy chained together in parallel, with more than one output on a single 5v or 12v wire. You can always extend that.<br><br>If you plan on using higher amperage, e.g. to power a car amplifier, it's better to wire all the outputs of same voltage into a single output. This mimics the effect of a thicker gauge wire, and eases the load on the psu.</p>
<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.</p><p>As far as I can tell you are the first to start this madness on instructables, congrats.<br> </p>
Hi Russ, thanks for the note. That was a great idea to collect all the different approaches to this project!
<p>nice work...</p><p>I found that my power supply did not follow the standard colour coding.</p><p>i was able to look at the original ATC motoerboard connectors and the table at <a href="http://pinouts.ru/Power/atxpower_pinout.shtml" rel="nofollow">http://pinouts.ru/Power/atxpower_pinout.shtml</a> and found that the purple, blue and white were non-standard on mine. double check with a meter before trusting the colours.</p>
I need 13.6v @0.6amp from 12v rails and +5v @1.5 amp from a pc smps for my project. Please help me here. Do I need to add few turns of winding to 12v coil with additional rectifier with 12v as it is (for sensing)? Or by connrcting some dummy load to 5v, can increase voltage from 12v? So that I can get desired voltage?
5v@1.5A should be no problem for any ATX PSU. Getting 13.6 volts will be more tricky, and I would not personally mess around with anything inside the power supply. I would suggest buying a 16-20 volt power supply (you may even have one already as a wall-wart AC-DC adapter) and then add an output regulator to get the 13.6v you need. Since it's only 0.6 amps, even an inefficient linear regulator should work, such as the LM317. Good luck!
I have 20 pin ATX supply and I need 12 volts and at least 50 amperes from it for my experiment. So can a connect all yellow wires of my supply(12volts, 16amp each) to make a single 12 volt, 50 amp power supply? I have a total of 5 yellow wires in my supply...
I don't think the 16A rating is for each wire...Even though you likely have many red 5v wires, the current rating on the label isn't for each wire, but for 5v overall. Most newer ATX power supplies have two or more separate 12v supplies, which complicates things to the point where I am not sure if you can just connect all the wires together like that and expect it to work. If you need a single voltage, high-current like that, I would suggest searching for a dedicated 12v power supply, instead of trying to rig up something this way. Good luck.
Still experimenting if it works correctly or not then i'll add switches and variable dc converters.<br>The problem is after i plug it it wont start working for another 10/15 seconds and when it starts the voltages are rather high 12-14;5-6;3-3,8<br>Is it because of the missing ressistor?
I have heard that if your voltages are rather high, that that is due to having zero or very little load on those voltages, and that if you add a proper load that they will drop down to the correct voltages right away. A switching power supply is sort of like trying to keep an air-filled balloon at a certain height above the ground by bumping it upwards periodically. If there is very little gravity (very little load) the balloon (voltage) doesn't go down very quickly, and even the smallest bump upwards will tend to overshoot the desired height (output voltage). Also, I think the specification is actually for 3,3 volts, not 3,0 volts, but still 3,8 volts is too high.<br><br>I'm not sure what is causing the start-up delay. You have tied the green wire (PS On) to a black wire (GND)? You might try adding a resistor load to the 5v or 12v wires.
<p>I'm having a problem with one. I have the brown connected to the orange as it was in the plug, and the resistor on the red and grnd as it should be. But when I try to fire it up it will only spin the fan for a second and then quit. It won't stay running. Any suggestions? Much appreciated.</p>
What do you mean by &quot;the brown connected to the orange&quot;?<br><br>Perhaps try moving the power resistor to a different voltage rail, such as 12v or 3.3v? Maybe your particular power supply needs the minimum load on a different voltage line? Or it needs a minimum load on two or more of the voltage lines? I've never seen any specifications for this sort of thing, so you might need to experiment with your power supply. Let us know if you figure out what's going on.
<p>There was a brown wire piggybacked onto an orange in the 20 pin plug so I after removing the plug I tied the brown wire to an orange. Also, I've tried the resistor on the 3.3v, the 12v, and the 5 volt. Also tried two resistors. One on 3.3v and one on the 5v at the same time. I've set this one aside for now pending a solution. If I can figure it out I'll post my results.</p><p>Thanx!</p>
Did you use 3 fuses as you have shown in the schematic because I can only see 1 fuse on pictures
Yes, sorry for the confusion. When I originally created this project and instructable (and took the photos) I only had one fuse, and it was on the ground wire. After that, I learned more about best practices and a lot of people suggested that it's a bad idea to fuse the ground wire, as you always want that one connected. Instead, it's better to put individual fuses on each power rail. I updated the schematic to show a fuse on each power rail, but I haven't rebuilt my power supply.
<p>Thank you for your reply. I really needed that to finish building this power supply, as it will be only power supply as the &quot;lap power supplies&quot; are much more expensive. :</p>
Will a 10W 100Ohm Resistor work as the power resistor ? <br>When I use the 10W100Ohm Resistor and connect the green line with the black line, Some noise comes for a second and then stops .. Sometimes the noise comes for 5 seconds and then the Fan starts but then immediately stops ! .. Is there something wrong with the Power Supply I am using or is it the resistor ?
Hi! Somewhat counter-intuitively, the larger the resistance you have, the less current it was draw from the power supply. A 10 ohm resistor will draw 500 milliamps (mA), a 100 ohm resistor will draw 50 mA, and a 1000 ohm resistor will draw 5 mA. Presumably there's some minimum current draw that your power supply requires on the 5v line (and maybe the 3.3v line too?) and if you don't draw enough current (resistor is too large) then it won't stay on. <br> <br>It really sounds like the behavior you describe is due to insufficient load current, or perhaps some other problem. I would try a power resistor with a smaller resistance value to try and draw more current. You could try connecting two of your 10W 100 ohm resistors in parallel, producing an equivalent resistor of 20W 50 ohms that will draw 100 mA, and dissipate P = V * I = 5 * 0.1 = 0.5 watts shared across both power resistors. Good luck!
<p>I always wondered why all tutorials use or suggest to use specifically a 10 Watt 10 Ohm resistor for dummy load. In high school I was taught: P=V*I =&gt; P=V*V/R<br>In this case: P= 5*5/10 = 2.5 Watts So I always wondered, why use a 10 Watt resistor instead of 5 Watt. Even considering possible spikes the resistor wouldn't be damaged. A constant load that produces 5 Watts of heat or more would be necessary to damage it.</p>
Yep, only 2.5W burned at 5v. I would guess that 10W 10Ohm resistors are more standard and easier to find, if you can find a power resistor at all. They are not used for many applications and can be difficult to find.
<p>[head-bulb.jpg]</p><p>What about use a incandescent bulb instead? Maybe hader to find soon becouse of led lamps tough.</p>
<p>Car light bulbs (back up/brake/dome, etc) are incandescent, durable, and run at ~12v. They also put off significantly less heat (less power waste) than that fat resistor.</p>
<p>As far as I know, the whole point of the constant minimum load is to always draw a small amount of current from the PSU to keep it from turning itself off. If the PSU requires at least, say, 500 mA current draw at 5v to keep it running, then it doesn't really matter whether you dump that power into a resistor as heat, or an incandescent bulb as a little bit of light and the rest as heat.</p><p>The real question is to figure out how much minimum current draw is needed on one or more of the power supply rails. I've never seen any definitive &quot;minimum load&quot; data, and for a hobbyist level project like this, the power resistor(s) aspect is much more of a &quot;try things until it works reliably&quot; process than anything else :-) Maybe there are some power supply datasheets we could check? I'm not really sure, but that would be useful.</p>
<p>Extraneous heat matters to the components inside of an enclosure. I realize we can use cooling fans to expel it, etc., which is using more energy to expel wasted energy (and another noisy fan harshing my zen-like bench experience). Using an external incandescent &quot;Power On&quot; indicator as a load seems appealing to both heat management as well as my tree-hugging sensibilities. ;) <br><br>I agree that &quot;minimum current&quot; data would be useful.</p>

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