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A DC power supply can be hard to find and expensive. With features that are more or less hit or miss for what you need.

In this Instructable, I will show you how to convert a computer power supply into a regular DC power supply with 12, 5 and 3.3 volt outputs. For about $10!

Why use a computer (ATX) power supply? Well, they're available everywhere, and they can output tremendous amounts of power in a small form factor. They have overload protection built right in, and even a 500W model can be reasonably priced with high efficiency. The voltage rails are incredibly stable. Giving nice, clean DC current even at high loads.

Plus, it's likely that many of you simply have an extra one lying around doing nothing. Might as well get the most value for your investment.

Step 1: Getting Started

The first order of business is that of safety. While I'm reasonably sure that there isn't enough residual energy to stop your heart, those capacitors can still bite, and that can cause significant pain and maybe even burns. So be paranoid when getting close to the internal circuitry. It would probably be a good idea to put on some insulating gloves. Also (obviously) make sure the thing is unplugged. You are responsible for your own safety!

Here are the tools/parts needed:

Drill
Needle-nose pliers
Soldering iron
3 x "Banana Jack" Insulated Binding Post sets
1 x bag of "#6" Ring Tongue Terminals (16-14 gauge)
Rubber feet
Small bit of heat shrink.
Screwdriver
Wire strippers

Ok, let's get to voiding some warranties!

Step 2: Opening Up

Open the PSU and make an assessment of the space you have to work with. Make sure that there won't be any clearance issues for the binding posts or wires.

Once you have decided how your PSU will be configured, mark with pencil where you want to drill the holes later on. This will help you in cutting the wires to the appropriate length.

Step 3: Wires, Wires Everywhere

You will be met with the daunting task of sorting through a hundred wires of different colors. The only colors we care about are Black, Red, Orange, Yellow and Green. Any other colors are superfluous and you can cut them at the circuit board.

The green wire is what tells the power supply to turn on from stand-by mode, we want to just solder it to a ground (black) wire. Put some heat shrink on this so it won't short out on anything else. This will tell the PSU to be constantly on without a computer.

Cut all of the other wires down to about a foot, and remove any zip-ties or cable organizers. You should have a forest of wires with no connectors.

The colors represent:

YELLOW = 12 Volts
RED = 5 Volts
ORANGE = 3.3 Volts
BLACK = Common Ground.

Now, theoretically, you could be done. Just hook the wires to 4 large alligator clips (one for each color set) or some other terminals. This might be handy if you're just going to be powering one thing, such as a ham radio, electric motor or lights.

Step 4: Grouping Wires

Group the 4 wire colors together and cut them to length to where you marked where the posts would go. Use the wire strippers to take off the insulation and stick about 3-4 wires into one tongue terminal. Then crimp them. The exact number of wires per voltage rail depends on the wattage of the PSU. Mine was a 400W and there are about 9 wires per rail. You need all these wires so that you can get all of the current rated for that rail.

Step 5: Holes

Now we come to the drilling. With most power supply units, you won't be able to completely remove the circuit board from the chassis. But you should be able to remove it partially and wrap it in plastic so that it doesn't get contaminated by metal shavings.

Onces you have the holes drilled, file away any rough spots and wipe down the chassis with a damp cloth.

This might be a good time to figure out something for that hole the old wiring harness used to go through. I used a washer and the head of a bolt to make a cap, and epoxied it in there. But this is purely cosmetic and unimportant.

Step 6: Putting It Together

Now comes the fun bit. Install the binding posts while using a small screwdriver to make sure they're all orientated right when you're tightening them down.

Install the tongue terminals onto the back of the binding posts and tighten them down good and snug with the pliers. This might be tricky if you have a high-wattage PSU as you will have more wires. The most the posts shown in these pictures can take is 4 tongue terminals.

After that's done, close up the power supply.

I had some clearance issues with mine- the 90mm fan just wouldn't fit. I figured since it will not be acting as the exhaust fan for a computer anymore, it wouldn't be needed anyway. So I removed it.

Step 7: Make It Pretty

You need some way of clearly marking which post is which voltage. You could go super polished and make a color-coded decal in Illustrator and print it at your local print shop, but I'm lazy... and cheap. So I used some permanent markers.

You could also take some plastic or vinyl paint and color each post. Whatever puts a bee in your bonnet.

Lastly, stick on the rubber feet on what you want to be the bottom.

Step 8: Conclusion

My 400 Watt power supply can deliver 23 Amps through the 12V rail, and 40 Amps through the 5V. That's very good for something that, aside from the initial cost of the PSU, cost about $10.

Step 9: Updates


Originality

This project is not necessarily original and has been done by many people.

The most "together" project is that of this guy: http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

There are a multitude of other projects, but I feel mine and his are the best I've seen so far.

Issue of the Resistor

Power supplies need a certain minimum load to work properly. The min. load for mine is around 0.8 amps. Thus if you plan on powering LED's or other such low-power device exclusively, you'll need a resistor to provide a load. Otherwise you will damage the PSU.

A meaty 10-Ohm, 10 watt resistor from Radio Shack is a good choice. Wire it across 12 volt and ground.

-12V and -5V lines

It has been brought to my attention that the -12V and -5 lines are pretty handy for diversifying the voltages this thing can produce. These are the white and blue wires I told you to cut earlier.

Of course, adding them is simple, it's just a matter of getting two extra binding posts and connecting the wires to them. It's just a question of "Do I need these?"

I didn't, all I really needed was the 12V line. But as I said, if you need them, they're easy to install.

UPDATE 12-1-11

Still going strong! This little PSU has been super handy. 
Oh, I forgot to suggest that a good repurpose for the large hole is to insert a nice LED or other ref light to indicate the unit is powered up or not!
Really enjoyed this instructable!!<br>The only problem I ran into was that one yellow wire came right out of the pcb! I tried to examine the soldering on the bottom of the pcb but was unable to find where the one yellow wire came from. <br>My PS is rated at 500 W so I'm not too alarmed about this one wire.<br>I'm being extra careful with the &quot;sea&quot; of wires.<br>Question: what to do with brown, purple, blue light pink, and gray wires? I'm going to finish the build and then explore these wires with VOM. Maybe then I'll understand what they are for..<br>Thanks again for an excellent tutorial!<br>Joe Gates <br>Clarksville, TN<br>
<p>Is there any way to get a higher voltage out? I would love to get 24 and higher.How do you determine which bundles connect to which rail? Could a potentiometer, mini multimeter and an ammeter be added without too much more effort? </p>
<p>I think it should be possible to combine a 12V and -12V to get 24V.</p>
<p>You can change the output voltage. Here is a link,<br>use translator http://www.webx.dk/oz2cpu/radios/psu-pc400-mod.htm</p>
<p>I made it using the instructions, connected each of the colors together and put a 10 watt 1 ohm resister between the 12 volt wires and ground thats all I could find and everything works fine I have the correct voltages on the wires but when i connect up a 12 volt light strip to it they light up fine but I get a buzzing kinda high pitch whine noise im assuming thats not good sorry but im new to this and have no idea whats goin on can anybody help me out ? thanks</p>
<p>I think i made it. (soft of...) I measure voltage from each post is fine. Get 12.31v, 5.24v, 3.42v. The only issue is the metal casing give me a &quot;shock&quot;. I measure voltage from ground to casing is 0 voltage but it still shock me. Any idea?</p>
<p>Probably your unit is not properly grounded</p>
<p>If you get no DC voltage but still get shocked, measure AC voltage to ground. That might be what's biting you. </p><p>If you have AC voltage, look for opens to ground from the green wire of the AC plug or the neutral (white) wire being open. It can also be your outlet, you house wiring. Before three prong plugs or polarized plugs on everything, this was very common. A little line checker like this: ( </p><p>https://www.amazon.com/Electrical-Receptacle-Outlet-Ground-Tester/dp/B0012DHVQ0 ) can tell you the AC line is OK. (No relation to Amazon or whoever makes those. I have something like it that's 35 years old, and when you need it, they're very simple to use.) </p>
<p>Hi ! I've done this to plug a lipo charger but i have a problem. When i plug the lipo charger, the screen light for a second and the atx cut off. On the voltmeter i have 12v showed, so no problem, but it seems that when i apply some little load (just the charger without charging anything) it goes away.</p><p>Any idea ? :/</p>
<p>did u ever add a load resister?<br></p>
<p>I'm part-way through making this, and I'm new to this kind of project. The 300w power supply I'm using has 7 yellow wires labelled 12v 8.0A, and 2 black-and-yellow wires labelled 12v 14.0A. If I connect these two lots of wires separately (ie. to separate binding posts), does that mean I can have two 12v supplies with different current (ie. one at 8.0A and one at 14.0A)? Also, I successfully ran the power supply for more than half-an-hour with no load before turning it off, so does that mean I can get away with not having a resistor to provide a load?</p>
Many power supplies have different output ratings. For example, I have a crappy 250w from an old Acer. It has only 1 12v+ supply rated at 15a total. Although it does have a pair of yellow/black wires and a handful of yellow. The yellow/black pair come from their own thoughts hole on the board and the gaggle of yellow come from 1 other larger hole. However they are all tied to the same trace on the circuit board. Your PSU on the other hand has different board terminations and most likely different traces. I would NOT tie these together, rather use them separate as you assumed. You should never tie power sources together that have different ampacities. Think of it like tieing multiple breakers in your home together to run things. The wires may not be able to handle the load on the smaller breaker. <br>Electricity does NOT follow the path of least resistance. It follows ALL paths, however obviously, the least resistant path carries more voltage. <br>So.... After my A.D.D. guided post...<br>Yes you should (MUST if you want to follow proper electrical theory) separate each circuit per it's ampere rating. <br>Nice catch!! This is something the OP SHOULD ADD to his thread!
How do you determine which load resistor to use? I'm new to all this. Learning slowly but surely. I couldn't find any 10w 10ohm resistors so I bought a 8ohm 20w &quot;non-inductive&quot; resistor and a 10w 50ohm wire wound resistor. Thanks!
<p>Thanks I think I made it... but I just have a little problem with my 12v... it seem that I got 16v.. Is that okay?</p>
<p>Made it!</p>
I am new at this ...... i didn't understand why we can not use atx directly in our circuit when our cpu's mother board and all things work with it.....what is diffrence b/w atx and regulated power supply? <br>please someone help me....
<p>Atx is not fabricate to use it as regulated power supply, it contain IC to contol underload or overload current, see links</p><p> <a href="http://hackaday.com/2013/08/30/disabling-underover-voltage-protection-on-atx-power-supplies/" rel="nofollow">http://hackaday.com/2013/08/30/disabling-underover...</a></p><p> <a href="http://planetimming.com/atx_mod/atx_mod.html" rel="nofollow">http://planetimming.com/atx_mod/atx_mod.html</a></p><p>but you can use it, after some modification :</p><p>1. connect black with green to run it.</p><p>2.connect load (resistor 10W 5,0oms) in any red wire to stabilise all other outputs.</p>
Thanks to instuctables.
<p>Hi,</p><p>Are you using standard binding posts? The cheap ones aren't rated for the maximum amperage of the PSU. Is it OK to use any old post?</p>
<p>Could we used this PSU for car battery charging? If not, what does we need to enable that?</p>
<p>Hello</p><p>I have an old power supply from a LinotronicRIP50 that is not working.</p><p>The power outputs are 12V (6 amp) and 5.1v (20 amp) would I be able to use a modern psu?</p>
<p>If you hook the +12v too the +3.3v would that give me +15.3v ? And would it make it 41amps ?</p>
No, by connecting those wires in series you will get 17+ volts, but the amperage that can be drawn safely is the lowest of the two outputs. 23 amps, in this case.
<p>why it will give 17+ V instead of 15.3V?</p>
<p>ok, thanks for the help :)</p>
<p>Can it kill me if I touch it? Because I heard that 30 mAmps can kill you, so ...</p>
Yes. Amps kill, not voltage. So long as you don't have it plugged in while working on it you're fine. Once work is done and cover back on, these are well grounded and safe.
<p>nope not enough voltage .....i wouldnt stand and pee on it holding a cold water pipe tho either ....gettiing ur hands across a 12 v car battery makes a burning sensation at 12 v ....if u have alot of cuts on ur hands then u could have a problem ...a guy in a junk yard carrying a half dead car battery fell down both hand on the terminals they determined 6v and half an amp stopped his heart ...thats blood contact ...</p><p>but if ur at the work bench dont worry much about it ..just watch the sparks when u mess up</p>
Last one working so well I made another from a unit that came bundled with a case I bought. Lack of pci'e connectors made it useless for my pc.
<p>I made mine some time ago, using an old PC cd-rom and car amplifier and added two usb from an old motherboard, looks ugly but it does the job</p>
Made mine fron a 500W Dynex unit.
<p>I built my power supply with the similar instructions as yours using only the 12Volt output to power my Car Airpump (12V 14Amp max).I joined all 5 12V wires (Yellow) together and same for the ground connections to get the max current on the 12V rail which is rated for 17A on this supply.</p><p>With no load connected, the SMPS turns on and works fine but as soon as i connect the Car Airpump it trips (shuts itself down) not sure why ?</p><p>Now if i connect the same Power supply with a Digital Ampere meter connected in series, the Air Pump works perfectly. Could you help me understand the possible cause for this behaviour and how could i go about fixing or diagnosing it.</p>
<p>Hay I'm having the same problem with my power supply that you where. Mine just shuts off if I connect much of anything to it! Did you ever figure out what the problem is?</p>
<p>i solve my problem by connecting sensing voltage wire(brown) to 3.3v rail. </p>
<p>i got same problem ,i did't use resistor is that cosing shutdown?</p>
I think the issue some folk may be experiencing with power lines not powering up is down to lack pf the power resistors on the lines. More modern power supplies need a load resistance on the 12v, 5v and 3.3v lines. If they aren't given that load, the power supply can't stabilise the outputs. Worth a try if you're struggling to get your supply up and running.
<p>Correct, a switch-mode power supply will need a load to operate properly.</p><p>I use a 10ohm, 25watt power resistor from the 5v rail to ground. </p>
<p>I have a little problem here I keep getting double voltages on each group of cables . For example in 3.3 v i get 6.6 v on 12.v i get 24.v etc.. What is happening ?</p>
<p>Make sure you're meter is set to measure Volts DC, not Volts AC. </p>
<p>Mine doesn't have a green wire!</p>
<p>Great article and great idea. I've used these PSUs in the past for various purposes, but the idea of adding the banana plugs, etc. is so much better!</p>
Hi there i tried a lot but there is no power in orange red and yellow. Only live wire is purple. Can u help me
<p>You need to short the green wire with any black wire to make the PSU power up. The purple is a constant +5v supply no matter if the PSU is on or off. Once you have shorted the green and any black wire, the PSU fan will spin up and you'll receive power on all rails.</p>
<p>I made it! Thanks for providing such an easy-to-follow Instructable -- this is something I've wanted to make for a while and was finally able to take the plunge. Thanks!</p>
<p>Great tutorial, it does a lot of help esp to the newbie like me.</p><p>I'll just ask what if I have multiple devices that required 12V DC supply and different ampere supply? </p><p>My 12V DC-requierd devices are the ff:</p><p>12V DC Arduino</p><p>12V DC DC-geared motor</p><p>12V DC Relay (2 relays)</p><p>12V DC LED Strips</p><p>I also have 5V devices like Microswitch, Honeywell water Level sensor and LCD.</p><p>Guys, please help me how to connect these devices to the 500 Watts PSU.</p><p>Thank you and you may all have a great day :)</p><p>Any response will be appreciated.</p>
<p>Can anyone tell me if one of these resistor will work ? i want to add just to be sure ? it wont blow op :D</p><p>http://www.ebay.com/itm/2x-5-10w-10-ohm-10-J-Ceramic-Cement-Power-Resistors-Flame-Resistance-Brand-New-/401045504802?hash=item5d602cc322:g:NrIAAOSwT5tWGjvK</p><p>http://www.ebay.com/itm/Chassis-Mounted-10-Watt-10-Ohm-5-Aluminum-Case-Wirewound-Resistor-/191680189623?hash=item2ca10794b7:g:WycAAOSwHnFV5mMN</p><p>Thanks :D</p>
<p>A follow-up question.. how can I use this power supply for a circuit requiring 12v and 6 amps?</p>
<p>I have a question on volts, amps and watts... Is 400w variable depending on whether 12v or 5v used? When you are using 12v, the total wattage will be 276 and with 5v, 200W. Am I understanding this correctly? What if I use both 12v and 5v at the same time?</p>
I had a little project which required 12v dc current to power it... But couldn't find a power supply/adapter powerful enough for the job.. So I decided to use my old computer PSU... This guide has been very useful.. Thank you!

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