A Simple Bench Variable Power Supply




Introduction: A Simple Bench Variable Power Supply

As I have started messing around with electronics more I had struggled a bit with a whole lot of different adapters for testing. After doing some research I came to a conclusion that building one from an old PC power supply would be more than adequate, being how it gives 12, -12, 5 and 3.3v available (some older ones have -5v also but mine being newer didn't) making it great for testing most electronic projects.

Step 1:

Tools and materials required:

  • old (or new) working PC power supply to repurpose (mine was 300W)
  • amplifier binding posts (got mine here cheap)
  • on/off switch
  • wood
  • screws
  • heat shrink tubing or electric tape
  • wood glue
  • hand saw
  • jigsaw
  • sandpaper
  • soldering iron and helping hands
  • drill and few bits
  • pliers

Step 2:

First I cut the base for the bottom, then cut the holes in the back section for the fan, power plug and the indicator LED. I also made screw holes and screwed the panel to the back.

Step 3:

I cut off all except the 24 pin wire cable, leaving about 5 cm of each wire before wrapping them in electric tape. This is made so in case later I decide that for some reason I want to use those wires again.

Step 4:

Next I cut a piece of wood for the side, attached it to the bottom part and the back side. I also cut a smaller piece for the other side and attached it to bottom (not pictured here).

Step 5:

Cut a piece for the front and drilled the holes for binding jacks and on/off switch. Space between the holes is 2.5cm (1 inch). I also drilled the pilot holes and countersinks for the screws.

After that I put all the binding posts in and tested if all the cables would fit in the front when the panel is mounted.

Step 6:

I cut the wires to size and soldered them to the binding posts; isolated them with heat shrink tubing to make sure there would be no unwanted contacts. All the wires of the same color give out same voltage so some of them are bundled together. You don't have to do that, you can can just pull one of the appropriate color wires to the jack and cut off the rest. Green wire and ground is connected to the on/off switch; grounding the green wire will turn on the power. Colors have outputs as follows:

  • Yellow +12v
  • Blue -12v
  • Red +5v
  • White -5v (non existent on this PSU)
  • Orange +3.3v
  • Black Ground
  • Green Power ON
  • Gray Power on indicator (attach 330 ohm resistor and led to it and ground it if you want an indicator)
  • Purple Standby (not used)
  • Brown (older PSU) 3.3v check, must be connected to orange wires if you want 3.3v output

Step 7:

I cut some more wood for the top, bearing in mind to keep the ventilation holes open. I checked all the connections and finally plugged in the power.

Step 8:

After checking the voltages I wrote them down on the front panel. Hope you enjoy the project and good luck making your own.



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    57 Discussions

    So to make sure, will the color of the wires you mentioned here be universally applied to every brand? I'm thinking of trying this, but I'd rather not find out halfway through that my values are all wrong.


    2 replies

    The wire colors shoud be universal. Some PC power supplies have just one main connector, others have two half-size. In addition, there should be one to three smaller connectors that supply 3.3 and 5 vdc.

    Most all power supplies I've seen had a label on the side announcing the wire values and amps.

    I don't think this was asked already. Would it be improper and/or not work to daisy chain one ground to each thing that requires a ground? I mean if multiple 5v and one or two 12v power sources each require ground can they use a common ground?

    Or do all them need their own separate ground wire for each rail? I'm kind of a beginner (sorry).

    1 reply

    You can daisy-chain them together but you may overheat the wire if you draw too many amps total. Better safe than sorry and run just one to each ground... or better yet two if you have enough wires. I've seen some people run as many as the can to each post, including the 3V, 5V and 12V lines.

    Many voltage combinations are possible. This was the face plate design for the one I made 2 yrs ago. At the top right of the photo you will find a list, and a very important notice about current rating. The current limit for combined voltages is the lowest of the two rails used.

    1 reply

    1 year ago

    Nice one! I've been doing a bunch of electronics projects too and think ill go down this route. However (and this may be a bit of a n00b question) what's the difference between a -5V connection and just reversing the polarity of a 5V connection? i.e. if I put a multimeter red to red it would read 5V, if i put it on red to black it would read -5V... if voltage is "potential difference" then it's all relative isn't it? Or for that matter, folks suggesting combining +12v with -12 v... if you had two pairs of red/black +12V, could you not just "patch" from red to black and get +24V from the remaining terminals?

    2 replies

    SiDawg, I believe part of your question was left unanswered.

    The last part about TWO (2) pairs of +/- red/black 12v wires...

    I think you were asking about 2 separate rails of 12v wires... the colors you mentioned were confusing given that in this device the 12v rails are yellow. One is Yellow and the other is Yellow w/a Black stripe (2nd 12v rail only on modern PSUs).

    The answer is no, you cannot use those two 12v rails together to make 24v because they share the same black wire. So, if you try what you said, it would simply short out one of the rails, in which case the rail is shut down and you will never get your 24v by using these rails in series.

    Sadly, using +12v(yellow) and -12v(blue), typically, will give you only about one amp...

    Theoretically, there is another way to get 24v at a reasonable current by combining those two 12v rails in parallel and feeding them into a boost converter. Because they are in parallel, they combine their current rating, and although some of that is lost in the step-up process of the boost converter, you are still getting reasonable current if the boost converter is rated for high amperage. Of course those are expensive...

    Alternatively, a cheaper solution is to just buy a 24v supply and get on with your life. ;-)

    The negative rails do not provide nearly the same amperage so you could end up overloading your PSU. Also, since there is only one -12v wire you wouldn't be able to combine it with others.

    I think I understand what you mean but I don't see a way you could do that with the PSU.


    1 year ago

    I think it's one of the best cover concept I ever seen. I plan to build something similar based on Your idea. Thank You for inspiration.


    1 year ago

    thanks for this very intersecting it's a shame it's only for testing items if you could use it power them up that would be great but testinfis fine lol,I,have a few things around the place that I need to work it if they are worth keeping ?|


    1 year ago

    Sorry if you've already gone over it somewhere, but what would a person use this for?

    1 reply

    For powering on electronic components and circuits before you connect them to other stuff (and checking if they're faulty).

    love it great idea for recycling old computer parts into a workshop great tool

    My psu has two yellow wires with a black stripe and well as the ones listed above. It is labeled as +12v2. What is the difference between this and the solid yellows labeled +12v1?

    4 replies

    The 12V1 and 12V2 notation indicates that there are (2) 12v outputs. 12V1 differs from 12V2 in that they are derived from separated secondary winding of the isolating transformer. They are intended for different functions as specified in the end product and generally will have different current ratings. One may find some supplies with more than two. For example, 12Vn where n is any integer (1,2,3...).

    +1'ing this question too.

    I thought it meant that the power supply contained two different 12V sources, but I was never able to find a second or third, in some.

    What is that?

    +12V2 is second rail that supplies processors voltage regulator circuit on motherboard. while +12V1 supplies the graphics card, fans and motors of HDD and CD/DVD. They each have an overload sensor to protect the various circuits; a short on those will immediately shut down the PSU.

    Thank you! I also have two solid black wires that are separate from the others. I can't see what they are labeled on the board. they should still be ground right?