Simple ATX bench power supply


For some time I had this old ATX power supply gathering dust and getting moved from one place to another. I kept wanting to transform it into a bench supply. ATX supplies are switched power supplies that can provide a lot of current. They have a set fixed of voltages (5V, 3.3V, 12V, -12V), nonetheless very useful for most electronics and low voltage high-power projects. Once nice thing is that you can combine outputs to get less conventional voltages, for instance you can use 3.3V as GND and 5V as VDD to get an effective 1.7V potential difference which could come in handy in some projects; you could also use -12v as GND and 12V as VDD to get a 24V range for your project. Be warned if  you try that though – if your project has other GND sources that are connected to mains ground/earth you could potentially cause some sparks/smoke/flame/fireworks, since all the grounds in this supply are directly connected to mains earth – as revealed by a simple connectivity test between the supply input and output ground terminals.

Recently I was inspired by Phil’s excellent ATX bench supply video at JumperOne, and that motivated me to get my act together and get this overdue thing done. He does a great job explaining the theory behind ATX supplies so please check out his video for details on how to wire the inside, it’s really well made. Essentially all the voltages you care about are color coded and you can’t really make mistakes when wiring. I cutoff all the other non important wires and bundled together all the voltages I cared about (purple: -12V, yellow: 12V, red: 5V, orange: 3.3V, black: common ground). The green wire is for switching the supply on, it will go to a switch that connects it to ground for convenient front panel ON/OFF action.

So I ordered parts and started planning how I will place everything inside the box. One important aspect is to make sure that when you put everything back in the case, none of the wires or terminals touch the metal casing of the supply. For this reason I bought terminals that have a plastic retainer that gets sandwiched between the inside/outside of the case, such that the terminal metal core could never touch the casing wall. See pictures for what I mean. Also it really helps to have terminals with a tab so you can solder the wires to that  tab instead of directly to the terminal screw.

Not all ATX supplies are the same size and design, and in my case the space inside was not particularly friendly. My supply turns ON with no load on any rails, so even though I wanted to use a 10W resistor between 12V-GND (this is important if you have a newer ATX supply), I decided to leave it out since my supply is stable without it (I guess older supplies have built in load resistors for this purpose).

See my WARNINGs and DISCLAIMER before you attempt to do anything involving electricity or chemicals.

 I had to cut a rectangle on the side for the supply switch, and make holes for 2 status LEDs (one for mains power ON status, and one for supply turned ON status), and holes for the therminals. Here are some photos of the progress.

2 thoughts on “Simple ATX bench power supply

  1. nicely done! i’ve seen phil’s version and i too have been inspired to give it a try. so far so good on the build. my power supply already came with a mains switch but i have another power supply [which i am working on simultaneously] that i would like to wire a mains switch to it. im having a bit of trouble and was wondering which wires to use to wire a switch. any tips or suggestions would be really helpful. thanks in advance.

    • You should wire the switch on the HOT mains wire, NOT the neutral/ground. That way there’s no current in your supply when it’s switched off.

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