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DIY 12V Power Supply for Testing Headlight Components

If you are going to be testing car electronics and don’t want to have to install them on the car every time you need to test something you are going to need a 12V power source.

A cheap, easy, and flexible way is to use an old ATX desktop power supply. These have multiple +12V outputs that work great for testing headlight components. I can light up highs, lows, and other accessory lighting in a headlight simultaneously.


  • An ATX power supply for a desktop computer
  • A toggle switch
  • Quick connectors, solder and soldering iron, or Suitcase taps
  • Heat shrink or electrical tape
  • Step 1:

    Find a power supply. Being a Computer Engineer, I always have spare computer parts laying around because you never know when you are going to need them, right? So I was able to just pull one from an old tower I had laying around. It’s nothing special, just a 250W ATX power supply.

    Step 2:

    Find out the +12V wires. Should be all the yellows, there should be a label someplace on the casing that says the output values of the different colored wires.

    Step 3:


    Find the wires that run to pin 3 and 4 of the ATX connector. This is the power wire that the power switch for the motherboard uses to turn on the power supply. In the above picture, it’s the green and black wires you see clipped and spliced. We need to attach a toggle switch between these two wires to make it easy to turn on the power supply. You can just splice them together, but then you have to plug in and unplug every time you want to turn on or off.

    Step 4:

    This step is technically optional. I actually didn’t do it until I decided to write this up. Now that I have this convenience, I don’t know why I put it off so long. Adding a toggle switch or button to control the power. You just need a switch or button that stays on or stays off for this. I used an aftermarket fog light relay button. They will have 3 terminals. You will need to ground 2 of them and run the power wire to the wire that was on pin 4 of the ATX connector.

    Step 5:

    Plan your connections and solder/splice them in place. There are adapters for 9006 to just about everything else and since I’m a dealer, I have all those adapters so I soldered up two of my +12V outputs to 9006 pigtails. This won’t work for everything though, sometimes you need more flexibility so I recommend keeping a couple for using with alligator clips. This way you can just clamp on to unusual connectors, swap the power from low to high on a dual filament bulb, or send power to individual headlight housing pins.

    Step 6:

    Clean up. It’s a real pain to move a power supply around with all those wires hanging off it. Chop off all the wires you don’t need. Tape or shrink wrap all your solder joints and exposed wires.