This is a DIY to turn your old PC ATX power supply into a 12v source of power. This idea is not my own, it was taken from a much more complicated one on HIDPlanet (which focused on the older AT power supplies). I figured I should put it out here as well, modified to make it much simpler to understand than anything I have found there.
Disclaimer: You're working with electric parts, and especially if you open the power supply to tidy up the wires leaving only the two we need, it is advisable to be prudent with handling.
Difficulty: Very easy.
Application: Ever wanted to test your HIDs before putting them in your car in the comfort of your home? See if they work? Test lens spacing on retrofits? Just want to blind yourself in your home?
1x ATX Power Supply (I actually bought one on ebay for 19 dollars shipped for a new one, but you can easily find them out of old computers and the like)
1x Paper Clip
2x Alligator Clips
1x Wire stripper/crimper
1x Ballast and Bulb
Picture of the ballast and bulb that I used:
Step One: Get a paper clip. Trim and cut some of the insulation off on each side, leaving bare wire.
Step Two: Identify the green and black wires on the 20 plug ATX. Insert your paper clip into the two holes on each end. There are several black ground wires- For simplicity's sake I just chose the one next to the green.
Step Three: Identify the 12V wire on the ATX connector. This will be denoted by a yellow color. Cut the wire and affix your alligator clip (or other tool) to that wire. I simply crimped it together. Do the same for another black wire (this will be the ground)
Step Four: Attach the positive yellow wire to the positive of the ballast, and the ground black wire to the negative of the ballast.
Step Five: Turn on the Power supply switch. (You can also put a switch between the black wire and green wire from step two and use that as the switch)
That's how I did it. Others surely have better ways, but I'm a DIY noob, and I like to keep my projects simple.