Recently, I’ve been scouring the web for a cheap power supply. Building my own was definitely a consideration, but I never considered ripping one straight from a computer. Even my rusty old 386dx can supply ±5V, ±12V, and GND.

My journey led me to eHow’s wiki page that describes how to convert an ATX power supply into a workbench power supply. The instructions are extremely thorough. Unfortunately there is no 3.3V tap, but you can figure out how to add that on your own!

There is a similar conversion that explains the engineering and need for a power resistor in greater detail. Basically, the “cheap” circuitry in the ATX supply requires some (significant) current in order to “turn on.”