Designing a DIY Bench Power Supply from ATX PSU

An ATX PSU is one of the best free lab power supplies if you know what you're working with. Before starting: treat the mains-side wiring with respect even when the PSU is unplugged. Capacitors on the primary side can hold lethal charge for minutes after disconnection.

The trick for turning an ATX PSU into a bench supply: the PS_ON pin (green wire) must be pulled to ground to enable outputs. A simple toggle switch between PS_ON and a black GND wire does this. A 10-ohm power resistor across the 5V rail as a dummy load keeps the PSU regulation stable — many ATX supplies behave erratically with no load.

The rails you get: +12V (typically 15-30A), +5V (20A), +3.3V (20A), -12V (0.5A). Binding posts on a scrap panel with color-coded banana jacks make a clean front panel. Add a voltage/current display module on the +12V output to complete a very capable bench supply for under $20.