Step 2: The diagram

Picture of The diagram
The diagram at right shows the main output connector of the power supply when viewed from the end. The colors represent the different colored wires going into it. Common colors represent common functions, ie all red wires are +5 volts, all black wires are common and so on. The connections most useful to us as haunters are the +5V (red wires) , +12V (yellow wire) and the Common or ground (black wires). Both 5 and 12 volt lines normally deliver ample current for our needs.

Of the other voltages available, the +3.3V connection delivers ample current, it's just not a very useful voltage. The +5VSB (5 volts, always on), -12V and -5V are normally very low current lines and are of little use to us.

The green wire, pin 14, is the on/off switching line. To turn the power supply on, the green line needs to be shorted to a Common line. An easy way to do this is to insert a jumper between pin 14 and pin 13.

Most power supplies require a load across one or more of the outputs to operate. The link I give above shows how to add a resistor across the 5 volt side of the supply to act as a load.

The smaller connectors coming out of the power supply use the same color codes. As an example, a connector with a yellow, a red and two black wires will have +12 volts (yellow), +5 volts (red) and two commons.

To use the power supply, for 12 volts, you'd connect a yellow wire to the + input of your project and a black wire to the - input. For 5 volts, you connect a red wire to the + and a black wire to the -.
Gelfling62 years ago
Might not work with all power supplies.. there are a few Dell supplies, which have a different wiring scheme for the ATX connector.. (PG or the constant (standby) +5V, can't remember which.) occupy the wrong pin, and connecting them will end in a loud POP as the switching IC will blow its top. (been there, done that, everyone came running, wondering why they smelled burnt epoxy.) Also, not all supplies will start off with zero load. the Arduino is hardly enough.. Some of the older ATX supplies required a minimum load to maintain the switching circuit.. a 33-ohm, 5W 'Sandbox' resistor (ceramic block, with sand/epoxy holding a wire-wound resistor), across the +5V to GND has always worked for me. The supply will see the load, but match to maintain the voltage. One more little tip, try this.. If you have an old 3.5" floppy disk drive that is fried, desolder the power connector, and keep the matching mini connector on the power supply side that matches it.. You can then, from a breadboard, run jumpers from the VCC on the arduino to +12V (Yellow), and run +5V external devices off the +5V (red).. One final tip, if the supply cooling fan draws air in, forcing it into the box, flip it around! drawing air out is more efficient for cooling..

I've converted two supplies like this myself, and run arduino, raw breadboard projects, and even a Raspberry Pi off them.
Nice thing about the ATX supply, is you have access to 3.3V, which is great if you're running projects for the Pi's GPIO bus... I still haven't put together a 3.3V<-->5V level handler, so been slowly learning to run things from the 3.3V level.