In another thread, someone said it would be nice to be able to use an old PSU to power the r-pi. I tought it would be nice to share the information to the rest of the community.
It's possible to convert PC PSU's into stand alone units which may supply 3.3, 5, 12 and 7 volts. The color code for these PSU's is as follows: RED: +5V, YELLOW: +12V, BLACK: GND, ORANGE: +3.3V PURPLE: +5Vsb . That means that you can get 7V between the red and the yellow wires. On top of that, it can provide a small amount of power to a 5V standby connection.
But how do you power on a PSU without a computer attached to it? This can be done by connecting the GREEN wire with a BLACK wire on the 24 pin ATX connector.
For a tidy PSU you could open the unit up an trim all the cables. Drill 6 holes in the case: 5 for banana connectors or something similar (GND, 3.3V, 5V, 12V, 5Vsb) and one for an off/on switch. Connect the switch between the green and black wire. When the PSU is off, the 5Vsb should still be available!
Some PSU's require a minimum load to function properly. Therefor, it's safer to solder a 10ohms, 10Watt sandbar resistor between one of the +5V wires and GND. For better cooling of the resistor, you could zip-tie it to the enclosure, like so.
The 5Vsb should be able to power an r-pi. The psu itself could be used to power 250 r-pi's
I *KNOW* that this is VERY inefficient to just power 1 R-pi. This is actually intended for use on test benches or on a whole bunch of r-pi's and their accessories (hard drives, etc, etc)
If you are not familiar with electric/electronic components and do not have experience with high-voltage circuitry, then DO NOT OPEN UP the PSU. If you do open it up, then NEVER EVER touch the capacitors inside (you should be fine if you leave the PCB in the enclosure.
You can do this hack without opening up the PSU by using the external cables. I usually open the PSU up, so I can solder all of my connections out of side and so I can screw my connectors through the enclosure.