Thread: PC Power supply Switch

Older form factor desktop PC/XT cases had the power switch at the back of the machine, usually on the right side of the case. This switch was actually inside the power supply itself, with a hole cut out in the case so that it could be reached from the outside. Users hated having to reach to the back of the machine to turn it on or off! The positioning of the switch also meant that the PC could not be onented with its right side towards a wall or partition.

Starting with the AT form factor, tower cases changed to a remote, physical toggle power switch that' was connected to the power supply using a cable. AT desktop cases retained the old style case in the back of the PC, but clone manufacturers soon began to use a remote switch on these units as well. The switch is normally mounted to the front of the case.some "slimline" (LPX) systems actually use a mechanical plastic stick (!) that is' pushed on by the button on the front of the case, and presses against the real power switch on the power supply itself, in the back of the machine.



The remote switch cable has four leads that run into it (with a fifth ground lead, to ground the power supply to the case, optional) . One pair of these (the brown and blue) run to the power cord on the back of the power supply. The other pair (black and white) run from the switch to the power supply circuitry. When the switch is on, brown connects to black, and blue connects to white, and the power supply is energized. These wires attach to spade connectors on the body of the switch.


Starting with the A TX/NLX form factor, the way the power switch works has been changed altogether. Instead of using a physical toggle switch connected to the power supply, on modern systems the power switch is electronic. It connects to the motherboard, ml,lch the way that the reset switch does, using a feature called soft power. So on an A TX system, when you press the power switch, you aren't really turning on the power supply; it is more like sending a "request" to the motherboard to turn the system on. As a result, the switch is a simple affair and the wires carry only