Power Supply Unit

A power supply unit (or PSU) converts mains AC to low-voltage regulated DC power for the internal components of a computer. It receives power from an electrical outlet and converts the current from AC (alternating current) to DC (direct current), which is what the computer requires Modern personal computers universally use switched-mode power supplies.

In the personal computer world the two main standard power supply types are the AT and the ATX types. AT is a very old standard that was created by IBM in the late 1970's. AT computers had their power switches connected directly to the power supply while in ATX systems, the switch is connected to the motherboard.

see also: PC Power Supply Voltage Data and Connector Types which includes instructions to Hot-Starting and ATX Power Supply Not Connected to a Motherboard.