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Power Supply Unit


A power supply is installed in the back corner of the PC case, next to the motherboard. A power supply unit (PSU) converts mains AC to low-voltage regulated DC power for the internal components of the computer. Modern personal computers universally use a switched-mode power supply.

Most modern desktop personal computer power supplies conform to the ATX form factor.

ATX power supplies are turned on and off by a signal from the motherboard. They also provide a signal to the motherboard to indicate when the DC power lines are correct so that the computer is able to boot up.

First-generation microcomputer and home computer power supply units used a heavy step-down transformer and a linear power supply. Modern computers use switched-mode power supplies (SMPS) with a ferrite-cored high frequency transformer. The switched-mode supply is much lighter and less costly, and is more efficient, than an equivalent linear power supply.

Computer power supplies may have short circuit protection, overpower (overload) protection, overvoltage protection, undervoltage protection, overcurrent protection, and over temperature protection.

Recent power supplies have a standby voltage available, to allow most of the computer system to be powered off. When the computer is powered down but the power supply is still on, it can be started remotely via Wake-on-LAN and Wake-on-ring or locally via Keyboard Power ON (KBPO) if the motherboard supports it.

Power supplies may have passive or active power factor correction (PFC). Passive PFC is a simple way of increasing the power factor by putting a coil in series with the primary filter capacitors. Active PFC is more complex and can achieve higher PF, up to 99%.

A 20 conductor cable carries +5vdc, -5vdc +12vdc, -12vdc and ground to the motherboard.

Another pair of cables, each with four conductors and two 4-pin connectors daisy-chained along it, carry +5vdc, +12vdc and ground to the drives (hard, floppy and CD/DVD).

Typical PC power supplies are rated at 200-250 watts. Higher wattage supplies are available.


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