AT Power Supply (original IBM PC XT AT standard)


Ground (0V)

+5 V

+12 V (typically for disk drives and cooling fans)

-5 V (typically for peripherals on the ISA bus)

-12 V (typically for RS-232 serial port negative rail)

Power Good – pin used to prevent digital circuitry operating while the power supply turns on and output is not yet properly stable.

Typical power rating 63.5 watts (most of it on the +5 V rail)

Power is turned on and off at the mains (no energy saving low-power mode).


ATX Power Supply


Ground (0V)

+3.3 V

+3.3 V sense (should be connected to the +3.3 V at the motherboard to allow for remote sensing of the voltage drop in the power supply wiring)

+5 V

+12 V

-5 V – absent in current power supplies (optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3)

-12 V

Power good (PWR_OK) – low when other outputs have not yet reached, or are about to leave, correct voltages (typically remains low for 100–500 ms after the PS_ON# signal is pulled low)

Power on (PS_ON#)- pulled up to +5 V by the PSU, motherboard drives low to turn on the PSU.

+5 V standby (+5VSB) – provides a small amount of standby power when the computer is “off”

Positive supply voltages must be within ±5% of their nominal values at all times. Negative supply voltages must be within ±10%.



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