20 pin MOLEX 39-29-9202 connector at the motherboard
20 pin MOLEX 39-01-2200 connector
ATX specification includes not only Power Supply Unit, but also
interface to case and motherboard. In addition to the old AT standard,
ATX 2.0 has one extra voltage line available (+3.3V), a connector
chain-lined to the single 20-pin and a power-on wire that allows
Software to turn off the PSU.
The ATX specification requires the power supply to produce three main
outputs, +3.3 V (±0.165 V), +5 V (±0.25 V) and +12 V (±0.60 V).
Low-power −12 V (±1.2 V) and 5 VSB (standby) (±0.25 V) supplies are also
required. A −5 V output was originally required because it was supplied
on the ISA bus, but it became obsolete with the removal of the ISA bus
in modern PCs and has been removed in later versions of the ATX
Originally the motherboard was powered by one 20-pin connector. Current
version of ATX12V 2.x power supply provides two connectors for the
motherboard: a 4-pin auxiliary connector providing additional power to the CPU, and a main 24-pin ATX 2 power supply connector, an extension of the original 20-pin version.
ATX connector pinout
Power Ok is a status signal generated by the power supply to notify
the computer that the DC operating voltages are within the ranges
required for proper computer operation (+5 VDC when power is Ok)
+5 VDC Standby Voltage (max 10mA) 500mA or more typical
|14||/PS_ON||Green||Power Supply On (active low). Short this pin to GND to switch power supply ON, disconnect from GND to switch OFF.|
|18||-5V||White||-5 VDC (2002 v1.2 made optional, 2004 v2.01 removed from specification)|
/PS_ON activated by pressing and releasing the power button while the power supply is in standby mode.
Activating /PS_ON turns on the power supply.
In several power supply units pin-12 may be Brown (not Blue), pin-18
may be Blue (not White), and pin-8 may be White (not Gray). In
addition, some PSU violate color coding of wires.
Pin 9 (standby) goes from 0 to 5V and pin 14 goes from 0 to 3.7 when PSU switch is turned on.
Shorting pin 14 (/PS_ON) to GND (COM) causes power supply to switch ON and PWR_OK to change to +5V.