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20 pin MiniFit Jr 5566-20 male (MOLEX 39-29-9202) connector layout
20 pin MiniFit Jr 5566-20 male (MOLEX 39-29-9202) connector
at the motherboard
20 pin MiniFit Jr 5557-20 female (MOLEX 39-01-2200) connector layout
20 pin MiniFit Jr 5557-20 female (MOLEX 39-01-2200) connector
at the cable
v 1.x with 20 pin connector was widely used at PC motherboard. Replaced by v 2.x with 24 pin 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 standard.

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

Pin Name   Color Description
1 3.3V   Orange +3.3 VDC
2 3.3V   Orange +3.3 VDC
3 COM   Black Ground
4 5V   Red +5 VDC
5 COM   Black Ground
6 5V   Red +5 VDC
7 COM   Black Ground
8 PWR_OK   Gray 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)
9 5VSB   Purple

+5 VDC Standby Voltage (max 10mA) 500mA or more typical

10 12V   Yellow +12 VDC (may sometimes have a colored stripe to indicate which rail it's on)
11 3.3V   Orange +3.3 VDC
12 -12V   Blue -12 VDC
13 COM   Black Ground
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.
15 COM   Black Ground
16 COM   Black Ground
17 COM   Black Ground
18 -5V   White -5 VDC (2002 v1.2 made optional, 2004 v2.01 removed from specification)
19 5V   Red +5 VDC
20 5V   Red +5 VDC

/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) supply 5V even when PSU is turned off. 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.

 

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Pinouts.ru > ATX, WTX and other standard and proprietary power supply (PSU) cables connectors > Pinout of ATX power supply connector and layout of 20 pin MOLEX 39-29-9202 connector and 20 pin MOLEX 39-01-2200 connector
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