Power Supply Connector Pinout ATX 24 Pin

Changes to ATX standard were made to support 75 watt PCI Express requirements. Most power is now provided on 12 V rails and the power on 3.3 V and 5 V rails was significantly reduced. The standard specifies that two independent 12 V rails (12 V2 for the 4 pin connector and 12 V1 for everything else) with independent overcurrent protection are needed to meet the power requirements.
New ATX v 2.2 uses new connector, but most motherboards nowdays allow to use an old ATX v 1.x power supply with ATX 20 pin connector - it connects to 24 pin motherboard receptacle.

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, max 2A in ATX 2.2 spec)
10 12V   Yellow +12 VDC
11 12V   Yellow +12 VDC
12 3.3V   Orange +3.3 VDC
13 3.3V   Orange +3.3 VDC
14 -12V   Blue -12 VDC
15 COM   Black G
16 /PS_ON   Green Power Supply On (active low). Short this pin to GND to switch power supply ON, disconnect from GND to switch OFF.
17 COM   Black Ground
18 COM   Black Ground
19 COM   Black Ground
20 -5V   White -5 VDC  (this is optional on newer ATX-2 supplies, it is for use with older AT class expansion cards and can be omitted on newer units)
21 +5V   Red +5 VDC
22 +5V   Red +5 VDC
23 +5V   Red +5 VDC
24 COM   Black Ground
 
 /PSON activated by pressing and releasing the power button while the power supply is in standby mode. Activating /PSON connects the power supply's /PSON input to ground, thereby switching the power supply to full-on condition. 
 
18 AWG is recommended for all wires except pin 11, which should be 22 AWG. For 300W configurations 16 AWG is recommended.

Power Supply Pinout, AT Motherboard

AT PSU's were used before ATX become to be standard.
An AT power supply delivers +5 V, +12 V, -5 V and -12 V voltages using two six-pin connectors. A few newer boards used an aditional connector for +3.3 V. Note, that you must install these connectors to the motherboard in a way that the black wires are placed on the center, or your equipment will be damaged.

P8 AT power supply connector



Pin Name Color Description
1 PG Orange Power Good, +5 VDC when all voltages have stabilized.
2 +5V Red +5 VDC (or n/c)
3 +12V Yellow +12 VDC
4 -12V Blue -12 VDC
5 GND Black Ground
6 GND Black Ground

P9 AT power supply connector



Pin Name Color Description
1 GND Black Ground
2 GND Black Ground
3 -5V White or Yellow -5 VDC
4 +5V Red +5 VDC
5 +5V Red +5 VDC
6 +5V Red +5 VDC

Cable connector is Molex 90331.
The original PC debuted in 1981 and used two cables to connect the PSU (power supply) to the motherboard. The two cables plug side by side into the motherboard connectors. Sometimes they are keyed so they only plug in one way and sometimes they aren't. Even if they're keyed you can insert them the wrong way if you put a little effort into it. You always have to remember to plug them in so the black wires are next to each other.
In old PCs, almost all of the chips ran directly off of the 5 volt rail. As a result the PSU delivers most of its wattage at 5 volts. There are three or four lines dedicated to the 5 volt rail. The other main rail is 12 volts. That was used primarily to run disk drives, motors, and fans. The two negative rails are bias supplies which only have to provide small amounts of current.

P10 AT aux power supply connector (rarely seen)
Pin Name
 Color Description
1 GND Black Ground
2 GND Black Ground
3 GND Black Ground
4 3.3v Green +3.3 VDC
5 3.3v Green +3.3 VDC
6 3.3v Green +3.3 VDC
The aux power cable was added to provide extra wattage to motherboards for 3.3 and 5 volts.