ATX Power Supplies

From HotDec

Normal Power Supplies

You must short pin 14 (/PS_ON) to GND to make the power supply turn on. (See diagram)

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 (+5V & +3.3V is ok)
9	5VSB		Purple	+5 VDC Standby Voltage (max 10mA)
10	12V		Yellow	+12 VDC
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
19	5V		Red	+5 VDC
20	5V		Red	+5 VDC

(source)

Dell Power Supplies

In addition to grounding pin 11, you must suppy a minimum load of ~3 Amps to turn on the power supply. Dell power supplies generally do not have power switches, so it may be a good idea to place a switch on pin 11 instead of a simple jumper. The easiest way to do this is to cut the two lines (black and grey) away from the header, then solder these directly to a switch.

This resistor is generally recommended to supply the load. Placing it in front of the PS fan may be a good idea.

Color	Signal	Pin	Pin	Signal	Color
Gray	/PS_On	11	1	+5V	Red
Black	GND	12	2	GND	Black
Black	GND	13	3	+5V	Red
Black	GND	14	4	GND	Black
White	-5V	15	5	Power_Good	Orange
Red	+5V	16	6	+5VSB (standby)	Purple
Red	+5V	17	7	+12V	Yellow
Red	+5V	18	8	-12V	Blue
KEY (blank)	-	19	9	GND	Black
Red	+5V	20	10	GND	Black

(source)