PC Power Supplies

    There are several types of power supply form factors: PC/XT, AT, Baby AT, LPX, ATX(NLX), SFX, and WTX. Each form factor is dependant on the case it is used in, and the motherboard it is connected to! The most common form factor is the ATX, as it provides a cleaner power to the system. Unlike AT P.S.'s, the ATX must be connected to a motherboard in order for it to be turned on.
    All current motherboards require an ATX power source and few support the older AT P.S.'s. ATX power supplies are superior as they deliver a more regulated current to the motherboard. ATX cases also typically have space for a second and third fan (for additional cooling) and second P.S. (to support additional high-powered devices).
    SFX and ATX P.S.'s can generally be interchanged in systems sized to hold them because their 20-pin main motherboard connectors are almost identical. They are not however exactly identical: the SFX P.S. does not provide the -5 V signal that may be required for some systems that use certain ISA bus expansion cards.

Power Supplies
Form Factor	Typical Dimensions (W x D x H, mm)	Usual Style(s)	Motherboard Connectors	Match to Case Form Factor	Match to Motherboard Form Factor
PC/XT	222 x 142 x 120	Desktop	AT Style	PC/XT	PC/XT
AT	213 x 150 x 150	Desktop or Tower	AT Style	AT	AT, Baby AT
Baby AT	165 x 150 x 150	Desktop or Tower	AT Style	Baby AT, AT, AT/ATX Combo	AT, Baby AT, AT/ATX Combo
LPX	150 x 140 x 86	Desktop	AT Style	LPX, some Baby AT, AT/ATX Combo	LPX, AT, Baby AT, AT/ATX Combo
ATX/NLX	150 x 140 x 86	Desktop or Tower	ATX Style	ATX, Mini-ATX, Extended ATX, NLX, microATX, AT/ATX Combo	ATX, Mini-ATX, Extended ATX, NLX, microATX, FlexATX
SFX	100 x 125 x 63.5 *	Desktop or Tower	ATX Style	microATX, FlexATX, ATX, Mini-ATX, NLX	microATX, FlexATX, ATX, Mini-ATX, NLX
WTX	150 x 230 x 86 (single fan) 224 x 230 x 86 (double fan)	Tower	WTX Style	WTX	WTX

ATX Power Connector
Pin	Signal	Wire Color
1	+3.3Vdc	Orange
2	+3.3Vdc	Orange
3	GND	Black
4	+5Vdc	Red
5	GND	Black
6	+5Vdc	Red
7	GND	Black
8	PWR-OK	Gray
9	+5Vdc VSB standby Voltage	Purple
10	+12Vdc	Yellow
11	+3.3Vdc	Orange {brown is 3.3Vdc sense]
12	-12Vdc	Blue
13	GND	Black
14	PS-ON	Green
15	GND	Black
16	GND	Black
17	GND	Black
18	-5Vdc	White
19	+5Vdc	Red
20	+5Vdc	Red
There's also a 5v source that stays live when the power is off (purple wire) but output is very low and the power varies with each model of power supply. FYI, you can power up an ATX P.S. by shorting the green wire with any black ground wire. As you know, the ATX P.S. will not turn on normally without a motherboard connected to it. There are times when you may want to test an ATX P.S. or use the voltage from one but don't really want to hook up a motherboard. There's only one green wire and there are plenty of ground wires so it's easy to do. To use a bent paper clip and insert one end in the green wire connector and bend it around and put it into a black wire connector.

AT Power Connector
Pin	Signal	Wire Color
1	Power Good *	Orange
2	+5Vdc	Red
3	+12Vdc	Yellow
4	-12Vdc	Blue
5	Ground	Black
6	Ground	Black
7	Ground	Black
8	Ground	Black
9	-5Vdc	White
10	+5Vdc	Red
11	+5Vdc	Red
12	+5Vdc	Red
* Power good pins gives +5V output when the voltage in the P.S. outputs is according the specs. This power good pin is usually used for resetting purposes so that the motherboard reset signal is kept active until the power good signal is raised to +5V. This keeps the motherboard not trying to do anything before the operating voltages have stabilized. Some motherboards will only work if +5V is applied to the power good pins slightly after the power is turned on on other pins.

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