ATX Power Supply Modding

This post is details, for my own reference, on how to mod an ATX power supply so that it can be used as a source for 12VDC and 5VDC.

To turn on an ATX power supply that isn't connected to a motherboard use a wire or paper clip to short the green wire (pin 14, PS_ON) to any one of the black wires (COM).

The power supply shuts down if it doesn't receive a power good signal within about 3 secs. You need to short "Power OK", pin 8 on the diagram to a +5v line to keep the P/S operating.

The following is from Wikipedia.

ATX 2.0 Connector

24-pin ATX12V 2.x power supply connector (20-pin omits the last four: 11, 12, 23 and 24)

Color	Signal	Pin	Pin	Signal	Color
Orange	+3.3 V	1	13	+3.3 V	Orange
				+3.3 V sense	Brown
Orange	+3.3 V	2	14	−12 V	Blue
Black	Ground	3	15	Ground	Black
Red	+5 V	4	16	Power on	Green
Black	Ground	5	17	Ground	Black
Red	+5 V	6	18	Ground	Black
Black	Ground	7	19	Ground	Black
Grey	Power good	8	20	Reserved	N/C
Purple	+5 V standby	9	21	+5 V	Red
Yellow	+12 V	10	22	+5 V	Red
Yellow	+12 V	11	23	+5 V	Red
Orange	+3.3 V	12	24	Ground	Black
Pins 8, 13, and 16 (shaded) are control signals, not power: "Power On" is pulled up to +5V by the PSU, and must be driven low to turn on the PSU. "Power good" is low when other outputs have not yet reached, or are about to leave, correct voltages. The "+3.3 V sense" line is for remote sensing.[7] Pin 20 (formerly −5V, white wire) is absent in current power supplies; it was optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3. The right-hand pins are numbered 11–20 in the 20-pin version.

Four wires have special functions:

• PS_ON# or "Power On" is a signal from the motherboard to the power supply. When the line is connected to GND (by the motherboard), the power supply turns on. It is internally pulled up to +5 V inside the power supply.^[8][9]
• PWR_OK or "Power Good" is an output from the power supply that indicates that its output has stabilized and is ready for use. It remains low for a brief time (100–500 ms) after the PS_ON# signal is pulled low.^[10]
• +5 V_SB or "+5 V standby" supplies power even when the rest of the supply lines are off. This can be used to power the circuitry that controls the Power On signal.
• +3.3 V sense should be connected to the +3.3 V on the motherboard or its power connector. This connection allows for remote sensing of the voltage drop in the power supply wiring.