Power Supply Notes

A computer power supply typically is designed to convert 110 V or 230 V AC power from the mains to usable low-voltage DC power for the internal components of the computer. The most common computer power supplies are built to conform with the ATX form factor. This enables different power supplies to be interchangeable with different components inside the computer. ATX power supplies also are designed to turn on and off using a signal from the motherboard (PS-ON wire), and provide support for modern functions such as the Standby mode of many computers.

Computer power supplies are rated for certain wattages based on their maximum output power. Typical rated wattages range from 200 W to 500 W, although units used by gamers and enthusiasts usually range from 500 W to 800 W, with the highest end units going up to 1 kW (although 1.1kW and higher power supplies are available for extreme performance computers with multiple processors and graphics cards (eg. ATI CrossFire or NVIDIA SLI)

Pentium 4’s require a power supply that is called an ATX12V. These power supplies are backwards compatible, but have a +12V DC connector. This input is near the processors DC-DC converter, enabling more responsive and efficient power delivery.

Intel does not recommend the use of adapters.

The older AT power supplies converted 110V to 12V and 5V of DC (Direct Current) power. With the ATX power supplies it also has an output of 3.3V which is used to power the motherboard. Power supplies do essentially the same thing as the AC-DC converters that hang out of the wall for small electronics, but more efficiently. These are linear converters. PC’s use switching power supplies. Switching power supplies use a transistor switch and a closed feedback loop to properly regulate regardless of the load.

In the picture shown above, the wiring color codes are:

Yellow – 12V+ - Runs disk drive motors and graphics cards

Red – 5V+ - Runs motherboards and processor on older motherboards

Orange – 3.3V+ - Runs the processor and motherboard for newer boards

Black – Ground

White – 5V- - Runs the floppy drives

Blue – 12V- - Used on old serial port circuits

These are known as molex connectors and are used to power computer

components such as hard drives and CD drives. The colors on a molex

connector are coded as well.

Yellow – 12V

Black – Ground

Red – 5V

When a power supply is first turned on, it needs a few seconds to stabilize. Once it has stabilized, it sends a “Power Good” signal to the motherboard.

Here is the pin-out for the ATX12V power supply connection

Pin	Name	Color	Description
1	3.3V	Orange	+3.3 VDC (Runs the motherboard and processor (Newer))
2	3.3V	Orange	+3.3 VDC (Runs the motherboard and processor (Newer))
3	COM	Black	Ground
4	5V	Red	+5 VDC (Runs older motherboards and processors)
5	COM	Black	Ground
6	5V	Red	+5 VDC (Runs older motherboards and processors)
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
9	5VSB	Purple	+5 VDC Standby Voltage (max 10mA) Used to keep keyboard and USB devices turned on. This is why you see the green light on the keyboard.
10	12V	Yellow	+12 VDC (Runs disk drive motors and graphics cards)
11	12V	Yellow	+12 VDC (Runs disk drive motors and graphics cards)
12	3.3V	Orange	+3.3 VDC (Runs the motherboard and processor (Newer))
13	3.3V	Orange	+3.3 VDC (Runs the motherboard and processor (Newer))
14	-12V	Blue	-12 VDC (Used on older serial port circuits)
15	COM	Black	Ground
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 (Powers the floppy disk drives)
21	+5V	Red	+5 VDC (Runs older motherboards and processors)
22	+5V	Red	+5 VDC (Runs older motherboards and processors)
23	+5V	Red	+5 VDC (Runs older motherboards and processors)
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.

Some motherboards require an auxiliary connector that looks like

The 4 pin connection is used to power the Intel Pentium 4 processor.

The voltages on this connector are

Yellow – 12V

Black – Ground

The other connector in the picture is the auxiliary power to help supply more power to the newer 3.3V RAM on newer motherboards. The pin-out for this connection is

Black – Communication

Orange – 3.3V

Red – 5V

As you may notice – All of the wiring codes are the same. All red wires are 5V, etc.

The older AT connectors

With these connectors you have to make sure you put the black ground connectors to the middle. If you do not connect them in this order, your motherboard goes up in smoke.

Pin arrangement was