A power supply unit (PSU) supplies direct current (DC) power to the other components in a computer. It converts general-purpose alternating current (AC) electric power from the mains (110 V to 120 V at 60 Hz [115 V nominal] in North America, parts of South America, Japan, and Taiwan; 220 V to 240 V at 50 Hz [230 V nominal] in most of the rest of the world) to low-voltage (for a desktop computer: 12 V, 5 V, 5VSB, 3V3, −5 V, and −12 V) DC power for the internal components of the computer. Some power supplies have a switch to select either 230 V or 115 V. Other models are able to accept any voltage and frequency between those limits and some models only operate from one of the two mains supply standards.

Most modern desktop computer power supplies conform to the ATX form factor. ATX power supplies are turned on and off by a signal from the motherboard. They also provide a signal to the motherboard to indicate when the DC power lines are correct so that the computer is able to boot up. While an ATX power supply is connected to the mains supply it provides a 5 V stand-by (5VSB) line so that the standby functions on the computer and certain peripherals are powered.

Computer power supplies are rated based on their maximum output power. Typical power ranges are from 500 W to lower than 300 W for small form factor systems intended as ordinary home computers, the use of which is limited to web-surfing and burning and playing DVDs. Power supplies used by gamers and enthusiasts mostly range from 450 W to 1400 W. Typical gaming PCs feature power supplies in the range of 350–800 W, with higher-end PCs demanding 800–1400 W supplies. The highest-end units are up to 2 kW strong and are intended mainly for servers and, to a lesser degree, extreme performance computers with multiple processors, several hard disks and multiple graphics cards.

it is possible to overload a PSU on one rail without having to use the maximum rated power.

This may mean that if:

PSU A has a peak rating of 550 watts at 25°C, with 25 amps (300 W) on the 12 volt line, and
PSU B has a continuous rating of 450 watts at 40°C, with 33 amps (396 W) on the 12 volt line,

This is because power or wattage is current times voltage or amps times voltage.  This is the calculated rating.
PSU A can handle 300 W
PSU B can handle 396 W

So don’t trust the peak or continuous rating… Only trust the calculated rating.

Typically, power supplies have the following molex connectors.

PC Main power connector (usually called P1): This is the connector that goes to the motherboard to provide it with power. The connector has 20 or 24 pins. One of the pins belongs to the PS-ON wire (it is usually green). This connector is the largest of all the connectors. In older AT power supplies, this connector was split in two: P8 and P9. A power supply with a 24-pin connector can be used on a motherboard with a 20-pin connector. In cases where the motherboard has a 24-pin connector, some power supplies come with two connectors (one with 20-pin and other with 4-pin) which can be used together to form the 24-pin connector.

ATX12V 4-pin power connector (also called the P4 power connector). A second connector that goes to the motherboard (in addition to the main 24-pin connector) to supply dedicated power for the processor. For high-end motherboards and processors, more power is required, therefore EPS12V has an 8 pin connector.

4-pin Peripheral power connectors: These are the other, smaller connectors that go to the various disk drives of the computer. Most of them have four wires: two black, one red, and one yellow. Unlike the standard mains electrical wire color-coding, each black wire is a ground, the red wire is +5 V, and the yellow wire is +12 V. In some cases these are also used to provide additional power to PCI cards such as FireWire 800 cards.

4-pin Molex (Japan) Ltd power connectors (usually called Mini-connector or “mini-Molex”): This is one of the smallest connectors that supplies the floppy drive with power. In some cases, it can be used as an auxiliary connector for AGP video cards. Its cable configuration is similar to the Peripheral connector.

Auxiliary power connectors: There are several types of auxiliary connectors designed to provide additional power if it is needed.

Serial ATA power connectors: a 15-pin connector for components which use SATA power plugs. This connector supplies power at three different voltages: +3.3, +5, and +12 volts.

6-pin Most modern computer power supplies include 6-pin connectors which are generally used for PCI Express graphics cards, but a newly introduced 8-pin connector should be seen on the latest model power supplies. Each PCI Express 6-pin connector can output a maximum of 75 W.

6+2 pin For the purpose of backwards compatibility, some connectors designed for use with high end PCI Express graphics cards feature this kind of pin configuration. It allows either a 6-pin card or an 8-pin card to be connected by using two separate connection modules wired into the same sheath: one with 6 pins and another with 2 pins.

A IEC 60320 C14 connector with an appropriate C13 cord is used to attach the power supply to the local power grid.

AT stands for Advanced Technology when ATX means Advanced Technology eXtended.

There are two basic differences between AT and ATX power supplies: The connectors that provide power to the motherboard, and the soft switch. On older AT power supplies, the Power-on switch wire from the front of the computer is connected directly to the power supply.

On newer ATX power supplies, the power switch on the front of the computer goes to the motherboard over a connector labeled something like; PS ON, Power SW, SW Power, etc. This allows other hardware and/or software to turn the system on and off.

The motherboard controls the power supply through pin #14 of the 20 pin connector or #16 of the 24 pin connector on the motherboard. This pin carries 5V when the power supply is in standby. It can be grounded to turn the power supply on without having to turn on the rest of the components. This is useful for testing or to use the computer ATX power supply for other purposes.

AT power connector (Used on older AT style mainboards)
Color Pin Signal
P8.1 Power Good
P8.2 +5 V
P8.3 +12 V
P8.4 −12 V
P8.5 Ground
P8.6 Ground
P9.1 Ground
P9.2 Ground
P9.3 −5 V
P9.4 +5 V
P9.5 +5 V
P9.6 +5 V
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, 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.
  • Pin 13 supplies +3.3 V power and also has a second thinner wire for remote sensing.[8]
  • 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.

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