There are various types of PSUs of which the ATX form factor is the most popular. This form factor allows different power supplies to be interchangeable with different components inside the PC.
ATX computer power supplies (PSUs) are the switching types that are turned on and off by signals from the motherboard. This feature enables the standby mode present in most computers.
The present ATX standard is now ATX Ver.2.31.
Power supplies are rated based on their maximum power output. These devices typically output power in the ranges of 350 watts to 500 watts for small form factor PCs, 550 watts to 700 watts for medium range PCs and for extreme gaming and multimedia PCs, 750 up to 1400 watts.
Always unplug the system before servicing your Computer Power Supply
CAUTION! - It is never recommended to repair a power supply since it does not contain user-serviceable parts. Repairing power supplies can be dangerous and hazardous.
Never open it to repair it because it stores electricity even when the computer is turned off and unplugged!
Power Supply Connectors
Below are some of the main connectors of a power supply unit:
PC Main power connector or P1 is the connector that attaches to the motherboard to provide it with power. This connector usually has 20 or 24 pins one of which is the PS-ON wire which is usually green.
Whichever connector is used, the P8 and P9 or the 20/24 pin, there is one wire present in each that carries the power good signal.
This signal tells the motherboard that the current from the power supply is within the acceptable range thus preventing accidental damage to the system from erratic current.
Power supply units with 24-pin connectors can be used with motherboards with 20-pin connectors. Motherboards that have 24-pin connectors can be used with power supplies that have 2 connectors - one with 20-pin and the other with 4-pin. These two can be used together to produce a 24-pin connector.
ATX12V 4-pin power connector or P4. This is another connector in addition to the main 24-pin connector that plugs into the motherboard to supply dedicated power to the CPU.
In high-end motherboard and CPU systems, extra power is needed hence the addition of an Entry Level Power Supply Specification (EPS12V) 8-pin connector.
4-Pin Peripheral power connectors or Molex as they are sometimes called are smaller connectors that supply power to the disk drives of your PC.
These mostly consist of 4 wires - 2 black, 1 red and 1 yellow. The black wire represents ground, the red wire is +5 volts and the yellow wire is +12 volts.
4-Pin Mini Connector or mini Molex is the smallest connector that supplies power to the floppy.
Auxiliary Power connectors are designed to provide additional power if required.
Serial ATA power connectors are 15-pin connectors for devices that use SATA power and provide power at three different voltages - +3.3, +5 and +12 volts.
6-Pin Connectors found on most modern power supplies is used to power PCI Express video cards but recently this is upgraded to 8-pin. Most 6-pin connectors provide a maximum of 75 watts.
C-14 IEC Connector connects your PC to the local power grid via a C13 cord.
Computer Power Supply Form Factors:
Just like how a computer case and a motherboard belong to a particular form factor, power supplies also come in different form factors. There are approximately four industry standard form factors. AT, Mini-ITX, SFF and ATX.
One type such as the AT is rarely used or obsolete today. The ATX is now the standard for both desktops and towers even though there are exceptions - one being the Small Form Factor (SFF) made by Shuttle and the Mini-ITX. Many different power outputs are available with the different power supplies.
Selecting a Computer Power Supply
For general mid-range PCs, 500 - 650 watts is sufficient but, if you venture into the world of heavy gaming, get yourself a power supply of 1,000 watts or more. I would recommend that it should be Intel or AMD certified.