Function

• The purpose of a computer power supply is to convert AC power from your wall switch to lower voltage DC power that your computer can use. In other words, computer power supplies deliver electricity to your motherboard and every other part of your computer in a form that it can use. The electrical outlet in your house delivers power at 115 volts in the U.S. or 230 volts in Europe. If the electricity were delivered to your computer at that voltage, it would fry the delicate circuits on the circuit boards. To avoid this, the power supply in your computer uses a switching technology to convert the higher voltage current into three different voltages--3.3- and 5-volt to power circuit boards and 12 volts to run disk drives and fans.
  
  

Identification

• The power supply in a desktop computer is generally fairly easy to find. From the outside, you can locate the power supply by finding where the power cord plugs into the computer. The power supply also has a grate-covered opening for the computer fan and a switch that allows you to change the line voltage on the power supply for different countries. If you open your computer, you'll see that the power supply is a metal box generally located at the back of the computer composed of a bundle of multi-colored wires with various types of power connectors that emerge from its front.
  
  

Types

• Over the years, there have been several different types of computer supplies in different "form factors," a term that the computer industry uses to describe standardized sizes and shapes of computer cases. Originally, computer makers all manufactured their own parts, including power supplies. That often meant that only a power supply manufactured by the same company could be used in a computer. In fact, it often meant that the power supplies for one model of a computer could not be used in any other model made by the same company because it would not fit in the space supplied for the power supply.
  
  Eventually, the computer industry came up with standard sizes to fit the standardized computer boxes and motherboard requirements. Over the years, there have been eight popular standardized power supplies, each of them developed to meet the space and electrical requirements of different kinds of computers. These have included PC/XT, AT, Baby AT, Slimline, LPX and WTX. Most computers today use the ATX power standard.
  
  

Connectors

• Computer power supplies typically provide the following power connectors to supply power to different computer components. They include:
  
  The main power connector, often labeled P1, supplies power to the motherboard. In older computers, this connector may be split into two connectors, labeled P8 and P9.
  
  A 4-pin connector, labeled P4, which supplies dedicated power to the CPU.
  
  Molex power connectors, which are 4-pin power connectors, supply power to the disk drives. They can be identified by the plastic case with rounded corners. They are designed to only fit into the drive the right way.
  
  Berg power connectors, also called mini-connectors, fit into floppy disk drives. They may also supply power to AGP video cards.
  
  Serial ATA power connectors are 15-pin connectors that supply power to components that use SATA power plugs.
  
  

Warning

• Inside computer power supplies you'll find capacitors, diodes, transistors and transformers along with the various wires that carry electrical power. In most power supplies, you'll also find a heat sink and a fan to dispel heat that can cause damage to the computer. The main capacitor in a power supply can hold a charge long after the computer has been turned off and unplugged from an electrical outlet. Because of this, you should not ever open a power supply to service it.