The power supply must provide enough power for the components that are currently installed and allow for other components that may be added at a later time. If you choose a power supply that powers only the current components, you might need to replace the power supply when other components are upgraded.

The power supply, shown in Figure 1, converts Alternating Current (AC) power coming from a wall outlet into Direct Current (DC) power, which is a lower voltage. DC power is required for all components inside the computer. There are 3 main form factors for power supplies, Advanced Technology (AT), AT Extended (ATX), and ATX12V. The ATX12V is the most common form factor used in computers today.

A computer can tolerate slight fluctuations in power, but a significant deviation can cause the power supply to fail. An uninterruptible power supply (UPS) can protect a computer from problems caused by changes in power. A UPS uses a power inverter. A power inverter provides AC power to the computer from a built-in battery by converting the DC current of the UPS battery into AC power. This built-in battery is continually charged via DC current that is converted from the AC supply.


Most connectors today are keyed. A keyed connector is designed to be inserted in only one direction. Each power supply connector uses a different voltage, as shown in Figure 2. Different connectors are used to connect specific components to various ports on the motherboard.

NOTE: If you have a difficult time inserting a connector, try repositioning it, or check to make sure that no bent pins or foreign objects are in the way. If it is difficult to plug in a cable or other part, something is wrong. Cables, connectors, and components are designed to fit together snugly. Never force a connector or component. If a connector is plugged in incorrectly, it can damage the plug and the connector. Take your time and make sure that you are handling the hardware correctly.