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You should be aware of the following facts about power supplies:

  • Power supplies must be matched to the motherboard and case form factor. If you have an ATX motherboard, purchase an ATX power supply.
  • The power supply converts AC current to DC current.
    • AC (alternating current) is the type of current distributed through wall sockets. With AC, the voltage alternates (at a quick rate) between a negative and a positive charge. This type of current is good for appliances requiring a high current. 
    • DC (direct current) is the type of current used inside a computer. Negatively charged particles being drawn toward a positive charge create a direct current flow. This type of predictable reliable current is ideal for an application where a lower current is required.
  • Power supplies provide + 3.3 volts, +/- 5 volts, and +/- 12 volts (DC power). Most modern components require +12 volt output.
  • Each separate voltage output circuit is referred to as a rail. To avoid overloading one circuit, many newer power supplies have multiple +12 volt rails. Much like a circuit breaker in a house, separate rails allow you to distribute the power load between multiple circuits to prevent any one circuit from becoming overloaded. Each rail can power multiple devices.
  • Most power supplies have the capacity to receive both 110 and 220 volt power just by toggling a switch (typically red) on the power supply casing. You can use this switch when using the power supply in other countries. When troubleshooting, make sure this switch is set to the correct voltage.
    • 110 volts is used in the United States.
    • 220 volts is used in many parts of Europe.
  • Power supplies are rated in watts. The watt describes how much work or how much power can be supplied to various devices. The more devices you have in your computer, the more wattage you will require.
  • You can calculate the system's wattage requirements with the following method:
    • Find the wattage requirements of each individual circuit by multiplying volts by amps (W = V x A).
    • Add the circuit wattage requirements together to find the total system wattage requirement.
  • Power supplies include a fan that helps to cool the system.
    • On older ATX systems, the fan direction blows air into the case and across the CPU.
    • Newer (all current) ATX systems reverse the fan direction to pull air from inside the case (blow air out).
    • System case fans help improve airflow. Current ATX cases typically pull (cooler) air in from the front, where the power supply and additional fans at the rear blow the (warmer) air out.
  • An ATX power supply provides soft power. This is a condition where the motherboard always has power, even when the computer is turned off. This feature enables the operating system to power off the system and enables other features such as power on for network or other events.
  • The power supply includes connectors for powering various computer components. When choosing a power supply make sure it includes the necessary connectors for your motherboard. Specifically, some motherboards and processors require an extra 4-pin and/or 8-pin connector in addition to the main 20- or 24-pin power connector.
  • Power supply connectors are standardized following the ATX specifications. However, some computer manufacturers, such as Dell, produced power supplies with proprietary connectors. In some cases, the connectors are the same as ATX connectors, but the wiring positions might be different. When replacing a power supply, identify whether a standard ATX or a proprietary power supply is required.