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Be aware of the following facts about power supplies:

  • The power supply converts AC current to DC current.
  • Most power supplies have the capacity to receive both 110 and 220 volt power just by toggling a switch 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 provide 3.3, 5, and 12 volts.
  • 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.
  • 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.
  • You must match the power supply with the correct voltage and connectors for your motherboard.
  • 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.
  • Many power supplies have a switch on the back that turns the power on or off.
  • Symptoms of bad power supply include:
    • The system does not start
    • The system shuts off
    • The system reboots
    • Fan does not run or is noisy
  • Never ground yourself when working on a power supply.
  • Use a voltmeter (multimeter) to measure the voltage on internal connectors.
  • Power supplies store dangerous voltages. Never open a power supply. Instead, replace the entire power supply.

The following table shows the common power supply connectors.

Connector Description

20-pin

The 20-pin connector is the main motherboard connector and supplies 3.3, 5, and 12 volts to the motherboard. On older motherboards, the CPU is powered through a 5 volt wire in this connector.
24-pin (20+4 pin)


The 24-pin motherboard connector replaces the 20-pin connector in older motherboards. The additional 4 pins supply an extra wire for 3.3, 5, and 12 volts.
  • Some power supplies use a 20-pin connector with an extra 4-pin connector.
  • Some 24-pin connectors allow you to disconnect 4 of the pins.
  • You can plug a 24-pin connector into a motherboard using a 20-pin connector (leaving the 4 extra pins unconnected).

4-pin +12 volt power (P4)

Starting with the Pentium IV processor (P4), CPUs required more power than could be provided through the main motherboard connector. In addition, processors are powered using 12 volts instead of 5 volts. The 4-pin 12 volt connector supplies 2 additional wires of 12 volt power.

Note: This 4-pin connector is not the same as the 4-pin connector used in conjunction with the 20-pin motherboard power connector.

8-pin EPS +12 volt





The 8-pin EPS connector provides 4 lines of 12 volt power.
  • This connector is used with some older dual processor systems or some newer quad-core processors.
  • Depending on the processor and the motherboard, you might be able to use a single 4-pin connector instead of the 8-pin connector (all 8 pins are typically required for quad-core processors).
  • Some power supplies have two 4-pin connectors (4+4) that are meant to be used together in the 8-pin connector.

6-pin PCI Express




Newer video cards require more power than can be supplied through the PCI Express bus and from the main motherboard connector. The 6-pin PCIe connector, also known as a PEG connector (PCI Express Graphics), plugs in directly to the video card to supply the additional power. The 6-pin connector provides up to 75 watts.

Instead of a 6-pin connector, some PCI Express cards require an 8-pin connector which provides up to 150 watts. Some power supplies have a combined 6-pin and 2-pin connector (6+2).

4-pin accessory power

The 4-pin accessory power connector (often called a 4-pin Molex connector) is used by IDE hard drives, optical drives, and other accessory devices. The connector provides both 5 volts (red wire) and 12 volts (yellow wire).
  • Each power supply cable typically has multiple 4-pin connectors on the same cable.
  • When connecting devices, try to balance the devices connected to each cable.

SATA power cable




The SATA power cable has 15 pins and provides 3.3, 5, and 12 volts. As its name implies, it is used for powering SATA devices.
  • You can use a special adapter to convert a 4-pin Molex connector to a SATA connector.
  • When using an adapter, or on some power supplies, the connector only supplies 5 and 12 volts.

4-pin mini-Molex



The 4-pin mini-Molex connector provides both 5 and 12 volts and is used by floppy drives.

Note: If your power supply does not have some of the required connectors (such as for the CPU, video card, or SATA devices), you can purchase adapters to convert from one connector to another.