Be aware of the following facts about power supplies:

1

• 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.

Be aware of the following facts about power supplies:

2

• 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.

Be aware of the following facts about power supplies:

3

• 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.

Be aware of the following facts about power supplies:

4

• 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.