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Power Supply Unit (PSU)

A quality power supply is essential to a PC, especially for those who overclock or have numerous components, extensive cooling systems, and multiple hard drives installed. A power supply is the small, metal box usually located at the back of the PC that converts the AC current from your home to the DC current needed by the PC. The power supply is plugged into an eletrical outlet and then steps down the 110 volts (or 220) into the appropriate voltage and supplies it to the components in a PC. ATX-based power supplies are the standard now used in the PC industry. For help with power supply upgrades, see our Upgrading Power Supply Guide.


There are four units of measure for electricity:

  • Voltage
    Potential differences in an electrical charge creates a force called voltage. Voltage is measured in units called volts.

    An ATX power supply provides five voltages:
    • +12v
    • -12v
    • +5v
    • -5v
    • +3.3v
  • Wattage
    The total amount of power needed to operate a component is wattage. Wattage is measured in watts with the formula Wattage = Volts * Amps. For example, 110 volts multiplied by 10 amps is 1,100 watts. Most new video cards and PC components recommend a power supply rated 350 watts and above.
  • Amps
    The amount of electricity flowing through an electrical system is called current. Electrical current is measured in amperes or amps. An ammeter is used to measure electrical currents in amps.
  • Ohms
    Electrical resistance is measures in Ohms. Resistance is a property that opposes the flow of electricity. The flow of electrons decreases as resistance increases and vice versa. Resistors are used to resist the flow of electricity.

Dell Power Supply Facts

Dell has, in the past, been known to use non-standard pinouts for a number of their power supplies. For systems with these power supplies a standard ATX PSU will not work. You will need to either purchase the correct PSU from Dell, or a Dell specific PSU from PC Power & Cooling. However, their are a few models of Dell PCs that use a standard ATX power supply. They are listed below. Note: Even if your Dell uses a standard ATX PSU, the dimensions of your case may not allow for a ATX size PSU (specifically, the on/off switch if the PSU has one) without modifications to the case. Keep this in mind when searching for a Dell power supply replacement.

Dell Models Known to Use Standard ATX Power Supplies

  • Dell Dimension Models
    • 2200
    • 2300
    • 2350
    • 2400
    • 4300
    • 4400
    • 4500
    • 4550
    • 8200
    • 8250
  • OptiPlex Models
    • GX60
    • GX150
    • GX240
    • GX260
    • GX270
  • Other Models
    • PowerEdge 600SC
    • WorkStation 340
Standard ATX Power Connector (P1)
The text color represents the color of the wire. Compare this ATX power connector with a Dell's and if they match you have a standard ATX PSU.

EMI (Electromagentic Interference)

EMI can cause problems with PCs as well as radio and television reception. Power supplies are shielded to prevent any problems with EMI. EMI is caused by the magnetic field created from the flow of electricity.

Warning: You should never open the casing of a power supply. Capacitors in a PSU can hold a deadly charge for long periods of time after they have been removed from a PC. If for some reason you need to have a power supply serviced, it should only be performed by a qualified and experienced electrical technician.

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