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Power Supply Voltages

Refer to Figure 4.21 and notice how +3.3, +5, –5, +12, and –12 volts are supplied to the motherboard. The motherboard and adapters use +3.3 and +5 volts. The –5 volts is seldom used. If the motherboard has integrated serial ports, they sometimes use +12V and –12V power. Hard drives and optical drives commonly use +5 and +12 volts. The +12 voltage is used to operate the device motors found in drives, the CPU, internal cooling fans, and the graphics card. Drives are now being made that use +5V motors. Chips use +5 volts and +3.3 volts. The +3.3 volts are also used for memory, AGP/PCI/PCIe adapters, and some laptop fans. The negative voltages are seldom used.

A technician must occasionally check voltages in a system. There are four basic checks for power supply situations: (1) wall outlet AC voltage, (2) DC voltages going to the motherboard, (3) DC voltages going to a device, and (4) ground or lack of voltage with an outlet tester. A power supply tester can be used to check DC power levels on the different power supply connectors.

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