The system's power supply unit provides electrical power for every component inside the system unit.
It converts commercial electrical power received from a 120-Vac, 60-Hz (or 220-Vac, 50-Hz outside the U.S.) outlet into other levels required by the components of the system. In desktop and tower PCs, the power supply is the shiny metal box located at the rear of the system unit.
There are two basic types of power supplies to be aware of, traditional AT power supplies -designed to support AT-compatible mother boards, and ATX power supplies-designed according to newer ATX design specifications. The AT power supply has two 6-pin mother board power connectors (P8/P9), while ATX power supplies use a single 20-pin power connector. In the AT-compatible power supply, the cooling fan pulls air through the case from the front and exhausts it out the rear of the power supply unit. Conversely, the ATX design pulls air in through the rear of the power supply unit and blows it directly on the ATX mother board.
The desktop/tower power supply produces four (or five) different levels of efficiently regulated dc voltage. These are; +5V, -5V, +12V, and -12V (the ATX design also provides a +3.3V level to the mother board). The power supply unit also provides the system's ground. The +5V level is used by the IC devices on the mother board and adapter cards. The +3.3V level is used by the microprocessor.
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