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The power supply is one of the most
important parts of the computer to understand. The power supply unit
provides electrical power for every component inside the system unit.
In the past, it also supplied Alternating Current (AC) to the display
monitor. Some power supply units that can supply AC power can still be
found. These units are identified by the existence of two power plugs
at the rear. As mentioned in the previous chapter, the computer power
supply plays the critical role of converting commercial electrical
power received from a 120-volts AC, 60-Hz (or 220-volts AC, 50-Hz
outside the United States) outlet into other levels required by the
components of the computer. The power supply unit also provides the
system ground.
In both the desktop and tower style case, the power supply is a
shiny metal box that is located at the rear of the system unit. It
has a large bundle of cables (red, yellow, black, and so on)
emerging from it.
There are two basic types of power supplies:
There are two major distinctions between the legacy AT and the new ATX power supplies. The AT power supply has two 6-pin motherboard power connectors (P8/P9), while ATX power supplies use a single 20-pin power connector (P1). In the ATX-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 AT design pulls air in through the rear of the power supply unit and blows it directly on the AT motherboard. Table
The power supply produces four (five in the ATX) different levels of well-regulated DC voltage for use by the system components. These are +5V, -5V, +12V, and -12V. In ATX power supplies, the +3.3V level is also produced and is used by the second-generation Intel Pentium processors. The IC devices on the motherboard and adapter cards use the +5V level. Table 
summarizes the use of each DC voltage level produced by computer power
supplies, and the power supply form factors where these are produced.
It is important to be able to identify the uses for each voltage level and the corresponding color-coded wire. This will allow testing of the wires using a multimeter to determine if there are problems with the power supply. It is important to note that the computer power supply produces a voltage only when it has a load. In other words, some component must be running on the machine before a voltage can be found in the power cable connectors that supply power to the internal components. Never attempt to repair a defective power supply. Capacitors inside a power supply box store electricity and will discharge through the body if touched. The capacitor holds the electricity even if the unit is turned off and disconnected from a power source. Generally, defective power supplies are replaced rather than repaired. The voltage levels, mentioned earlier, are available for use through the motherboard's expansion slot connectors. Motherboard power connectors provide the motherboard and the individual expansion slots with up to 1 ampere of current each. The power supply delivers power to the motherboard and its expansion slots through the motherboard power connectors. The ATX motherboard connector is a 20-pin (P1) keyed connector. It is keyed so that it cannot be connected incorrectly. Note that the Pentium 4 type connectors are different from the normal ATX (that is, Pentium II). This information is typically contained in the motherboard manual from the manufacturer or automatically detected by the on-board BIOS.
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The cables provide power to the components of the system
unit and its peripherals.
gives a summary of some important factors to be
considered when shopping for a power supply. The Chapter,
"How Computers Work," can be reviewed for additional
information. 
