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III Power Supplies
The term power supply
in the PC is actually a bit of a misnomer. Todays power supply
is a sealed box, with a cooling fan and power
leads. Inside the power supply box is a step down transformer
that converts 120 or 240 volts AC to approximately
12 and 5 volts AC. From this point, the AC voltage
is run through four diodes filtering off the negative
voltage. This is known as a bridge rectifier.
Newer power supplies use switching transformers
in place of the diodes to accomplish this. They are known as
switching power supplies.
From there, the power is passed
to a capacitor, which outputs clean DC power.
Inside the case of the power supply are only two
more parts. They are a fuse and a fan. The fan
is used to pull air through the computer case as well
as to cool components of the power supply itself.
Figure 18: How power is converted from household current to 5 and 12 Volts DC. Voltage steps down from transformer on the left to 12 Volts AC, then the four diodes in the center (the diamond shape) convert to positive only voltage. The --| (-- is the symbol for the capacitor that filters the AC ripple to straight DC voltage. The same process occurs for the 5 Volts.
(Actually, the modern PC power supply is actually a switched power supply. See http://www.smpstech.com/tutorial/t01int.htm#SMPSDEF for details. The process described above is still popular with wall warts used for telephones, small CD players, etc.)
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