In a standard desktop computer, the power supply converts AC line voltage to DC, providing several carefully regulated low voltages at high current currents. Power supplies can be constructed in a number of ways, but linear and switching power supplies are two technologies relevant to this discussion. (More information on outdated technologies, such as large mechanical motor-generator systems and ferroresonant transformers.) Typical linear power supplies use bulky power transformers to convert 120V AC to a low AC voltage, a diode bridge to a low DC, and then a linear regulator to reduce the voltage to the desired level. A linear voltage regulator is an easy to use, inexpensive transistor element that converts overvoltage into waste heat, resulting in a stable output. The design and construction of a linear power supply is almost trivial. However, a big drawback is that they typically waste around 50-65% of their power as heat, usually requiring large metal radiators or fans to remove the heat. The second drawback is that they are big and heavy. On the plus side, components in a linear power supply (except transformers) handle only low voltages, and the output is very stable and noise-free.
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