Power Supply Inefficiency Comes From Its Design

A power supply is a multistep operation. The first step is to reduce the voltage from 120 volts wall power to the level required by the device. The device's plug that you push into the wall receptacle connects the power lines to the computer's power supply transformer.

A transformer is a set of coils. The primary coil is connected to the incoming power, and the secondary coil is connected to the next step in the power supply. Based on the number of windings in the coils, the voltage can be stepped down (in the case of the computer) to the lower level required by solid-state electronics. The amount of reduction is based on the number of turns in the coil.

The alternating current leaves the transformer and is passed to the rectifier. The rectifier uses a switching transistor (and a few other components) to produce unregulated direct current. This output is then filtered by capacitors and other circuitry to minimize electromagnetic interference (EMI) emissions.

There are other components that further shape the power and filter out line noise, but the important thing to understand is that the lowly power supply is a significant user of energy within your computer. Because of the heat it generates, a fan is required to keep it cool.

The typical mean time between failures for a power supply is 100,000 watt hours. The higher the power supply efficiency, the longer the life. Heat is the number one killer of power supplies, and the primary compo nent to fail is the cooling fan. Once the fan stops turning, the power supply components will fail soon thereafter. This is why a computer with a loud fan (which indicates that the fan's ball bearings are failing) needs to be promptly addressed. Often the smell of something burnt signals an imminent power supply failure.

Refer to Figure 4-1 which illustrates a 60 cycles per second wave form. The unused portions of the wave (anything above the dotted line or anything below 0) can be stored briefly in capacitors and/or inductors and released during the down cycle periods. The result is the value at the dashed line - a smooth voltage and current.

The output cables from the power supply are color coded and may vary. However, they are generally 5 and 12 volts. The motherboard and the circuit cards that attach to it are powered by the 5-volt line. The CD drive, fixed disk drives, and cooling fans use the 12-volt feed.

HEAT LOSSES: A TRIPLE WHAMMY

Heat in a power supply is the result of the electrical resistance on the components. The balance to strike is between the use of energy efficient materials and the cost to the consumer.

Most computers do not approach the maximum potential of their power supplies and operate at around a 30% load. Computer power supplies are

Figure 4-1

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