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Electronic power supply

An electronic power supply, often referred to somewhat incorrectly as an AC adaptor, is an electronic device that produces direct current of a particular voltage and current from wall-socket power. The direct current is usually used to power the internal circuits of an electronic device, such as a computer.

A simple linear power supply uses a transformer to convert the alternating current from the wall to the desired voltage. An array of diodes then rectifies the alternating current to direct current. Finally, a low-pass filter smooths out the high-frequency ripple that is left after the rectification.

In a switching power supply the incoming power is passed through a transistor array that inverts (changes the positive voltage to negative and vice-versa) hundreds of thousands of times per second. This means that a smaller, less expensive, lighter transformer can be used, because the alternating current is being made to alternate faster, and thus there is less time for the magnetic core of the transformer to saturate.

The most exotic power supplies are used in aviation to enable reliable restarting of a stalled engine.

In jet transports, the engine is restarted from the power produced by the other engine(s), 400 Hz three-phase AC. Most of the starting torque generated by the engine's motor generator is provided by the current at the peak of the alternating current.

If the aircraft electronics used simple linear power supplies, they would steal current from this peak, because the diodes would conduct only during the peaks of the voltage. This could prevent the pilot from restarting an engine in an emergency.

Therefore, aircraft power supplies take energy evenly from all parts of the AC. They do this by using a switching power supply, but leaving the transistors on longer near the bases of the wave, and shorter on the peaks. This trick is called "power factor correction," and is the reason why some modern power supplies are computer-controlled.

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