Electrical Power Supply

Voltage-regulated power supplies feature circuitry that monitors their output voltage. If this voltage changes because of external current changes or because of shifts in the power line voltage, the regulator circuitry makes an almost instantaneous compensating adjustment.

Two common approaches are used in the design of voltage-regulated power supplies. In the less-common scheme, a shunt regulator connects in parallel with the power supply's output terminals and maintains a constant voltage by wasting current the external circuit, called the load does not require. The current delivered by the unregulated part of the power supply is always constant. The shunt regulator diverts almost no current when the external load demands a heavy current. If the external load is reduced, the shunt regulator current increases. The disadvantage of shunt regulation is that it dissipates the full power the supply is designed to deliver, whether or not the external circuit requires energy.

The more-common series voltage regulator design depends upon the variable resistance created by a transistor in series with the external circuit current. The transistor's voltage drop adjusts automatically to maintain a constant output voltage. The power supply's output voltage is sampled continuously, compared with an accurate reference, and the transistor's characteristics are adjusted automatically to maintain a constant output.

A power supply with adequate voltage regulation will often improve the performance of the electronic device it powers, so much so that voltage regulation is a very common feature of all but the simplest designs. Packaged integrated circuits are commonly used, simple three-terminal devices that contain the series transistor and most of the regulator's supporting circuitry. These "off the shelf" chips have made it very easy to include voltage regulation capability in a power supply.