Electrical Power Supply

Poor power line voltage regulation causes lights in a home to dim each time the refrigerator starts. Similarly, if a change in the current from a power supply causes the voltage to vary, the power supply has poor voltage regulation. Most electronic equipment will perform best when it is supplied from a nearly constant voltage source. An uncertain supply voltage can result in poor circuit performance.

Analysis of a typical power supply's performance is simplified by modeling it as a constant-voltage source in series with an internal resistance. The internal resistance is used to explain changes in the terminal voltage when the current in a circuit varies. The lower the internal resistance of a given power supply, the more current it can supply while maintaining a nearly-constant terminal voltage. An ideal supply for circuits requiring an unvarying voltage with changing load current, would have an internal resistance near zero. A power supply with a very-low internal resistance is sometimes called a "stiff" power supply.

An inadequate power source almost always compromises the performance of electronic equipment. Audio amplifiers, for example, may produce distorted sound if the supply voltage drops with each loud pulse of sound. There was a time when the pictures on television sets would shrink if the AC-line voltage fell below a minimum value. These problems are less significant now that voltage regulation has been included in most power supplies.

There are two approaches that may be used to improve the voltage regulation of a power supply. A simple power supply that is much larger than required by the average equipment demand will help. A larger power supply should have a lower effective internal resistance, although this is not an absolute rule. With a lower internal resistance, changes in the current supplied are less significant and the voltage regulation is improved compared to a power supply operated near its maximum capacity.

Some power supply applications require a higher internal resistance. High-power radar transmitters require a power source with a high internal resistance so that the output can be shorted each time the radar transmits a signal pulse without damaging the circuitry. Television receivers artificially increase the resistance of the very high voltage power supply for the picture tube by adding resistance deliberately. This limits the current that will be delivered should a technician inadvertently contact the high voltage which might otherwise deliver a fatal electrical shock.