Variable AC Power Supplies

Because controlled electrical energy is useful for a number of things, power supplies are one of the most popular types of electronic test equipment. Let’s take a look at variable alternating current, or AC, power supplies.

It’s important to analyze equipment that is exposed to under- and over-voltage conditions when testing electronics which are powered by the AC line. When many heavy loads are simultaneously using the line AC line voltage variations can exceed plus or minus ten percent, which is why designers often want to test beyond ordinary AC line voltage variations in order to account for potential stresses and to discover weaknesses.

A variable AC power supply is valuable in these situations. These types of power supplies are also very useful for brownouts — i.e. low line-voltage conditions — as they can be used to boost line voltage to regular levels. Another application for variable AC power supplies is boosting the voltage when a given load is connected to a long extension cord, causing the voltage to drop substantially.

Various AC voltages are supplied by a transformer or an autotransformer. The power supply uses switches to change voltage when the transformer has multiple windings or taps. In other situations you use a variable transformer or adjustable autotransformer (also known as a variac) to change the voltage. Certain variacs have meters for monitoring current, power, and voltage.

Some power supplies combine isolated transformers and variable transformers. This type of power supply enables you to test for AC leakage and includes an adjustable power supply for soldering irons.


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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