12.1: Theory Overview

A power transformer is used to change an AC voltage from one amplitude to another, ideally without power loss. This is accomplished through a magnetic circuit consisting of a metallic core wrapped with primary and secondary windings of wire. The ratio of the number of primary windings to secondary windings is called the turns ratio. The voltage at the secondary can be increased or decreased depending on this ratio. In the ideal case, or lossless transformer, the product of secondary voltage and current will equal the product of primary voltage and current. That is, the ideal transformer does not dissipate power itself, but rather transforms power from one scenario to another. Real transformers dissipate some power because the copper wires have finite resistance and the magnetic coupling is not 100% efficient. The lost energy is often found in the form of heat. Another important characteristic of the transformer is that it creates electrical isolation between the primary and secondary. In other words, the circuit common points do not have to be the same potential or tied together between the primary and secondary sides.

Typically, power transformers are rated for a given input voltage and frequency (120 VAC/60 Hz in North America) which yields a specified secondary voltage under load. If the load current is minimal, the secondary voltage tends to increase beyond the rated value. This is due to the resistance of the windings and can be reduced by using a larger gauge although this results in a larger transformer. Also, it is common for secondaries to be split or to have a center tap. A center tap allows the secondary to be treated as two symmetrical halves. This is useful for circuit rectification circuits. Finally, dots drawn on the transformer’s schematic symbol and connections indicate like instantaneous polarity on the primary and secondary. That is, when the primary voltage is positive at its dot, the secondary voltage will also be positive at its dot.