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The basic purpose of a transformer, as its name would suggest, is to transform AC (alternating current) from one voltage to another voltage. Transformers can be "step up" in which case the voltage output is greater than the input to the transformer or "step down" in which case the output voltage is less than the input voltage. A transformer works by the process of magnetic induction. An AC voltage at the input creates a magnetic field inside the transformer, and this magnetic field induces an AC current to flow at the output. In the most basic design, a transformer will have "primary windings" at the input and "secondary windings" at the output. These windings can be nothing more than coils of wire, with the greater the number of turns in the coil representing the higher voltage. In this manner, a step up transformer will have more turns in its output coil (secondary windings) than in its input coil (primary windings), and vice versa for a step down transformer.
Transformers are generally rated by an intended input voltage which yields an expected output voltage, and also by the size of the load (in amps) that the transformer can be expected to be able to handle. A transformer can have multiple connections, or taps, on both the input and the output, each at a different voltage, and it can also be variable by way of an adjustment dial. The standard transformer has separate primary and secondary windings and thus offers a certain level of isolation between the input and the output. An autotransformer is a type of transformer whereby the input and the output are both connected to the same coil at different places on the coil, and so it does not afford the same level of isolation as a standard transformer. One common form of autotransformer is the variac, which is also a variable transformer. Typically, you might input your standard 120 VAC into a variac, and its output could be anywhere between 0 and 130 VAC by virtue of its adjustment dial setting. The term "Variac" was a variable autotransformer trademark belonging to General Radio up until 2002, and Instrument Service Equipment received a trademark for the term in 2004. However, the term variac remains in general usage to refer to almost any variable autotransformer.
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