Autotransformer

An autotransformer (sometimes called autostep down transformer) is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting on itself and not to any kind of automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where electrical connections are made. Autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but autotransformers have the disadvantage of not providing electrical isolation.

Autotransformers are often used to step up or step down voltages in the 110-117-120 volt range and voltages in the 220-230-240 volt range—for example. providing 110 or 120V (with taps) from 230V input, allowing equipment designed for 100 or 120 volts to be used with a 230 volt supply (as in using US electrical equipment in with higher European voltages).