autotransformer

An autotransformer (sometimes called autostep down transformer) is an electrical transformer with only one winding. The "" (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 sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which are not connected.