autotransformer

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was it that difficult?

http://en.wikipedia.org/wiki/Autotransformer

Typically, a power transformer will consist of (at least) two electrically isolated coils wound on a common iron core.

An autotransformer consists of a SINGLE coil on an iron core. Typically input voltage is applied across the full coil. the autotransformer output voltage is developed between a third terminal and one of the input voltage terminals. It is the magnetic equivalent of a three terminal restance potentiometer.