Power transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled electrical conductors without changing its frequency. A changing current in the primary circuit creates a changing magnetic field which induces a changing voltage in the secondary circuit and the changing magnetic field also induces an electromotive force (EMF) across each winding. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF". By appropriate selection of the numbers of turns, a transformer thus allows an alternating voltage to be stepped up or stepped down.