Transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer's core, which induces a varying electromotive force (EMF) across any other coils wound around the same core. Electrical energy can be transferred between separate coils without a metallic (conductive) connection between the two circuits.

Transformers are used to change voltage levels, such transformers being termed step-up or step-down type to increase or decrease voltage, respectively. Transformers can also be used to provide isolation between circuits as well as to couple stages of signal-processing circuits. transformers have become essential for the transmission, distribution, and utilization of alternating current electric power.