Transformer is an energy transfer device. It has an input side (primary) and an output side (secondary). Electrical energy applied to the primary is converted to a magnetic field which in turn, induces a current in the secondary which carries energy to the load connected to the secondary. The energy applied to the primary must be in the form of a changing voltage which creates a constantly changing current in the primary, since only a changing magnetic field will produce a current in the secondary.

A transformer consists of at least two sets of winding wound on a single magnetic core. There are two main purposes for using transformers. The first is to convert the energy on the primary side to a different voltage level on the secondary side. This is accomplished by using differing turns counts on primary and secondary windings. The voltage ratio is the same as the turns ratio. The second purpose is to isolate the energy source from the destination, either for personal safety, or to allow a voltage offset between the source and load.

Transformers are generally divided into two main types. Power transformers are used to convert voltages and provide operating power for electrical devices, while signal transformers are used to transfer some type of useful information from one form or location to another.