A simple transformer consists of two coils and a core. The coils are called the primary and the secondary. These coils are wound over a common core. Transformers can have either air or magnetic cores. It depends on the frequency at which they are to be operated. The primary winding is connected to the voltage source. It receives energy. The secondary winding is connected to the load resistance and supplies energy to the load.
There are two types of transformers: a step-up transformer and a step-down transformer In a step-down transformer the number of turns in the primary is greater than the number of turns in the secondary. This transformer steps down the voltage In a step-up transformer the number of turns in the secondary is greater than in the primary.
There are 3 main classifications of transformers: for power-frequency circuits, for audio-frequency and for radio frequency.