Primary and Secondary Coil

In its most elementary form, a transformer consists of two coils wound of wire and inductively coupled to each other. The left-hand winding is assumed to be connected to a voltage source and is called the primary winding. The right-hand winding is connected to a load and is called the secondary.

Voltage induced in the secondary coil may be greater than or less than the primary voltage, depending on the ratio of primary to secondary turns. A transformer is termed a step-up or a step-down transformer accordingly.