Most transformers consist of two coils of insulated wire. One coil, known as the primary winding, is connected to the source of the voltage that is to be changed. This voltage is the input voltage of the transformer. The other coil, called the secondary winding, supplies the output voltage to the desired circuit. In most transformers, the primary and secondary windings are wound around a hollow core made of thin iron or steel sheets.
Most cores have the shape of a ring or a square.
The two wire coils are not connected to each other. When the input voltage is applied to the primary winding, it generates alternating current in the coil. As the current flows, it sets up a changing magnetic field in the core of the transformer. When this changing magnetic field passes through the secondary winding, it produces alternating voltage in the coil. If the secondary winding is connected to a circuit, the output voltage causes alternating current to flow through the circuit.