Transformer is a device that transform AC voltage, current and impedance. It uses the principle of electromagnetic induction to transform AC power of one voltage level into AC power of another voltage level. When an AC current is passed through the primary coil, an AC magnetic flux is generated in the iron core, causing a voltage (or current) to be induced in the secondary coil. It is composed of an iron core and a coil. The coil has two or more windings. The winding connected to the power supply is called the primary winding, and the remaining windings are called the secondary winding.
Transformer mainly uses the principle of electromagnetic induction to work. The specific process is when an AC voltage U1 is applied to the primary side of the transformer, and the current flowing through the primary winding is I1, the current will generate alternating magnetic flux in the iron core, which makes the primary winding and the secondary winding electromagnetically connections.

