The transformer is a device that uses the principle of electromagnetic induction to change the AC voltage. Its main components are the primary coil, secondary coil and iron core (magnetic core). The main functions are: voltage conversion, current conversion, impedance conversion, isolation, voltage stabilization (magnetic saturation transformer), etc. It can be divided into: power transformer and special transformer (electric furnace transformer, rectifier transformer, power frequency test transformer, voltage regulator, mine transformer, audio frequency transformer, intermediate frequency transformer, high-frequency transformer, impulse transformer, instrument transformer, electronic transformer, reactor, mutual inductor, etc.) according to the purpose. The circuit symbol usually uses T as the beginning of the number. For example, T01, T201, etc.