The principle of transformer is to use the principle of electromagnetic induction to transfer electric energy or signal from one circuit to another.

According to Faraday's Law of Electromagnetic Induction and Lenz's Law, this is simply stated as follows: when the current in the original coil (that is, the coil with electricity) increases, the magnetic field generated by the coil in the iron core increases (the direction of the magnetic field can be judged by the right-handed helix rule), then the secondary coil (that is, the coil without electricity) becomes stronger. The current in the coil is induced in the opposite direction to the current in the original coil (the result is that the current in the secondary coil produces a magnetic field in the opposite direction to the current in the original coil). When the current in the original coil decreases, the magnetic field generated by the current in the iron core decreases, and the current in the secondary coil is in the same direction as the current in the original coil. The magnetic field generated by the current in the iron core is in the same direction as the magnetic field generated by the original coil in the iron core. As the current changes, the current in the secondary coils is generated. This is the working principle of the transformer.