Transformers are made based on electromagnetic induction. It consists of an iron core stacked with silicon steel sheets (or silicon steel sheets) and two sets of coils wound around the iron core. The iron core and the coils are insulated from each other without any electrical connection.
The coil connecting the transformer and the power supply side is called the primary coil (or the primary side), and the coil connecting the transformer and the electrical equipment is called the secondary coil (or secondary side). When the primary coil of a transformer is connected to an AC power source, changing magnetic lines of force are created in the iron core.
Since the secondary coil is wound on the same iron core, and the magnetic line of force cuts the secondary coil, an induced electromotive force must be generated on the secondary coil, causing a voltage to appear at both ends of the coil. Because the magnetic field lines are alternating, the voltage of the secondary coil is also alternating. And the frequency is exactly the same as the mains frequency.
It has been proved theoretically that the voltage ratio between the primary coil and the secondary coil of the transformer is related to the turns ratio of the primary coil and the secondary coil, which can be expressed by the following formula:
Primary coil voltage / secondary coil voltage = primary coil turns / secondary coil turns
Explain that the more turns, the higher the voltage. Therefore, it can be seen that the secondary coil is less than the primary coil, which is a step-down transformer. The opposite is a step-up transformer.