What is the main purpose of a transformer?

A transformer increases (steps up) or decreases (steps down) alternating-current voltage using two coils of wire wound around a shared iron core. The ratio of turns between the primary and secondary coils determines how much the voltage changes. Transformers work only with AC because they rely on a changing magnetic field. They are essential to the power grid, stepping voltage up for efficient long-distance transmission and down to safe levels for homes. Note a transformer changes voltage but does not convert AC to DC (that is a rectifier's job, using diodes).