Design and use of transformers

A transformer is a device that can change the potential difference or voltage of an alternating current (ac):

  • a step-up transformer increases the potential difference
  • a step-down transformer reduces the potential difference

Structure of a transformer

A basic transformer is made from two coils of wire; a primary coil from the ac input and a secondary coil leading to the ac output. The coils are not electrically connected. Instead, they are wound around an iron core. This is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil.

An iron core transformer, with the primary coil on the left hand side and secondary coil on the right hand side.Components of a transformer

When a transformer is working:

  1. a primary potential difference drives an alternating current through the primary coil
  2. the primary coil current produces a magnetic field, which changes as the current changes
  3. the iron core increases the strength of the magnetic field
  4. the changing magnetic field induces a changing potential difference (voltage) in the secondary coil
  5. the induced potential difference produces an alternating current in the external circuit
Transformers can only work with alternating current.