Transformers are used to increase or decrease the voltage of alternating currents. A transformer consists of two coils of wire wound on a metal core.

Diagram of a transformer showing the a.c input, the magnetic filed in the core and the a.c. output.

An alternating voltage is applied to one coil (the primary coil). This causes a changing (alternating) magnetic field to be set up in the core. The other coil (the secondary coil) is in this changing magnetic field and so it has an alternating voltage induced in it.

The size of the secondary voltage depends on the number of turns on both the primary and the secondary coils, and on the size of the a.c. voltage applied across the primary coil.

The relationship linking the number of turns on each coil to the voltages is,

\frac{\text{V}_{1}}{{\text{V}_{2}}} = \frac{\text{N}_{1}}{{\text{N}_{2}}}


  • V1 is the voltage applied to the primary coil in volts
  • V2 is the voltage induced in the secondary coil in volts
  • N1 is the number of turns on the primary coil
  • N2 is the number of turns on the secondary coil

Worked example

Calculate the voltage obtained from the secondary coil of a mains (230 V) transformer, which has 11,500 turns on its primary and 600 turns on its secondary.

\frac{\text{V}_{1}}{{\text{V}_{2}}} = \frac{\text{N}_{1}}{{\text{N}_{2}}}

{\text{V}_{2}} = {230}\times\frac {600}{11,500} = {12{\text{ V}}}