Transformers use electromagnetic induction to change the voltage of alternating currents. The voltage and current changes can be calculated, as the power transfer is constant.
The ratio of potential differences on the transformerAn electrical device that increases, or decreases, the potential difference (voltage) of an alternating current. coils matches the ratio of the numbers of turns on the coils.
This equation can be used to calculate what the output might be from a particular transformer, or to work out how to design a transformer to make a particular voltage change:
Vp is the potential difference in the primary (input) coil in volts (V)
Vs is the potential difference in the secondary (output) coil in volts (V)
np is the number of turns on the primary coil
ns is the number of turn on the secondary coil
In a step-up transformer, Vs > Vp. In a step-down transformer, Vs < Vp.
Example
A mains (230 volt) transformer has 11,500 turns on its primary coil and 600 turns on its secondary coil. Calculate the voltage obtained from the secondary coil.
\(\frac{V_p}{V_s} = \frac{n_p}{n_s}\)
Rearrange to find Vs:
\(V_s = V_p \times \frac{n_s}{n_p}\)
\(V_s = 230 \times \frac{600}{11,500}\)
voltage from secondary coil, \(V_{s} = 12~V\)
The transformer in the example above is a step-down transformer. This is because there are fewer turns on the secondary coil, and there is a smaller voltage on the secondary coil.