Potential difference
The ratio of potential differenceThe potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. Units = volts (V). This is the voltage between two points that makes an electric current flow between them. 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 potential difference (pd) change:
\(\frac{pd~across~primary~coil}{pd~across~secondary~coil} = \frac{number~of~turns~in~primary~coil}{number~of~turns~in~secondary~coil}\)
This is when:
- the potential difference in the primary (input) coil is in volts (V)
- the potential difference in the secondary (output) coil is in volts (V)
In a step-up transformer, the potential difference in the secondary coil is greater than the potential difference in the primary coil. In a step-down transformer, the potential difference in the primary coil is greater than the potential difference in the secondary coil.
Example
A mains transformer (230 V) has 11,500 turns on its primary coil and 600 turns on its secondary coil. Calculate the potential difference obtained from the secondary coil.
\(\frac{pd~across~primary~coil}{pd~across~secondary~coil} = \frac{number~of~turns~in~primary~coil}{number~of~turns~in~secondary~coil}\)
First rearrange the formula:
\(pd~across~secondary~coil = pd~across~primary~coil \times \)
\(\frac{number~of~turns~in~secondary~coil}{number~of~turns~in~primary~coil}\)
\(pd~across~secondary~coil = 230 \times \frac{600}{11,500}\)
\(pd~from~secondary~coil = 12~V~(2sf)\)
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 potential difference on the secondary coil.