Transformers
A transformer is a device that has an AC voltage applied to its input to produce an
AC voltage with a different magnitude at the output. It consists of two coils of wire, known
as the primary and secondary, that are usually linked by an iron core. The magnitudes of the
voltages at the input and output are related to the number of turns of wire on the primary and
secondary coils by the following formula
V1 N 1
=
V2 N 2
Where
V1 is the input voltage of the transformer as applied to the primary coil
V2 is the output voltage of the transformer as provided by the secondary coil
N1 is the number of turns of wire on the primary coil
N2 is the number of turns of wire on the secondary coil
Key term
A transformer is a device that has two multi-turn coils wound on an iron core. It is used to
increase or decrease the magnitude of AC voltages.
Basic structure of a transformer
The transformer shown in figure U9.1 is known as a step-up transformer as it has a greater
number of turns on the secondary coil than on the primary coil. This means that V2 is greater
than V1.
A step-down transformer has fewer turns on the secondary coil than on the primary coil. This
means that V2 is less than V1.
Knowledge and understanding of the internal working of a transformer is not required in
VCE Unit 3 Physics. This topic is studied in Unit 4.
Sample problem
A transformer has 2000 turns of wire on the primary coil and 250 turns of wire on the
secondary. What is the output voltage if the input voltage is 240V AC?
Jacaranda Physics 2, 2nd edition
© John Wiley & Sons Australia, Ltd 2004
1
Solution
V1 = 240 V
V2 = ?
N1 = 2000
N2 = 250
V1 N 1
=
V2 N 2
V2 =
V1 N 2
N1
240 × 250
2000
= 30
=
So the output voltage is 30 V
Additional questions
1. What is the role of a transformer in an electric circuit?
2. A student wishes to obtain a 12 V AC voltage from a 240 V AC supply.
(a) Which type of transformer should he choose; step-up or step-down?
(b) Compare the number of turns of wire in the primary coil with the number of turns
on the secondary coil in this case.
Jacaranda Physics 2, 2nd edition
© John Wiley & Sons Australia, Ltd 2004
2