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Transformers
A transformer is a device that changes the
voltage. Transformers only work with AC
(alternating current) electricity. They do
not work with DC electricity, and they do
not change AC into DC.
There are two types of transformer:

step-up transformers increase the
voltage

step-down transformers decrease the
voltage
Step-down transformers are used in
everyday items such as mobile phone
chargers, radios and laptop computers.
Inside a transformer
A transformer consists of two coils of wire,
wound around an iron core. The AC input
passes through the primary coil, and the
AC output passes through the secondary
coil. Note that the two coils are not in
electrical contact with each other.
Step-up transformers
 In a step-up transformer, the number
of turns on the secondary coil is
greater than the number of turns on
the primary coil.
 The ratio of the number of turns
determines how much the voltage
will be increased. For example, the
voltage is doubled if the secondary
coil has twice as many turns as the
primary coil.
Step-down transformers
 In a step-down transformer, the
number of turns on the primary coil is
greater than the number of turns on
the secondary coil.
 As with the step-up transformer, the
ratio of the number of turns
determines how much the voltage
will be increased. For example, the
voltage is halved if the primary coil
has twice as many turns as the
secondary coil.
Isolating transformers
Isolating transformers are designed to
improve the safety of some electrical
circuits, rather than change the voltage.
Different transformers
You should be able to recognise and draw
the circuit symbol for a transformer.
For example, they are used in shaver
sockets found in bathrooms. Since there is
no electrical contact between the mains
supply to the primary coil and the
secondary coil, the sockets are not ‘live’.
This means that you cannot get
electrocuted if you touch them.
The number of turns on the primary coil
in an isolating transformer is the same as
the number of turns on the secondary coil,
so there is no change in voltage.
Transformers in the National Grid
The National Grid is used to distribute
electricity around the country from power
stations to consumers. Different types of
transformers are used:


Step-up transformers increase the
voltage from the generators at the
power station for supply to the
transmission cables in the Grid
Step-down transformers reduce the
voltage from the transmission cables
to a safer level for domestic use (in
homes) and commercial use (in
factories and shops)
The National Grid
Less power is lost when electricity is
transmitted through the National Grid at
high voltages. Take a look at the following
two sections - the equations and worked
examples can help you to understand this.
Power loss and current
The power loss is related to the current
flowing and the resistance of the wire:
power loss = current2 × resistance
where:
power loss is measured in watts, W
current is measured in amps, A
resistance is measured in ohms, Ω
For example, if the current is 3 A and the
resistance is 2 Ω, the power loss is:
32 × 2 = 9 × 2 = 18 W
On the other hand, if the current is
doubled to 6 A, the power loss is:
Electricity from a power station goes to:
62 × 2 = 36 × 2 = 72 W

Step-up transformers

High-voltage transmission lines

Step-down transformers
Note how much greater the power loss is
when the current is increased. This means
that it is best to transmit the electricity at
low currents so less power is lost. To do
this, a high voltage is needed.

Consumers
Some power is lost as heat by the
National Grid. This happens from the
transformers and from the cables.
Voltage and current
Assuming that a transformer is 100%
efficient:
Transformer calculations
This equation links the number of turns to
the voltages:
VpIp = VsIs
voltage across the primary coil/
voltage across the secondary
number of turns on the primary coil/
coil =
number of turns
where:
on the secondary coil
Vp is the voltage across the primary coil in
volts, V
For example, a transformer has 20 turns
on its primary coil and 100 turns on its
secondary coil. If the required output
voltage is 60 V, what is the input voltage?
Ip is the current flowing through the
primary coil in amps, A
Vs is the voltage across the secondary coil
in volts, V
voltage across primary coil =number of turns on
primary coil/
number of turns on secondary coil x
voltage across secondary coil
Is is the current flowing through the
secondary coil in amps, A
voltage across primary coil = 20/100 x 60
= 1200/100 = 12 V
For example, if the voltage across the
primary coil is 12 V and the voltage across
the secondary coil is 240 V, what is the
output current when the input current is
20 A?
Is = VpIp / Vs = 12 x 20 / 240 = 240 / 240 =
1A
Transformers - Alternating current
Transformers only work with AC
(alternating current) electricity. The
alternating current induces (causes) a
continually changing magnetic field
around the primary coil. The iron core
concentrates this field and transfers it
throughout the core.
The continually changing magnetic field
moving through the secondary coil
induces a continually changing voltage
there. If the secondary coil is connected
to an external circuit, an alternating
current flows