226
Electricity and Basic Electronics
reducing eddy currents, and eliminating
short circuits are the only ways to produce
an effi cient transformer.
Step-Up Transformers
Transformers are widely used in elec-
tronics because they can change the source
voltage to the voltage required for a circuit.
The transformer shown in Figure 12-24 has
100 turns of wire in the primary winding
and 600 turns in the secondary winding.
Remember that the number of turns of wire
in the coil of an inductor affects the amount
of inductance. If we connect a 110-V ac source
to the primary windings, 660 volts of ac are
induced in the secondary. This is an example
of a step-up transformer because the output
voltage in the secondary winding is greater
than the voltage in the primary winding. The
relationship between voltage and the number
of turns can be expressed by the following
equation:
NP
___
N
S
=
E
P
___
E
S
(12-4)
where N is the number of turns and E is the
voltage. Using the previous example, here
is how we fi nd the voltage in the secondary
windings (E
S
):
N
P
___
N
S
=
E
P
___
E
S
100
____
600
=
110 V
______
E
S
Cross multiply by multiplying 100 by E
S
and
600 by 110 V.
100 × E
S
= 600 × 110 V
100E
S
= 66,000 V
Now divide both sides of the equation by 100:
100E
S
______
100
=
66,000 V
________
100
E
S
= 660 V
With 110 V in the primary, this transformer
provides 660 V in the secondary. You can see
by this increase why this is called a step-up
transformer.
Example 12-4:
Suppose there are 150 turns in the primary
winding and 750 in the secondary winding
of a transformer. If the primary is carrying
80 V, what is the voltage of the secondary
windings (E
S
)?
N
P
___
N
S
=
E
P
___
E
S
150
____
750
=
80 V
_____
E
S
Cross multiply by multiplying 150 by E
S
and
750 V by 80.
150 × E
S
= 750 × 80 V
150E
S
= 60,000 V
Divide both sides of the equation by 150:
150E
S
______
150
=
60,000 V
________
150
E
S
= 400 V
The output of this transformer is 400 V.
Figure 12-24. A simplifi ed step-up transformer.
The output voltage (E) is greater than the voltage
in the primary since there are more turns in the
secondary.
Secondary
E = ?
600
turns
100
turns
Primary
E = 110 V
AC