Chapter 12 Electromagnetic Induction
227
Step-Down Transformers
Next, look at a step-down transformer,
Figure 12-25. In a step-down transformer,
the voltage in the secondary is less than the
voltage in the primary. Suppose we apply 100
V to a primary winding of 80 turns and the
secondary winding has 10 turns. We can use
the same formula to fi nd the output:
N
P
___
N
S
=
E
P
___
E
S
80
___
10
=
100 V
______
E
S
Cross multiply by multiplying 80 by E
S
and
10 by 100 V.
80 × E
S
= 10 × 100 V
80E
S
= 1000 V
Divide both sides by 80:
80E
S
_____
80
=
1000 V
_______
80
E
S
= 12.5 V
The secondary voltage is 12.5 V.
Example 12-5:
Suppose you have a transformer with 1000
turns on the primary windings and 250 turns
on the secondary windings. The primary’s
voltage is 600 V. What is the voltage in the
secondary winding (E
S
)?
N
P
___
N
S
=
E
P
___
E
S
1000
_____
250
=
600 V
______
E
S
Cross multiply by multiplying 1000 by E
S
and
250 by 600 V.
1000 × E
S
= 250 × 600 V
1000 E
S
= 150,000 V
Divide both sides of the equation by 1000.
E
S
=
150,000 V
_________
1000
E
S
= 150 V
The secondary voltage is 150 V.
The majority of the transformers that you
will encounter in electronics will be step-
down transformers. Circuitry with solid-state
devices requires low voltages. Phone circuits
use about 40 V, and battery chargers used
for home tools use less than 20 V. Step-down
transformers adjust the voltage to the proper
amount for these devices.
Tapped Transformers
If you want to use a transformer that can
cover a full range of voltages, you must use
a tapped transformer. A tap is a lead that
is connected or soldered to the wire in the
secondary. It allows the turns ratio between
the primary and secondary to be changed so
that a voltage lower than the full voltage of the
secondary can be used.
Look at the transformer in Figure 12-26. It
is designed to provide voltages that range from
3 V to 12 V. The voltages increase in incre-
ments of 1.5 V. To achieve these increments,
the transformer has taps that change the
ratio of winding between the primary and
secondary.
To understand how taps work, let us
examine the turns ratio and output of the
Figure 12-25. A simplifi ed step-down trans-
former. The output voltage (E) is less than the
voltage in the primary since there are fewer turns
in the secondary winding.
Secondary
E = ?
80
turns
10
turns
Primary
E = 110 V
AC