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Electricity and Basic Electronics
voltage by two. The voltage at the center tap
for this transformer is, therefore, equal to 12 V
divided by 2. The center tap would be labeled
6 V. You could use the ratio formula to get the
same answer.
N
P
___
N
S
=
E
P
___
E
S
2200
_____
120
=
110 V
______
E
S
2200 × E
S
= 120 × 110 V
Divide both sides by 2200 V.
2200 E
S
_______
2200
=
13,200
______
2200
E
S
= 6 V
Example 12-6:
For the transformer in Figure 12-27, deter-
mine the number of turns that would need
to be made before a tap would be soldered
in place to achieve 3 V. Use the following
formula:
E
P
___
E
S
=
N
P
___
N
S
Substitute the known values:
110 V
______
3 V
=
2200
_____
N
S
tapped transformer in Figure 12-27. It has a
110-V primary consisting of 2200 turns of wire
and a 12-V secondary with a center tap. The
center tap divides the number of turns in half.
First, let us calculate how many turns of wire
are on the secondary. Substituting the vari-
ables of the formula with the known informa-
tion, you would have
E
P
___
E
S
=
N
P
___
N
S
110 V
______
12 V
=
2200
_____
N
S
110 V × N
S
= 12 V × 2200
110 V N
S
= 26,400 V
Divide both sides by 110 V.
110 V N
S
________
110 V
=
26,400 V
________
110 V
N
S
= 240
The secondary of the transformer has 240
turns of wire.
Now, let us determine how many turns
would be made before the center tap is soldered
in place. The easy way to fi nd this answer is to
divide the number of turns on the secondary
by two. The center tap would be attached
after winding number 120. To determine the
voltage at the center tap, divide the secondary
Figure 12-26. Even small step-down trans-
formers are used to deliver a range of voltages.
Here is one with taps and an output from 3 V
to 12 V.
Tap
switches
Figure 12-27. This transformer has a center tap
that divides the number of turns in half.
Secondary
E = 12 V
Center
tap
2200
turns
Primary
E = 110 V