Chapter 12 Electromagnetic Induction
219
Inductors in Series
When an inductor is shown in a circuit
drawing, it is labeled with a capital L.
Inductors are found in series, parallel, and
series-parallel circuits. In Figure 12-12, three
inductors (L
1
, L
2
, L
3
) are used in series with an
ac power supply.
When analyzing a series circuit for total
inductance, we use the following formula:
L
T
= L
1
+ L
2
+ L
3
+
…
(12-1)
For the circuit shown in Figure 12-12, the
total inductance for this circuit is 9 mH.
L
T
= L
1
+ L
2
+ L
3
= 2 mH + 3 mH + 4 mH
= 9 mH
Example 12-1:
Suppose that there are two inductors in series
with the following values: L
1
= 7 μH and
L
2
= 4 μH. What is the total inductance (L
T
)
in the circuit? Start with the formula for total
inductance in series.
L
T
= L
1
+ L
2
= 7 μH + 4 μH
= 11 μH
The total inductance for this circuit is 11 μH.
Inductors in Parallel
If the same three inductors were found in
a parallel circuit, Figure 12-13, we would use
this formula:
1
___
L
T
=
1
__
L
1
+
1
__
L
2
+
1
__
L
3
+
…
(12-2)
Solving for total inductance, we would fi nd:
1
___
L
T
=
1
__
L
1
+
1
__
L
2
+
1
__
L
3
=
1
______
2 mH
+
1
______
3 mH
+
1
______
4 mH
=
6
_______
12 mH
+
4
_______
12 mH
+
3
_______
12 mH
=
13
_______
12 mH
8. Tape the fi nished coil, leaving both wire
ends outside the tape. Scrape the enamel
from wire ends and solder two hookup
wires to them. Tape the soldered joints to
the coil.
9. Epoxy the core to the cover so that it
cannot move. Allow 24 hours for drying.
10. Install and wire the other parts to the
underside of the cover. Observe the correct
polarity when connecting the silicon diode
(D
1
) and the electrolytic capacitor (C
1
).
To use the magnetizer-demagnetizer, plug
the unit into a 110-V ac outlet. Place the
toggle switch in the magnetizer position. While
depressing the push button, pass the small tool
across the core a few times to magnetize the
tool. To demagnetize a tool, put the toggle switch
in the demagnetize position. While depressing
the push button, pass the tool across the
core several times. The tool will no longer be
magnetic.
Project 12-1: Magnetizer-
Demagnetizer (Continued)
Figure 12-12. Inductors of three different milli-
henry values are used in this ac-powered series
circuit.
L1 = 2 mH
L3 = 4 mH
L2 = 3 mH