Inductors in series
As long as the magnetic fields around inductors do not interact, inductances in series
add like resistances in series. The total value is the sum of the individual values. It’s important
to be sure that you are using the same size units for all the inductors when you
add their values.
Problem 10-1
Three 40-μH inductors are connected in series, and there is no interaction, or mutual
inductances, among them (Fig. 10-3). What is the total inductance?
You can just add up the values. Call the inductances of the individual components L1, L2,
and L3, and the total inductance L. Then L = L1 + L2 + L3 +40 = 40 + 40 = 120 μH.
Problem 10-2
Suppose there are three inductors, with no mutual inductance, and their values are 20.0
mH, 55.0 μH, and 400 nH. What is the total inductance of these components if they are
connected in series as shown in Fig. 10-3?
First, convert the inductances to the same units. You might use microhenrys, because
that’s the “middle-sized” unit here. Call L1 = 20.0 mH = 20,000μH; L 2 = 55.0μH;
L3 = 400 nH = 0.400 uH. Then the total inductance is L = 20,000 + 55.0 + 0.400 uH =
20,055.4 μH. The values of the original separate components were each given to three
significant figures, so you should round the final figure off to 20,100 μH.
Note that subscripts are now used in designators. An example is L2 (rather than
L2). Some engineers like subscripts, while others don’t want to bother with them. You
should get used to seeing them both ways. They’re both alright. If there are several
inductors in series, and one of them has a value much larger than the values of the
others, then the total inductance will be only a little bit more than the value of the
largest inductor
Note that subscripts are now used in designators. An example is L2 (rather than
L2). Some engineers like subscripts, while others don’t want to bother with them. You
should get used to seeing them both ways. They’re both alright. If there are several
inductors in series, and one of them has a value much larger than the values of the
others, then the total inductance will be only a little bit more than the value of the
largest inductor.
