Measuring Inductance
Digital multimeters that measure resistance, voltage,
and current can be purchased at very low cost, and
hobbyists typically have at least one of these in their tool
chest. Inductance meters are more unusual and usually
more expensive.
I don’t have an inductance meter, but I do have a
signal generator and oscilloscope (as most dedicated
hobbyists have), so I use a resonance technique to
measure inductance (Figure 6).
A parallel combination of an inductance and
capacitance is often called a tank circuit by old-timers
since these circuits were often immersed in a tank of
insulating oil in high power transmitters.
Such a circuit will resonate at a characteristic
frequency F given by:
F = 1/(2p(LC)-1/2)
The resonance of the
parallel LC circuit will be
apparent by a peak in
the voltage on the
oscilloscope as the
frequency is changed
(Figure 7).
You don’t need to
carefully graph the
response (as in the
figure) if you’re just
interested in the
resonance frequency.
The capacitor had a
±5% tolerance, so the
calculated inductance
could be accurate only to
±5%. If you have access
to a capacitance meter,
you can do better.
To determine the
inductance, just parallel it
with a known
capacitance and plug the
values into the equation.
The one caveat here
is that capacitor values aren’t exact; they have a tolerance.
The 0.01 µF capacitor that I used in the measurement
shown in Figure 7 had a tolerance code J, which specified
a not-ideal ±5%, but better than most.
This means that the calculated inductance can’t be
any better than this. Some common capacitor tolerance
codes are shown in Table 1.
Reworking the resonance equation to give inductance
yields the following:
L = 1/(4p2L2F2)
Based on the capacitor value of 0.01 µF and the
observed resonance frequency of 72 kHz, the inductance
for the coil shown in Figure 4 was 492 µH.
Plugging in values for the toroid relative permeability
of 1,000, its cross-sectional area, radius, and number of
turns into the toroid inductance equation gives 476 µH, or
a deviation of about 3% — consistent with the capacitor
tolerance.
Conclusion
Winding inductors can be an easy task when you
know a few simple equations and measurement
techniques.
Now that you know the basics, the Internet is a good
resource for further information. NV
48 July/August 2018
FIGURE 7.
Frequency response
of a resonant
parallel
combination of the
toroidal inductor
shown in Figure 4
and a 0.01 μF
capacitor.
Table 1: Capacitor Tolerance Codes.
F ±1%
G ±2%
J ±5%
K ±10%
M ±20%
