Ferrite-core transformers used in switching power supplies can have large capacitances
effectively in parallel with a winding, which make it difficult to measure the
winding's inductance. Typically, if the capacitance is the result of a reflection
from some secondary, it's unremovable, and direct measurement of an inductance,
L_{x} , in the presence of a parallel capacitance, C_{x} , is
corrupted by C_{x} on many inductance meters. Moreover, direct measurement
of C_{x} in the presence of L_{x} is virtually impossible. But
with this AGC-controlled oscillator circuit, both L_{x} and C_{x}
can be measured, each without any corrupting influence from the other.

In the test circuit (*see the figure*),
switch S_{1} connects any one of four capacitors (C_{1} through
C_{4}) into the circuit. Then, with S_{2} open, rotate R_{3}
clockwise from its full CCW position until a 2-V pk-pk output signal is obtained.
If oscillation can't be achieved, return R_{3} to full CCW, close S_{2}
, and try again.

Next, the frequency is measured and recorded. Then switch S_{1} is
set for any one of the remaining capacitors and the aforementioned process is
repeated.

As an example, say that capacitors C_{1} and C_{2} were used
to obtain frequencies f_{1} and f_{2}. L_{x} and C_{x}
are found from the following equations:

j = 1, 2, 3, or 4

Four capacitors are provided so that this process can be repeated for up to
six combinations of capacitor values as a double check of the results. If any
of the six calculations differ significantly from the others, a frequency measurement
error may have to be corrected. Accurate knowledge of C_{1} through
C_{4} is a must—they must be measured carefully. It's recommended
that their given values be further refined to account for circuit capacitance,
as follows: Construct a test coil of approximately 200 turns of wire on a diameter
of approximately nine inches. This creates an inductance with an L_{x}
value that will remain essentially constant under variable levels of excitation.

Connect that coil in parallel with some capacitor of accurately known value,
say 0.01 mF. Measure the L_{x} and C_{x}
values of this combination as described earlier. Adjust the assumed values of
C_{1} and C_{4} for the narrowest range of calculated values
for L_{x} and the most accurate value of C_{x} possible. A simple
Basic program can be used to automate the calculations for the test results
(*see the program listing*).