The classic method for determining self capacitance of an inductor is to
measure the resonant frequency (freq) with several values of parallel
capacitance.(C)
Making a plot of 1/(freq)squared on the vertical axis v. resonating
capacitance on the horizontal axis yields a straight line(or should!!) that
intersects the HORIZONTAL or capacitance axis at a negative C value when
extrapolated.
The negative C value is the self capacitance of the inductor in question.
As well, the slope of the straight line is a measure of the TRUE
inductance value of the coil and can be taken from the relation L (henries)
= 0.0253M, where M is the slope of the line.(freq in MHz and C in pF).
A more practical method based on the above is the F/2F method where the
inductor is resonated at an initial frequency, F1, the added parallel C
value measured (C1). The inductor is then re-resonated at TWICE the first
frequency, say F2, and again the resonating C measured(C2).
The inductor self-capacitance is then given by
Cself = (C1 - 4C2)/3.
Measuring the inductance and then the parallel resonating capacitance and
trying to predict the stray C from the required result correction will only
give an approximate answer, as will direct measument of the self-resonant
frequency (and thence to the self-C) but for accurate work the F/2F method
is better.
73
Dave
ZL3FJ
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