# Measuring self-capacitance directly (Re: flat secondary)

```Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:

> Original poster: "Metlicka Marc by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <mystuffs-at-orwell-dot-net>

> Tonight i will sweep for fo, double check selfC with my general radio
> capacitance bridge and do some playing around, also try to get Paula to
> count turns for accuracy (after counting the 3k coils turns twice for
> me, this may be difficult?), I will post results.

How are you measuring the self-capacitance?

I was verifying if it's possible to measure directly something
related to the self-capacitance of a coil. I used a secondary
coil that I know that has a self-capacitance of 5.6 pF (obtained
by measuring the main resonance frequency of the coil, and
consistent with Medhurst's formula).

First I verified if my capacitance meter can measure capacitances
accurately with a high resistance in series with the capacitor.
I simply picked a 180 pF capacitor (a Leyden jar), connected one
terminal to the meter, and held with one hand one terminal of the
meter and with the other hand the free terminal of the capacitor.
I obtained correct measurements when measuring far from sources
of interference, as computers or TVs.

Then I put the coil over an insulating support with height similar
to the height of the coil above the wooden floor, connected one
terminal of the meter to one terminal of the coil, and held the
other meter terminal in my hand, while sitting on the floor (so
I was "grounded").

Before the measurement, I connected the meter terminal that would go
to the coil to an insulated area of the coil form, and set the meter
to zero with the wires at that position.

With the connection to the coil made, the meter read 10 pF, with the
coil in any position. At half the distance from the ground, the meter
reads 11 pF. The readings are stable and consistent. If I approach
my free hand to the coil, the capacitance increases, as expected.
The same if I mount the coil closer to the ground.

*****
Conclusion: It is really possible to measure the self-capacitance
of a coil with a direct measurement of the body capacitance of
the insulated coil. The body capacitance is about twice the "self-
capacitance" of the coil, as it should be (the correct lumped model
for a floating coil has two "self-capacitances", going to ground,
one at each side of the coil).
*****

This was discussed in the list some time ago, but I had not verified
yet.

By the way, a direct reading across the coil leads to overrange in
all scales in my meter (expected, as these meters typically try to
charge the capacitor with a DC current and measure the time taken to
reach a certain voltage).

Also, to use the power line ground as "ground" in the measurement
is useless. Too much interference, and the meter reads noise.

Antonio Carlos M. de Queiroz

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