# Re: distributed capacitance measurement?

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From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Thursday, September 04, 1997 2:48 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: distributed capacitance measurement?

Hi Jim,

> From:   bmack[SMTP:bmack-at-frontiernet-dot-net]
> Sent:   Wednesday, September 03, 1997 10:09 PM
> To:     tesla list
> Subject:    distributed capacitance measurement?
>
> To All,
>
> How the heck do you MEASURE the distributed capacitance
> of a secondary?

Measure its inductance with an L meter, measure f then calculate Cdist
from that. I don't know of any method of measuring it directly.

> I have a several methods to determine f-res and Q, but don't
> have a clue how you can separate the system into L and C for
> true empirical measurement.
> Unless you are calculating it from the approximate known value
> of L when resonance is determined? Thats what I use,-but that's
> NOT a measurement.

Agreed, but it has to be correct. There is at least one distributed
model around now that agrees with the lumped parameters :)
You could try a capacitance meter hooked between the base and ground.
You would have to compensate for extra lead capacitance. Guess I
should try this myself.

> Another indirect method might be to use lisajous figures
> on a scope to determine the 45 degree point in a LR circuit (the R
> being a variable) at a low and high freqency ( below Fr).  the
> reactance difference would represent it's distributed capacitance,
> at the higher freq used. For best results, get close to Fr.
> As you know, all electronic component values are frequency
> dependent-even resistors!  The problem gets worse as one progesses
> upward into hf -the uper limit for Tesla coils.
>
> The last paragraph is pure speculation on my part. Has anyone tried this?

No. One has to be careful about how the measurement is taken because
mounting the coil upright and connecting the base to a low impedance
turns it into a quite different animal from a non-grounded coil.

Malcolm

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