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Re: distributed capacitance measurement?




From: 	Geoff Schecht[SMTP:geoffs-at-onr-dot-com]
Sent: 	Friday, September 05, 1997 10:48 AM
To: 	Tesla List
Subject: 	Re: distributed capacitance measurement?

A caveat about using capacitance meters in gneeral, they're only really
accurate at the frequency of interest. In other words, don't use one of
those cheapo little digital C-meters and expect to get accurate answers;
they usually work at low frequencies (using something like an integrator
runup time measurement) and that doesn't account for field fringing effects
that occur at the actual frequency of operation. 

I agree with Malcolm' s comment below. I can't think of a way to measure
interwinding capacitance directly with a meter, myself. I don't think that
measuring the capacitace from the coil's base to ground is quite the same
thing but that would certainly affect the secondary's resonant frequency.

If you can latch onto an old Boonton or HP RX meter, you'll obtain the
"best" answers since they can be used to bridge an unknown at its actual
operating frequency. Malcolm's & Dr. Resonance's method (which I read after
this e-mail arrived) of just finding the resonant frequency (a grid-dip
meter's good for that) and then finding the inductance (on a good bridge at
that measured frequency for the best accuracy!) then allows you to back the
effective value of C out of the formula: C=1/(L*(2*pi*f)^2).

Geoff

----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: 'Tesla List' <tesla-at-poodle.pupman-dot-com>
> Subject: Re: distributed capacitance measurement?
> Date: Thursday, September 04, 1997 10:02 PM
> 
> 
> 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
> 
> 
>