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Re: distributed capacitance measurement? Isotropic?


From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Sunday, September 07, 1997 4:21 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: distributed capacitance measurement? Isotropic?

Hello Fr MaGahee,

> From:   Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
> Sent:   Saturday, September 06, 1997 10:50 AM
> To:     Tesla List
> Subject:    Re: distributed capacitance measurement? Isotropic?
> 
> 
> > > 
> > > 
> > > 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.
> > > 
> 
> You would be measuring something more akin to the isotropic
> capacitance of the coil surface (much as in measuring the isotropic
> capacitance of a topload). 

That is exactly what you want to measure because that is exactly how 
it works. The inter-turn capacitance plays a very small part in the 
overall scheme of things when you use a coil as a grounded resonator.
Look at the model for example. It works and it agrees with the real 
world.
 
> The distributed capacitance can only be determined indirectly using
> resonant frequency and L.
> 
> Also, the frequency at which the capacitance meter operates may skew
> the results. I would imagine that if the poor capacitance meter was
> using a frequency equal to the self-resonant frequency of the coil
> that it would give a large error in measured capacitance. There are
> several different methods used by capacitance meters to measure
> capacitance. Large values of C can be measured using a fixed current
> and measuring the resultant ramp time for the voltage to reach a
> given value. Smaller capacitance values are generally measured by
> passing a fixed frequency signal of known amplitude through the
> capacitor under test and measuring the resultant AC current. When
> measuring pure capacitance that works just fine. Unfortunately, it
> fails miserably if there are other factors involved, such as
> inductance or excessive leakage. At resonance the AC current flow
> will be QUITE different than that at any other frequency!
> 
> An interesting side issue here is what effect the isotropic
> capacitance of the coil has on the total distributed capacitance.
> Isotropic measurement would not measure inter-wire capacitance, which
> definitely has an effect on the total distributed capacitance. The
> question is how the isotropic capacitance of the coil relates to the
> total distributed capacitance. Is it small compared to the total
> distributed capacitance, or does it predominate?
> 
> Malcolm, I know you have done a fair amount of work with tight-wound
> and also with space-wound coils. Have you noticed anything odd about
> the total distributed capacitance of space-wound versus tight-wound
> Tesla secondaries? 

The self capacitance is virtually independent of wire spacing!! Yes, 
it is absolutely true. The lumped parameters always work. If you use
the measured lumped inductance and measured F you will find that for 
a given former, Cself always come out to be the same.
   IMHO, the key is that in using what _looks_ like a coil as a 
grounded resonator turns it into a different animal.

> Is there any correlation between distributed
> capacitance and exposed wire surface? Or is it primarily an
> inter-turn phenomenon? What does that have to say about the relative
> merits of tight versus space windings? My own work has led me to
> believe that the exposed surface area of a coil is very important,
> especially in regard to the electrostatic phenomena that accompany
> Tesla coil operation. Probably much greater in its overall EFFECTS
> than the inter-turn capacitance. By the same token, the proximity of
> the inside of the coil windings and the coil FORM result in
> interactions that greatly affect the Q of a coil. The losses are
> probably capacitive in nature, in that energy is capacitively coupled
> to the material that makes up the form.
> 
> For what it's worth...
> Fr. Tom McGahee

Malcolm 
<snip>