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

["Tesla list" <tesla-at-pupman-dot-com>]

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: "Paul Nicholson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
> 
> For the benefit of anyone trying to follow this thread, I'll just
> summarise Antonio's recipe for an equivalent circuit of a
> TC secondary:
>... 
> Feed a signal into the top of the coil and find the frequency in
> between F1 and F2 at which the impedance dips to a minimum.

Fp is the frequency where the input impedance has poles, a maximum,
not a minimum.
Measuring from the other side, with the base open, the same poles
would appear too, but there is no need to change the connection
of the signal generator at the base. This measurement is sensitive
to parasitic capacitances at the base, as you mentioned. The measurement
of the top voltage is also sensitive to parasitic capacitances.

>...
> Limitations:
> 
> This equivalent circuit does not reproduce the input or output
> impedance of the coil.  Nor does it correctly represent the
> relationship between the base current (or top volts) and the
> stored energy in the inductance (or capacitance).  Also it does
> not reproduce the relationship between the input impedance and
> Q factor.

The model reproduces well the input impedance, and quite well
the output impedance too, even without the Ldc-L1-L3 inductance.
As a lossless model, it doesn't model the Q factor, but with
a resistor added at the input (series) it would model it too.

>...
> [zeros of Zft due to mutual M]
> > Ok that they can be at high frequency, beyond the frequency band
> > where the model works.
> 
> Perhaps.   Requires more thought. I never use the model above about
> 9/4 waves.

What do you mean by "9/4"?
 
> > there is the problem of how are the two inductors coupled to the
> > primary. A 3 coils transformer? Too complicated. The only connection
> > point correctly modeled is the base, and so the model is suitable
> > only as model for a third coil of a magnifier.
> 
> Agreed.  I think we'd have to repeat the process by starting with
> a transfer impedance relating primary current to secondary top volts,
> and then fit in the various measured poles and zeros.

Seems possible.

>...
> I suspect that we end up with different k factors for the pri-sec
> and sec-pri coupling, and that the circuit behaviour will be as if
> the overall k was the square root of the product of these two
> separate k factors.  I'm unconvinced that this makes complete sense,
> so work is ongoing.

A transformer must be reciprocal. Different coupling coefficients
in the two directions cause it to be active, generating or dissipating
energy. A model containing one transformer and a c-l-c pi section
at the output side seems possible, obtainable from similar measurements
using the primary coil as input. 

Antonio Carlos M. de Queiroz