Re: RF inductor modeling for the 21st century

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Barry,

On 12 Oct 2001, at 8:09, Tesla list wrote:

> Original poster: "B2 by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<bensonbd-at-erols-dot-com>
> 
> Hi All,
>     Found an interesting article in the September 27, 2001 issue of EDN
> magazine on page 67.  Click on the URL below and look under "Design
> Feature" to read the article or download a PDF version.
> 
> http://www.e-insite-dot-net/ednmag/index.asp?layout=issueTOC&pubdate=9/27/2001
> 
> The new model uses an inductor in series with a resistor.  This is in turn
> in parallel with a capacitor in series with a resistor.  The core is
> represented by a resistor in parallel with it all.
> 
>     The article finds inaccuracy with the Coilcraft model on their website.
>  The Coilcraft data is considered to be accurate, though.
> 
>     The main features of the article are the graphs.  They are very
> interesting graphs of impedance and Q vs frequency.  There is an impedance
> peak at the self resonant frequency.  The Q drops but does not go to zero
> at the self resonant frequency (we know this from measurements).  Below the
> coil's self resonant frequency the impedance drops precipitously and the Q
> reaches a peak.  This implies that there is a definite size (huge) for the
> toroid to maximize the coil's Q (if the model obtains).  There could be
> drawn further inference that the best primary should have a peak Q where
> the secondary Q peaks to minimize system losses.  This may seem trivial in
> light of spark gap loss investigations.  It may be useful, though, when the
> art of designing triggered pressurized spark gaps is perfected.  The new
> 1kA, 15 kV SCRs are also interesting.
> 
>     This article may or may not apply to larger inductors like Tesla coil
> resonators.  It would be interesting if such a simple model could be
> "adjusted" to work.

It does apply to TC resonators. I found this phenomenon at work in a 
series of measurements a few years ago. The limiting factor in a TC 
resonator is wire size. Suppose you start out with a resonator whose 
wire is, for example, six skin depths thick at its non-topload 
resonant frequency. As you add topload capacitance, Q rises, then 
peaks, then falls away again as the wire losses are pushed up by the 
corresponding drop in frequency. From the Q consideration, there is 
definitely an optimum value for top capacitance for a given 
resonator. But also important is the L/C ratio. If this starts out 
being small, Q is compromised right from go.
     This leads to a less than arbitrary approach to coil design that 
I alluded to in a post last week. Starting point for a design becomes 
required output voltage and charge availability (derived from total 
secondary energy). 

Regards,
Malcolm