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Measurements of a cap's ESL, ESR (fwd)
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From: Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: Monday, May 25, 1998 11:56 PM
To: Tesla List
Subject: Re: Measurements of a cap's ESL, ESR (fwd)
Hi Gary,
> From: Gary Lau 25-May-1998 1447 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
> Sent: Monday, May 25, 1998 2:25 PM
> To: tesla-at-pupman-dot-com
> Subject: Re: Measurements of a cap's ESL, ESR (fwd)
>
> >> From: Gary Lau 22-May-1998 0834 <lau-at-hdecad.ENET.dec-dot-com>
> ><snip>
> >> I'm thinking that ESL is the more significant parameter in terms of being
> >> a predictor of a cap's usefulness in Tesla coil service. I was wondering
> >> if anyone else had made similar measurements of home-made or commercial
> >> caps that I could compare my figures to?
> >
>
> >From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
> >I would say that ESR is by far the more important parameter of the
> >two. It directly relates to internal dissipation in the cap. ESL is
> >only a tiny fraction of total tank L. Any R = a power loss. I haven't
> >measured any of my Tesla caps but will make a note to do so when time
> >permits.
> >
> >Malcolm
>
> Hi Malcolm:
>
> I agree, ESR is certainly related to internal power dissipation, but I
> thought the real culprit in cap self-heating was dielectric losses? I
> didn't mention this previously because I had no idea of how this is
> measured, given that it is frequency-dependant. My (flawed) technique
> for measuring ESR could only work at the cap's self-resonant frequency.
> Or is ESR and dielectric losses actually the same thing?
Right on. ESR is the equivalent series resistance and takes into
account *all* sources of dissipation in the circuit. It can be
transformed into an equivalent parallel resistance by 1+Q^2.
> While a cap's ESL is certainly small as compared to the tank inductor, I'm
> seeing in PSpice simulations that parasitic tank inductances outside of
> the primary (like ESL) have a profound effect on things.
>
> Terry Fritz's measurements using fiber optic probes have shown that a gap's
> conduction ceases briefly at each zero current crossing at the tank Fres.
> Each reignition of the gap excites high frequency oscillations due to
> parasitic L's and C-self of the primary. I believe significant losses
> occur due to this, and this is the reason it's so important to have very
> short and beefy tank wiring, minimizing stray L's.
>
> If ESR of a cap is high, then it may well be the more significant
> parasitic and I was wrong to discount it, especially if it's another name
> for dielectric losses.
I agree. You can see this in stark detail if you compare the behaviour
of TCs with primary caps which have different loss tangents. Total
loss impinges directly on circuit Q. If it makes a difference, it
matters and should be taken into account. Compare caps with the same
construction and same plate area, size etc. but with different
dielectrics. TC output shows the differences up all right :)
Cheers,
Malcolm