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Re: Measurements of a cap's ESL, ESR (fwd)



---------- Forwarded message ----------
Date: Mon, 25 May 1998 08:52:12 +1200
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Measurements of a cap's ESL, ESR (fwd)

Hi Gary,

> Date: Fri, 22 May 98 08:44:23 EDT
> From: Gary Lau  22-May-1998 0834 <lau-at-hdecad.ENET.dec-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Measurements of a cap's ESL, ESR
> 
> All:
> 
> I have done an experiment to determine the ESR and ESL parameters of my
> tank capacitor.  This is a surplus unit, .01 uF, 100KVDC, 2.5"x5"x10",
> marked F-C-I, KM14-1000-10, .01 MFD-100KVDC, DEC 1983.
> 
> To measure this, I hooked my cap to a 50 Ohm sine wave generator, in
> series with a 54 Ohm resistor.  I monitored the voltage across the cap
> with a scope and tuned for minimum voltage.  The applied voltage was 14V
> P-P.
> 
> I saw two minimums.  The sharpest minimum was at 1.9 MHz, with Vc = 0.3V.
> This corresponds to an ESL value of 0.702 uH.
> 
> There was also a lesser minumum at 15.4 MHz, Vc = 5.4V, with a slight
> maximum at 14.4 MHz, 15.0V.  Not sure what caused this.
> 
> For calculating ESR, the voltage across the 54 Ohm R at 1.9 MHz was 13.6V
> while Vc was 0.3V.  Assuming the Vc at resonance is due entirely to
> I*ESR, ESR comes out to 1.19 Ohms.  However, I think this measurement is
> flawed (way too high) as the 0.3V waveform was distorted and likely due
> to harmonics from the signal generator.
> 
> 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?

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