Re: Measurements of a cap's ESL, ESR (fwd)
---------- Forwarded message ----------
Date: Mon, 25 May 1998 09:03:15 +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)
> Date: Sat, 23 May 1998 05:47:53 PDT
> From: Bill the arcstarter <arcstarter-at-hotmail-dot-com>
> To: lau-at-hdecad.ENET.dec-dot-com, tesla-at-pupman-dot-com
> Subject: Re: Measurements of a cap's ESL, ESR
> Gary Lau wrote:
> >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.
> No kidding! I was doing something like this last night too!
> Synchronicity? :)
> My work last night served as a "reality check" using lumped
> low-frequency parts before I go measure some real (possibly
> distributed?!) coiling components. Gotta crawl before I can run...
> Here's what I did:
> Found a generic green polyester cap - measured 0.47 uf (not a pulse cap)
> Found a plastic spool of 20 gauge wire - used this as my air-core
> inductor. This measured as 0.293 mH. with a DC resistance of 0.51 ohms.
> The above measurements were performed using my Alfa LCR-24 meter which
> is within about 2% (according to the calibration shop at work).
> I placed the cap and inductor in parallel, and drove the cap via my
> 600-ohm output freq generator (with an additional 330 ohm in series for
> current monitoring purposes). I monitored the cap voltage using my Tek
> 543B oscilliboatanchoroscope.
> The computed vs. measured resonant frequency were mostly in agreement
> (at about 13700 Hz)
> At resonance, this LC network acted as a pure resistance of about 38
> ohms (measured input current and voltage etc). This did NOT initially
> match what theory would indicate - ie - net resistance is L/(RC)
The problem is that you are mostly measuring the ESR of your signal
generator in series with the 330 Ohm resistor. A parallel circuit
*must* be driven by an infinitely high impedance (current source).
Try driving the circuit with 1 MOhm in series with the generator and
wee what you get. A current transformer can be used in series with
the tank with a low value of burden resistor for monitoring
circulating currents (presents a very low ESR in the tank).
> (See my derivation of that result at:
> http://www.geocities-dot-com/CapeCanaveral/Hangar/6160/resonan1.html )
> To make a long story short - this particular cap has a resistive
> component measured as 15 ohms at 13.7KHz. Shockingly high.
> The resistive component of the inductor was measured (at 13700 Hz) as
> 1.5 ohms - so there was an additional 1 ohm due to (?) skin effect or
> eddy current in the wire... or something???
> Once I'd characterized these additional resistive components - I
> recalc'd the L/(RC) impedance value - which then fell into reasonable
> agreement with the observed results.
> Today I'll go measure my homemade poly plate cap - I'll report my
> >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?
> Really? Why wouldn't ESR be a better metric? FYI - Our Maxwell pulse
> cap has a stated nameplate ESL of 0.060 uH...
Quite agree :)