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



---------- Forwarded message ----------
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

Coilers,
 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)

(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 
findings.

>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...

Comments welcome!
"Reverse engineering the world
 one step at a time
 for fun and profit"
-Bill Pollack (arcstarter)

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