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Lumped resonance
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From: Bill the arcstarter [SMTP:arcstarter-at-hotmail-dot-com]
Sent: Thursday, May 28, 1998 8:36 AM
To: MALCOLM-at-directorate.wnp.ac.nz; tesla-at-pupman-dot-com
Subject: Lumped resonance
Malcolm, coilers,
Malcolm wrote:
>From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
>Hi Bill,
>> 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.
>
>> 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 )
>>
Malcolm - I have a hard time seeing this. Why would the net impedance
of the LRC circuit be affected by the driver's impedance? I made no
assumptions about any driver impedance in the derivation...
I would think that in my particular case, my LRC circuit will act like a
38 ohm resistance (at the specific resonant frequency) independent of
the driver's impedance.
Perhaps your comments assumed I was trying to measure the "Q" of the
circuit by observing a resonant voltage rise, etc. That wasn't the plan
- just wanted to check the impedance at resonance. I'd appreciate any
clarification!
PS - In his book "Troubleshooting Analog Circuits", (c) 1991, Robert
Pease published a letter by a Mr. Malcolm Watts of Wellington
Polytechnic, New Zealand. I assume that is you? :) Small world - ain't
it? :)
-Bill Pollack (arcstarter)