Re: A Question About LCR Circuits.

To: tesla-at-poodle.pupman-dot-com
Subject: Re: A Question About LCR Circuits.
From: Tesla List <tesla-at-stic-dot-net>
Date: Wed, 14 May 1997 00:43:38 -0500
Approved: tesla-at-stic-dot-net

Subject:     Re: A Question About LCR Circuits.
       Date: Wed, 14 May 1997 11:30:00 +1200
       From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization: Wellington Polytechnic, NZ
         To:  tesla-at-pupman-dot-com


Hello all,
            I decided to satisfy myself that I was correct in what I 
said on this subject so have done the necessary experiments (stuff 
the theory :).
Materials: L = 685uH gapped ferrite cored inductor
           C = 1nF polystyrene capacitor
           R = a selection of resistors
Method: 
        Build a resonant circuit using only L and C. The signal 
generator was shunt connected with a 100kOhm resistor in series with 
the hot lead and the scope was connected across the cap (probe R = 
10MOhm, probe C = 10pF).  SQRT(L/C) = 827 Ohms near as. Fres = 186kHz
near as.
    I then connected a range of resistors one at a time in series with 
the circuit and checked for resonance. Using 0.5 = SQRT(L/C)/R says
that the resonance should disappear when ESR = 1.6kOhms approx and 
that is exactly what I observed. I got a typical 1st order response 
when looking across the capacitor.
    For the unwary, there is a surprise in store if you put the scope 
across the coil however. The self-C of the windings plus scope showed
a resonance at around 540kHz. Disconnecting the 1nF cap and series-
R and probing across the inductor alone showed a strong resonance at 
560kHz since most of the damping resistance was now removed.

    I too would like to say that experiment and theory go hand in 
hand. I will not use formulae I have not personally verified or know 
not to be correct.

Malcolm