Re: measuring inductance (fwd)

Subject:      Re: measuring inductance (fwd)
       Date:  Wed, 30 Apr 1997 15:07:04 +1200
       From:  "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization: Wellington Polytechnic, NZ
         To:  tesla-at-pupman-dot-com

Hi Mark,
           Here's my explanation (I've done it too :)....

> At the moment I am building a tube (valve) Tesla coil. Today I
> decided to measure the inductance of the primary (which is 19 turns
> of wire on a 4 inch diameter piece of PVC ). I put a 0.047
> microfarad capacitor in parallel with the primary and a led across
> it as well. Putting in my signal generator caused the led to stay
> dark except  for at the resonant freq (which was 116 kilohertz
> giving as roughly expected a primary inductance of 0.57 micro
> henrys) when the led brightly lit up.

That'd probably 57uH perhaps?
> Instead of putting my signal generator away I tried doing the same
> measurement again but this time with the coil, capacitor and led
> in series. Since this is a series LCR circuit I expected the led to
> remain dark except for at the resonant freq when it would be
> brightly illuminated. However the led remained dark at all freqs
> (even at 116 kilohertz). Anybody out there know where I went wrong
> with my theory - I'm pretty sure all my connections were good.

What was the internal impedance of the generator?  Adding that 
additional series resistance may well have squished the Q of the 
circuit (Y, N, maybe)? 
    Actually, if there are two things that give a generator a hard 
time it is running off-frequency across a parallel circuit with no 
current limiting, and running in series with a tuned circuit with no 
current limiting. I sometimes feed mine straight into the base of the 
resonator for measuring Q etc. At resonance, and despite it having an 
output impedance of only 7 ohms, its output sags somewhat.