# Re: free standing coil

• To: tesla-at-grendel.objinc-dot-com
• Subject: Re: free standing coil
• Date: Thu, 12 Oct 1995 09:30:53 EDT
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```> The method I use to determine Q is the 1/2 power bandwith method:
> 1) find the resonant frequency f0 of your coil with/without terminal
> using a sine wave signal generator and note either the peak current
> into the coil

How do you measure the current into your coil?  I don't think you can
use a regular meter since the frequency response drops off as you go
above 60Hz (at least mine does - at kHz frequencies, my DVOM is
substantially down compared to my VTVM).   Dave Sharpe at the
biased led's to find f0 (a la Duane Bylund) and wiring in couple of
regular (silicon, zener???) diodes somehow to be able to measure the
current directly with a DC (I presume) meter.  For a real no-brainer, once
you find f0, you would simply adjust the output of your signal generator
to give a 10mA (or some other convenient power of 10) reading  on your
meter.  You then change your frequency until the meter reads 7mA (or
.7 x your initial reading).  Assuming that f1 and f2 are symmetric wrt/f0,
this will give you a df value directly to compute Q=f0/(2*df).

Any idea how you would wire the regular diodes up to be able to read
the AC current directly on a DC meter movement?  As an alternative,
and since I like playing with my o'scope, I was going to hook up a
resistor in series with my signal generator feeding the coil base and
measure the voltage across it to get the current (i=v/r).  Any comments
on this technique, either?