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Re: Inductance measurements of a flat spiral coil



Original poster: "Steve Greenfield by way of Terry Fritz <twftesla-at-qwest-dot-net>" <alienrelics-at-yahoo-dot-com>

See below

> > > I used to measure inductors the hard way: I had
> a
> > box
> > > with an oscillator in it, and two 5 way binding
> > posts.
> > > I could put just about any LC across it and it
> > would
> > > put out a nice sine wave. I used it, a frequency
> > > counter, and a calculator to measure inductors.
> > I'd
> > > usually use a 470pF or 1000pF cap with an
> unknown
> > > inductor, then add a 1% 100pF cap across it. I'd
> > then
> > > plug those two frequencies into a formula I'd
> > worked
> > > up from the standard resonance formula, and get
> a
> > > pretty accurate inductance reading.
> > 
> > But how accurate is that 1000 pF?  Capacitors are
> > very difficult to make to
> > high precisions (better than 0.1%). Lots of
> > confounding factors.  A coaxial
> > tube arrangement might work well, and the C can be
> > determined from
> > calculation (except for end effects, but you could
> > build two of different
> > lengths...).
> 
> But the way I do it, the 1000pF doesn't matter! :'))
> That is the beauty of this method. All that matters
> is
> the accuracy of the capacitance you add.
> 
> Ignoring resistive losses: A given inductor in an LC
> resonant circuit will shift frequency a fixed Hz
> when
> a 100pF cap is added, regardless of other parallel
> capacitance in the circuit, including parasitic,
> self,
> and circuit capacitance. Maybe I should do a webpage
> on this.
> 
> Just use some algebra with the resonant LC formula.
> One formula uses f1, the other f2. One uses L1*C1,
> the
> other L1*(C1+C2), where C2 is the precision 100pF
> capacitor.

Oh, yeah, I don't have to put up a website. Here's
one!

http://www-dot-netidea-dot-com/~fredn/theremin/thersens.htm

This was to figure frequency change for a Theremin,
but it works for this just the same. My algebra skills
have gotten a bit rusty. I hadn't realized just how
rusty until I tried to derive my equation again.

Steve Greenfield