Inductance measurements of a flat spiral coil

["Tesla list" <tesla-at-pupman-dot-com>]

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

--- Tesla list <tesla-at-pupman-dot-com> wrote:
> Original poster: "Jim Lux by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>
> 
> > > (more or less) from each other.  This resulted
> in three different
> > > inductances in each of the three wires.  The
> inductances were 6.20mH,
> > > 6.23mH, and 6.25mH.  Oddly enough, each of the
> three wires had pretty
> close
> > > to the calculated inductance!


> For all intents and purposes, if you told me that
> the three windings had the
> same inductance, your measurements would support
> that. 

I was thinking the same thing. Where he puts his meter
leads will affect measurements, too. What is the error
band on this? Those readings span only 0.8%, ie,
+-0.4%! I wish -I- could wind three coils to that
tight a tolerance. ;')

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. 

The idea behind all this was to negate the affect of
any self capacitance.

> Interestingly, if
> you hook the windings in parallel, since they are
> very tightly coupled, I'd
> ballpark the resulting inductance at 9 times that of
> the single winding, or
> around 50-60 mH, just on the N^2 principle.

But- you are connecting them in parallel, not series.
So since the coupling should be as near 1.0 as is
possible, you are ending up with exactly the same
number of windings, not 3 times as many. How is this
different from winding a coil with Litz wire? For
inductance calculations I've always seen Litz wire
treated as if it were just a single wire of equivalent
overall size.

Steve Greenfield

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