Re: Inductance measurements of a flat spiral coil

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

>
> 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. ;')

With a trifilar winding, you could, fairly easily... I'd almost suggest that
the trifilar wound coil could be used as a measurement standard, against
which you could check your measurement equipment.

On a measurement note, for calibrating it is fairly easy to generate very
precise magnetic fields.  A Helmholtz coil set (two parallel circular coils,
spaced the radius( or some such distance?) apart, generates a uniform field
in the center.  The magnitude of the field can be determined with an
accurate ammeter and a ruler.  If you build the coil form out of plywood,
and are reasonably careful with the dimensions and construction, accuracies
of much less than 0.1% are possible.  The accurate ammeter will be a problem
with casual equipment.  Perhaps a precision resistor measured with a
calibrated voltmeter.  There are very accurate inexpensive (<$50) bandgap
voltage references available for calibrating the voltmeter, although some
care is necessary if measurement technique errors aren't to dominate.



>
> 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...).

Actually, a precision inductor is probably easier to make (it's easier to
get rid of magnetic stuff near the coil than electrostatic stuff...), and
the theory is sufficiently well known that all the parasitic C effects can
be figured out.


>
> 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.

Oops again.. you're right, I was thinking series, and typing parallel...
too fast on the Send button.