[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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>

--- 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>
> 
> >
> > 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.

I know, my comment was meant as "I wish I could wind
separate inductors with that level of repeatability."
:')

> 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'm always keeping my eyes open for the occasional
lucky find. I have some 1% 2W 0.1 Ohm resistors I use
for current measurement, and a 0.1% capacitor, I think
it is 680pF.

> >
> > 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.

> 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.

There's my answer. Yes, it does seem to be much harder
to get rid of parasitic capacitance than parasitic,
uh, inductance? That doesn't seem like the correct
word. It is easier to avoid affecting the
electromagnetic field than to avoid affecting the
electric field.

I've been experimenting with antenna configurations
for a Theremin design I'm working on. This experience
makes me question David Thomson's relying on 0.2pF
difference in readings of several hundred pF when that
much capacitance can be from shifting a hand a few
inches closer even from several feet. The larger the
object, the further away you can affect it by the same
amount of capacitance change. So something as simple
as the curve in the test lead wire can easily affect
it by that much.

> > > 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.

I kinda figured, I even read over what you said
several times to see if I missed you saying series.

Steve Greenfield