RE: Dimensions of my flat spiral coil

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

Original poster: "David Thomson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>

Hi Malcolm,

>> It has a measured inductance of about 35mH.  The calculated inductance
was 5.7mH!

>When a discrepancy like that rears its head, it is wise to start looking
for a reason rather than immediately believe that something anomalous is
happening. Was your formula correct? You've previously stated that your
instrument had trouble reading one coil also.

The formula I used is the L=(NR)^2/(8R+11W).  I don't think this formula
works for large flat spiral secondaries.  Bart says he used acmi and got a
measurement close to the calculation.  I'm still learning this program.

>Oddly? I would have expected them to be roughly the same.

Not everybody did, and neither did I.  However, I now understand why this is
so.  The three wires are merely a single conductor with three times the
copper as one conductor.  It doesn't matter if one wire is used or all three
wires are used, the inductance of one will drive the others.  Essentially I
still have a single wound coil even though three wires are used in the
winding.

This partly explains why I have to run the coil at a higher frequency.

This is interesting in itself.  The three wires essentially add up to a
broad wire, three times broader than a normal wire.  I intend to wind
another coil with copper ribbon such that the wire is taller than it is
wide.  I was referred to a source of flattened magnet wire that appears will
do the job nicely.  Comparing the two coils may provide some useful
information.

>A single wound coil with the same inner and outer diameter *should* have a
much higher inductance than one with the same geometry and wound with three
wires in parallel. There will be three times the number of turns
contributing to the inductance.

That is clear to me now.

Dave