Re: Why the difference in Fo WinTesla & Fantc?

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

Original poster: "Paul Nicholson by way of Terry Fritz <teslalist-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Brad K wrote:

 > my coil is pretty well totally built off of Win tesla's program
 > to resonate at 500Khz.  Putting the same info into Fantc shows a
 > secondary resonate at 580-610Khz

Thanks for pointing out this discrepancy.  I've a couple of comments:

First, the primary inductance,

 > Win Tesla .0097.  Using formula.0054mH and I was answered fantc
 > showed .0055 I believe.

Something is not right there.  As far as inductance calcs go, there
isn't much room for variation, one program to another. I think we'd
expect better agreement between any two programs on this.

And the secondary Fres,

 > Fantc, (as long as I inputed everything right) shows the resonance
 > test at 589-610Khz!!!

That's quite a big discrepancy.  In principle, fantc should be more
accurate here, because it can take better account of the geometry.

But fantc can be in error for narrow secondary coils such as yours,
perhaps by 10% or more, tending to predict a higher frequency than the
actual resonant frequency of the coil.

The problem occurs when computing the self capacitance of the
secondary.  Fantc doesn't take account of the PVC coil former, and for
narrow coils, this dielectric will increase the C over and above the
value given by the program, hence the actual Fres will be lower than
fantc says.

WinTesla is also likely to be in error too, because it probably uses
the wrong value of inductance.  I suspect it uses the DC inductance
in its Fres calcs, whereas the actual inductance effective near
resonance is a bit less.  Therefore it will probably underestimate
the Fres.

So I think your actual Fres will come out higher than WinTesla says,
but lower than Fantc says. My guess is 5 or 8% higher than WinTesla,
and maybe 10% less than Fantc.  So 550kHz might be about right - but
that's just a guess.

 > sorry for asking so many questions.  I hope they don't sound dumb.

Not at all.  This is how progress is made.  As you can see, there is
still some way to go...best thing is probably to measure the secondary
Fres and tune the primary to match.  Allow plenty of range of primary
tuning.

Sorry I can't be more precise with the figures.  It's quite difficult
to calculate accurately for small coils, because some factors that are
negligible in large coils become significant in small coils.
--
Paul Nicholson,
Manchester, UK
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