Hi Finn,
Yes, I see now. I didn't look at your 180cm value correctly or
extrapolated the coil parameters correctly, thus lead inductance would
have had to be larger based on the parameters I was giving the
calculation. I also thought you were using a flat ribbon calculator,
but I guess not. I should have clarified first.
Anyway, this calculator is available for flat ribbon coils.
http://www.classictesla.com/java/flatribbon/flatribbon.html
Bart
Finn Hammer wrote:
bartb skrev:
Hi Finn,
The 1.5uH sounds correct (and I'm pretty sure it is). The ribbon is
only 2 turns and small. The inductance is therefore very small. I
would bet that you have nearly 0.8uH of lead inductance from the
meter affecting the measurement. It doesn't take much lead
inductance to add to the total inductance of the LC on such a small
coil inductance (and the value your off is about what you would see
with the leads). If you have some old leads around that will fit in
the Amprobe, cut those down to something short and measure again.
I am not using a meter.
Both circuits are closed and if you watch the picture closely, you
will see that I ring the circuits up with a sig. gen.
Both circuits, the flat spiral and the sanity check, are being
measured with the same gen., the same leads and the same scope.
I tried to twist the leads from the sig.gen. tigether, thes did not
change things.
I seem to remember that Antonio had a theory, that wide strip primary
would have a "internal mutual inductance effect" , due to current
repelling out to the edges of the strip, thus creating an effect
comparable to multible conductors in parallel inside the strip.
Kurt Schraner also has experienced this, and he has published about
it, but I never thought it could affect the inductance of a coil more
than say, 10% or so.
Also, metal sheeting directly under the coil can affect the
inductance (shorted turn affect - but this would cause L to
decrease). I doubt that is the issue but it can happen. This is easy
to see (place aluminum foil directly under coil base, measure L,
then slowly slide the foil out from under the coil and watch L
change in the process.
Again, look at the pics: All plastic construction, nylon, pvc and
etronax, apart from the copper busswork and coil.
I did another sanity check:
http://hammertone.com/sanity2nd.jpg
Another identical primary positioned 45mm higher than the first one:
Result: the combo ressonates @ 55kHz
So we have:
1 flat strip primary ressonating at 56kHz =>Much higher than
antissipated from calculating inductance
1 round conductor primary resonating at 43kHz =>Close to what is
antissipated from calculations of industance
2 of these, 45mm apart resonating at 55kHz => same as strip wound
primary.
I don`t have handy, charts or formula, of the inductance of 2
parallel connected, magnetically coupled inductors, but it seems like
a strip wound primary can be simulated as 2 independent rounf
donductor primary`s, spaced apart by a distance equal to the width of
the strip.
This opens some questions, which may only be answered by actual tests:
Calculating coupling: are these 2 virtual internal conductors equally
coupled to the secondary?
Can the coupling force current to prefer a path in the lowest virtual
conductor, the one that has least coupling, or will current sharing
be equal in the conductors. In other words, should the coupling be
calculated from a point in the midle of the strip, or closer to the
bottom of the strip.
This can become significant in close coupling, where the top of the
sheet might start to receive direct strikes from the secondary if the
strip is very wide.
Greg Leyh: I observe that you have gone from _very_ wide primary`s to
not so wide ones, in your twin prototype, although only one turn.
Have you done observations along these lines?
Cheers, Finn hammer
Take care,
Bart
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