# [TCML] Flat strip primary inductance

bartb bartb at classictesla.com
Sat Dec 20 15:50:43 MST 2008

```Hi Finn,

The java flat ribbon calculator was extracted from Kurt's Excel file
back when Kurt made the TCML aware of it. Kurt has made several
measurements with the Circa 74 calculation and I've done a few myself. L
works out very near the measured value (it's what helped me design the
flat coil used on my SISG). I used the approach of determining the L
desired with round conductor in Javatc and then used the ribbon
calculator to design L for the flat ribbon primary. Coupling could not
be looked at, so I measured and positioned accordingly.

Interesting thought using your dual round conductor test. It does appear
that the current in flat ribbon is concentrating toward the edge. It
looks as though the dual round conductor separation distance was similar
to the ribbon width. I assume this was on purpose? And it looks like a
parallel connection on the spiral in order to attain the value measured.

Anyway, an interesting and thought provoking insight. I never thought of
checking that. I only went as far as checking flat L to round L and then
of course realized the difference. But this test throws in a new twist.

Take care,
Bart

bartb wrote:
> 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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