Re: [TCML] Flat strip primary inductance

[Finn Hammer <f-h@xxxx>]

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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