Re: [TCML] Voltage Question

[Paul Nicholson <tcml88@xxxxxxxxxxx>]

Udo Lenz wrote:
> I can't resist another nitpick

Please nitpick - devils always lurk in the details...

> For a neglectable top capacitance, you would over estimate
> the voltage by a factor of pi/2 (i.e. a quarter wave along
> the secondary).

This is so.    Hence the use of an effective inductance 'Les'
in the calculation,

  V_top = 2 * pi * F * Les * I_base.

The value of Les is smaller (in a normal coil) than the low
frequency inductance 'Ldc' to take account of this effect.

Because of the coil's distributed self capacitance, not all
the current entering the base reaches the top.  Therefore the
total voltage induced is less than would occur if the current
was uniform all the way up.

With a large topload, the coil current is nearly uniform and
Les ~= Ldc.    With very short coils, the 'internal' capacitance
(between one part of the coil and another) creates circulating
currents and Les can be larger than Ldc.

JavaTC calculates Les correctly by working out the capacitance
distribution of the system, then the current distribution at
the operating frequency, and from that the effective inductance.

I should remark that Les is an attribute of the system as
a whole (coil + topload + any other sources of capacitive
loading), not an attribute of the coil itself.

> Why do you think the current measurement approach holds only
> below breakout?

Because then the loading on the top of the coil (and therefore
the current distribution) are no longer constants but vary
with the amount of breakout.

--
Paul Nicholson
--
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