Re: Measuring secondary Q Re: Success Re[TCML] Measuring secondary current

[paul <tcml88@xxxxxxxxxxx>]

I wrote:
> V_top = Q * V_base
> Z_in =  V_base / I_base

[measuring Z_in]

Andreas wrote:
> When you say V_base, are you talking about the voltage
> from the coil base to RF ground? I have a feeling it will be
> immeasurably low, assuming an I_base of ~140mA during operation
> and 1 ohm of resistance in the lead from base to the cold-water
> pipe that serves as ground.

Those two formulas only apply when driving a signal into
the base of the coil, for example when using the sig gen and
low-Z pad.  In normal operation (primary drive) the base is
effectively grounded (via your negligible 1 ohm) and those
formulas don't apply.

The base impedance (= loss resistance) will probably be in the
range 150 to 500 ohms.   The difficulty would be measuring an
I_base with only 0.1V of base drive from the pad.   You might
have to use a pad of 39 ohms and 10 ohms to get 1V drive from
the 5V sig gen.

[measuring V_top]

> The only way I see to accomplish this directly would be
> a capacitative divider; unfortunately, for a calculated DC
> capacitance (per JavaTC) of 11pF, I doubt I can make one that
> won't disturb the Q -- much less produce reliable measurements.

> I may use this as an excuse to finally make some Terry Fritz
> noncontact voltage / plane-wave antenna probes:

Which is a capacitive divider with a very low coupling
capacitance.

When testing with the sig gen it is safe to bring the 'plane
wave antenna' closer to the top load, which will increase
the coupling capacitance to a level where you get a usable
reading on the scope.   With V_base = 0.1V from the pad and
Q = 200 you would get about 20V on the topload.  Use a bit of
tinfoil or wire on the scope probe to make a capacitor plate
and bring the probe towards the topload until the coupling
capacitance is about 1pF.  With a 10pF/10Meg 1:1 scope probe,
you'll get about 2V on the scope, the resonant frequency will
be pulled down a few percent (retune sig gen to follow it),
and Q will lower only a little because the 10Meg is weakly
coupled to the resonator.  To calibrate this arrangement,
apply the sig gen directly to the top load to put 5V on it
and note the reading (about 0.5V) on the scope.

> http://www.classictesla.com/java/script/outQ.html

From the web page,

 Q = sqrt(Les/Ces) / Rac.

That doesn't quite take account of the energy storage when
the secondary current is non-uniform.   A slight adjustment
factor of Lee/Les (= Cee/Ces) is required -

 Q = Lee/Les * sqrt(Les/Ces) / Rac

This is obtained from

 http://abelian.org/tssp/pn2511.html

p22 equation 9.8.  But the difference is probably less than
the accuracy of most Q measurements or Rac calculations.
--
Paul Nicholson
--
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