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Re: Method of measuring self C of secondary

Hi Terry and all,   (combined reply)

It hard to keep with Terry's prodigious output.  Thanks for the measurement
and mods to T5 or is it T6 with an option of voltage weighted  self (vwsC or
medC) or  self C (true or LC bridge C).  Does it  now output the self C
(true)
along the coil if so can I have a copy.  Should very useful.   How about a
output for mathcad.  Does it now produce self C (true) of the topload. You
should set the primary V to 0v because you dont know the phase relative to
the secondary so zero will give the least error.

The measured self C (true) confirms a fiddle factor in Medhurst but Medhurst
may be for an isolated coil.  In which case  the difference between Medhurst
and self C (true) will be dependent on the separation from the ground plane.
Does anybody know what Medhurst assumes and if he assumes a ground
plain what was the separation.

The self C (true) or better the equation is required  for simulation and
maths models.

The use of a voltage weighted C in a lumped resonance calculation or any
calculation is totally new to me. As is the concept of a current weighted L.

The secondary can be shorted across the ends during C measurements to
decrease L to 1/4 and hence reduce errors.

Regards Bob

-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: 26 April 2000 19:55
Subject: Re: Method of measuring self C of secondary


>Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
>Hi Bob and Antonio,
>
>At 11:58 PM 04/25/2000 -0300, you wrote:
>>Tesla List wrote:
>>
>>> Original Poster: "Robert Jones" <alwynj48-at-earthlink-dot-net>
>>
>>>...
>>> Using your Variac controlled high voltage AC power supply  apply 2.5kV
AC
>>> between the coil and ground and measure the current flow to the coil.
Repeat
>>> this with 5kV. Wind up the volts slowly while checking the current does
not
>>> say go over 100uA.
>>>...
>>
>>I see a problem with this measurement. At 60 Hz, the voltage will be
>>the same along all the coil. In real operation the voltage is zero at
>>the bottom of the coil and maximum at the top. The difference in the
>>charge distribution leads to different values for the equivalent
>>"self-capacitance" of the coil. It would be interesting to see how
>>these measurements compare with the predictions using Medhurst's
>>formula.
>>
>>Antonio Carlos M. de Queiroz
>>
>
>I just simply connected my LCR meter between the base wire of my bare
>secondary and ground.  The meter gives a capacitance of 26.4pF.  The
>Medhurst (and actual value) is 9.28pF at resonance.  I think Antonio is
>correct that the voltage distribution on the secondary is far different at
>the meter's 1000 Hz than at resonance.  Antonio thinks it is constant, so I
>made the simple modification to E-Tesla5 and ran it.
>
>Assuming the voltage on the secondary is a constant 1 volt at all points,
>the capacitance value the program gave is 27.2pF (only 3% off from the
>measured reading of 26.4pF.  Even I was shocked at how close ET5 came!
>;-)).  The distance from the base of the coil to the foil ground plane has
>a dramatic affect on the capacitance.  I had to be sure that actual coil
>and the programmed values were exactly the same.  If I moved the coil 1/2
>inch closer to the ground plane (it was 1 inch) the capacitance would go up
>about 10pF.  It appears that the vast majority of the capacitance is
>between the base of the coil and the ground plane.  The field plot is shown
>below:
>
>http://users.better-dot-org/tfritz/site/misc/Bob30.gif
>
>Of course, in this case, the voltage at the top of the coil is no longer
>200X as driven by the coil's Q at resonance.  The voltage at the top of the
>coil and the bottom are just 1.  This makes the coil have a larger apparent
>capacitance.
>
>I assume the 1000Hz LCR meter is far enough away from any resonances that
>the voltage distribution on the coil is uniform as Antonio suggests.
>
>Cheers,
>
> Terry
>
>
>