[Prev][Next][Index][Thread]

Re: Method of measuring self C of secondary



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



References: