Method of measuring self C of secondary

• To: tesla-at-pupman-dot-com
• Subject: Method of measuring self C of secondary
• From: "Robert Jones" <alwynj48-at-earthlink-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Tue, 25 Apr 2000 20:23:41 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Hi all,

Here is a suggestion for measuring the C to ground of a coil that avoids the
accuracy problems caused by the L of the coil.

THIS EXPERIMENT MUST NOT BE PERFORMED BY ANYONE WITHOUT THE KNOWLEDGE OF
HIGH VOLTAGES TO PERFORM IT SAFELY (just a reminder for beginners)

Mount the coil in it normal position with the secondary in place and with
the secondary connected to ground. remove the earth connection to the coil
and be certain it is fully insulated from ground or any other connection.

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 suggest you only use a battery powered DVM to measure the current but even
some bat powered DVMs don't like to float at high voltages (CHECK THE
MANUAL) or  use one of those old fashioned things with moving needle and
scale (AVO)   Make certain all high voltage leads on the coil side of the
meter  are the well clear of any metal or the ground and mount the meter on
top of a plastic box for insulation.

DONT TOUCH THE METER WITH THE VOLTAGE ON

You will need a 50uA full scale for a typical coil.  With 5kV applied at
60Hz you will measure  1.885uA per pF of the self C of the coil.  It should
be possible to measure the C to better than 15% for say a 10pF coil.  The
same method can be used to measure the self C of a topload.

Check the current at both voltages if the current at the higher voltage is
say more than 10% greater than x2 of the lower voltage. If it is, you may
have brush discharge from the coil or leads so half both voltages and
repeat.
Brush discharge is very voltage dependent so halving the voltage should
eliminate it.

I am hoping to entice someone to do this measurement and post the results
with details of their coil.
Perhaps Terry one of our main experimenters will try it on the coil he has
been using for his recent tests, preferable above the same ground plain.

Regards Bob



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