Re: measuring secondary parasitic capacitance?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi!

On Sun, 3 Nov 2002, Tesla list wrote:
 > Original poster: "Laurence Davis by way of Terry Fritz 
<twftesla-at-qwest-dot-net>" <meknar-at-hotmail-dot-com>
 > just out of curiosity i tried to measure the capacitance of a secondary.
 > my meter kept giving the out of range error, then at one point started
 > to give a reading.

Probably a DMM? This means low frequency measurement, and the inductance
of the coil dominating the capacitance => high current from DMM => it
thinks there's a very high capacitance there.

Maybe adding a resistor in series with the coil (say at least 500kOhm)
will give you a better reading. But check first with a know capacitor that
the reading is still correct. :)

Real LC meters _might_ work better. Altough, the one I tested didn't, so
I'm not sure about this...

 > it wasnt stable.  the lowest I read was .150 nf.
 > I didn't do the tesla calc thing on the secondary, but I would expect
 > two things.
 >
 > should be .015nf (or 15pf) or rounds 'bout there.

As such, a good idea, but... Much depends on how you made the measurement,
and what exactly you want to measure.

The self capacitance consists of a lot of factors, see
  http://www.abelian.demon.co.uk/tssp/pn2511.html

Hooking up an LC meter right accross the ends of a TC secondary will give
you _some_ reading allright.
But, at 400Hz or so, the voltage distribution accross the coil is quite
linear, so you get a different reading for capacitance than you would get
at around f_res measuring frequency.

OTOH, all depends on what you take "true self capacitance" is assumed to
mean... ;o) Medhurst capacitance, free space self capacitance, etc.

 > I didn't do the tesla calc thing on the secondary, but I would expect
 > two things.
 >
 > should be .015nf (or 15pf) or rounds 'bout there.
 >
 > parasitic capacitance has to be measured with higher
 > voltage.

Concur there. It doesn't depend on the applied voltage, but on geometry
and surroundings and voltage (vs charge) distribution.

 > perhaps the lower voltage requires more sensitivity
 > to measure the capacitance than that of higher voltage.
 > or design a piece of test equipment to measure it.

Hmm, well, you're right there. If your LC meter doesn't extend into the
pF range then it's really hard to measure any pF's reliably. ;)

But if you'r LC meter or DMM measures small 22pF capacitors correctly, but
not the TC secondary, then there's something wrong in the measurement
setup.

Maybe the series resistor trick helps?

cheers,

  - Jan

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