Non-linear capacitance

• To: tesla-at-pupman-dot-com
• Subject: Non-linear capacitance
• From: "Lau, Gary" <Gary.Lau-at-compaq-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Thu, 06 Jan 2000 12:22:26 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

I am sloooowly gathering materials to build a fiber optic probe set.  Until
I do that, I decided to build a passive HV scope probe.  Knowing that stray
capacitance in high-value resistive dividers will cause probe anomalies, I
decided to use a capacitive divider approach.  For the "hot" end of the
divider I used a string of forty 680 pF/1KV ceramic disk capacitors.  When I
tested the probe with 120VAC input, the division ratio was as expected.  But
when I cranked the input voltage up, it appeared that the impedance of the
string decreased as the voltage increased.

My first thought was that since I did not immerse the cap string in oil,
perhaps corona was responsible for a change in apparent impedance.  I was
just about to try the oil immersion thing when I saw in a post today:

>This is sort of like those ceramic capacitors that have different
>capacitance at different voltage levels

Is this a real phenomenon with ceramic disc capacitors?  

Gary Lau
Waltham, MA USA

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