Re: Non-linear capacitance

• To: tesla-at-pupman-dot-com
• Subject: Re: Non-linear capacitance
• From: Ed Phillips <evp-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Fri, 07 Jan 2000 21:01:48 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Tesla List wrote:
> 
> Original Poster: "Lau, Gary" <Gary.Lau-at-compaq-dot-com>
> 
> 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

	Sure you've gotten response to this, but in case not the answer is yes,
the titanates used are "ferroelectric", and indeed do have capacitance
which is a non-linear function of voltage.

Ed

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