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

Non-linear capacitance



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