Re: Non-linear capacitance


Great post, Gary! Last year, I was thinking about doing the same kind of
thing, since HV ceramics are pretty inexpensive and easy to find at
Hamfests. However, after reading up a bit on ceramic capacitors, I
decided to look at other alternative. 

There are two different classes of ceramic dielectrics. The first,
called Class I, is a non-ferroelectric, lower-k dielectric. Capacitors
fabricated from this formulation do not show capacitance change versus
voltage, and have predictable capacitance versus temperature
characteristics. If you made the high-voltage section from this type of
capacitor, you shouldn't see the capacitance change as a funtion of the
applied voltage stress, and if you used NPO type (minimal change versus
temperature), they would work fairly well. 

The second type (Class II) are made from a higher k ferroelectric
dielectric (barium titanate). This formulation shows marked capacitance
changes with temperature and applied voltage, and (analogous to
ferromagnetic materials) they also show hysteris effects(!). The
capacitance of a Class II cap decreases significantly with increasing
applied voltage. For example, as the DC voltage stress is increased from
0 to 20 volts/mil, the capacitance may decrease by as much as -10% or

A better alternative for a measurement circuit may be to use one or two
cascaded fixed vacuum capacitors for HV front-end, since they are "self
healing" have relatively high standoff voltage for their size, and they
DON'T suffer from any C versus V or C versus F effects. You can often
find fixed vacuum capacitors at reasonable prices at Hamfests or on
ebay, many going up to 50 or 60 kV.  

Safe cappin' to you!

-- Bert --

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