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RE: Non-linear capacitance

Hi Bert:

You point out that in the non-linear, higher k ferroelectric (barium
titanate) dielectrics, the capacitance decreases with increasing voltage.
What I observed was, a 120V RMS signal read just that, but a 15KV RMS NST
gave me a reading of 33KV peak, or 23.3KV RMS.  I know that's way higher
than it actually is.  For the NST voltage to read so high, the capacitance
of my string would have had to increase with the increasing voltage.  And
the capacitive load imposed by the probes across the NST secondary is only
7.5pF, so I don't think that near-mains-resonance is at play to ring up the
voltage.

I built two such capacitive dividers so that I can do a true differential
measurement with the scope on A-B.  Today I encapsulated one of the
capacitor strings in melted Vaseline jelly to see if corona was responsible
for the apparent decrease in impedance with increasing voltage.  No
difference, both encapsulated and open strings performed the same.

Guess I better concentrate on getting those fiber optic probes built!

Regards, Gary Lau
Waltham, MA USA

>Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com> 
>
>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
>more! 

<snip>