Re: Doorknob capacitors for primary?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

Hi David,

A HV Ceramic doorknob cap is simply a flat plate capacitor that uses a 
thick, high k dielectric. The ceramic materials used in these caps has a 
high dielectric constant that can be in the range of 15-500 (for Class I 
ceramic dielectrics) and 2,000-20,000 (for Class II Ferroelectric 
ceramics). Class I HV caps are indeed usable for higher power RF, Tesla 
Coil, and pulsed power applications with some restrictions. However, Class 
II caps are intended only for DC filtering applications, since they have 
much greater capacitance change with temperature and applied voltage, and 
much higher losses at RF frequency.

Both types of capacitors outwardly appear similar. Internally, both types 
are constructed from a thick cylinder of dielectric material, with each of 
the flat ends being metalized. Pulse duty and RF power capacitors use 
heavier metallization to better withstand higher current. Both styles have 
similar plate surface area. Thicker material is used for higher voltage 
capacitors and, unlike small low voltage multilayer ceramics, they use only 
a single dielectric layer.

When used in Tesla Coil applications, Type II HV DC caps may overheat and 
die quickly, or the thin metallization may begin to gradually "disappear" 
in the high current environment. They also "broaden" primary circuit tuning 
because of their poorer Q, and this also causes significantly poorer coil 
performance than Type I or film capacitors/MMC's.

Best regards,

-- Bert --
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Tesla list wrote:
>Original poster: "David Trimmell" <humanb-at-chaoticuniverse-dot-com>
>Regarding "Door Knob Capacitors", just what, or how, do they work? I
>have broken open a few of my larger ones that have failed, and find they
>are nothing more than a ceramic material... Can anyone explain the
>physics of operation?
>Thanks,
>David Trimmell
>
>Snip>>>>>>>>>>>>>>>
>
>.