Fusion Message Board

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Subject: Re: Ceramic caps
Date: Oct 29, 2:17 pm
Poster: Richard Hester

On Oct 29, 2:17 pm, Richard Hester wrote:

The good manufacturers of ceramic caps make a series for extended pulse service. They use a lower K dielectric than the DC units, so that electrostriction is not a problem, and are constructed to take the discharge current. Excimer laser manufacturers use these capacitors in large quantities. Good manufacturers are typically TDK, Murata, and Ceramite.
I agree, though, if you get the wrong series of ceramic caps, they won't last very long in discharge service - had that problem myself...
>Ceramic caps are good as filter units, but large doses of fast discharge at max energy can ruin them quickly.
>I have had a lot exsperience here and have disassembled them following massive value change in the downward direction and other catastrophic failures to examine the methods of failure.
>Rapid discharge pulls large currents and RF is the order of the day in such systems. Ceramic caps are just slugs of Barium or Strontium titanate with a baby's breath of silver vapor on each end. A small central contact (usually threaded) is applied rather daintily at each end and the whole mess is sunk in epoxy as a casing and insulation medium.
> As high speed discharging proceeds huge Rf currents are formed and slowly and without fail, the silver is literally blown away, vaporized, etc. This reduces the effective plate area and ultimately a .005ufd 30kv cap used for just several hours in this terrible environment may be down to .003ufd! Worse still, is the possibility of blowing all silver away from the central contact in a small ring. This virtually disconnects the cap, but in a marx setup will continue to arc charge and work for a while, but the total failure is often an hour away. I wouldn't use these caps for anything other than a plain DC supply filter in a steady state supply or marx setup. Polyethylene discharge caps are demanded for pulsed power usage and are incredibly expensive and large (volumetrically) at voltages over 40kv.
>Richard Hull