Re: Is this capacitor okay ? ? ?

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

Original poster: "Bert Hickman by way of Terry Fritz <teslalist-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Tesla list wrote:
>Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><dhmccauley-at-spacecatlighting-dot-com>
>I'm looking for a capacitor to use with my MOT powered (2400VAC) dual 833A
>VTTC and was wondering what your thought were on the following capacitor?
>http://cgi.ebay-dot-com/ws/eBayISAPI.dll?ViewItem&item=2513287005&category=4662
>Its a 30xxxx series 15kv, 5uF capacitor.
>It would be used in conjuction with a bridge rectifier to provide high
>voltage DC to my VTTC for audio modulation.
>Thanks
>
>.

Dan,

Generally, you wouldn't use a DC Pulse Capacitor in a DC filtering 
application, since pulse capacitors are not designed for use in extended 
steady state DC applications. Internally, high voltage pulse caps (over 5 
kV) are constructed by connecting identical rolls in series/parallel so 
that any given roll never sees more than 3-5 kV across its dielectric. 
Pulse caps use no internal DC voltage balancing resistors across the 
capacitor rolls. Instead, they are designed to be charged and discharged 
rapidly, with a minimum time under full voltage. This assures that the 
applied voltage will be evenly distributed across the individual capacitor 
rolls in the chain, since the rolls have nearly identical capacitance.

Under an extended DC voltage, differences in the leakage resistance of 
individual rolls will lead to the highest voltage stress appearing across 
the best (least leaky) roll in the chain. This leads to overvolting of the 
best roll(s) in the chain. Once the overvolted dielectric fails in one 
roll, the higher voltage stress on the remaining rolls often causes a 
cascading failure of the next best roll, then the next, and so on - and 
spectacular failure of the entire capacitor.

Now the good news:
Since your application will operate the capacitor at a comparatively small 
fraction of its rated voltage (15 kV), this pulse cap should work fine in 
your particular application. Even better, since your application will have 
a significant amount of ripple current, this should also help equalize the 
internal voltage distribution between rolls. I'd go for it!

Best regards,

-- Bert --
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