RE: Safety gap resistor?

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Gary,

I was thinking that if one shorted, the full voltage would appear across
the other cap.  However, I think these caps are rated for the full primary
voltage anyway so maybe it would not be a problem.  Having the primary
capacitance suddenly double may cause resonant rise too depending on how it
was set up.

Cheers,

	Terry

At 02:48 PM 8/6/2001 -0400, you wrote:
>Hi Terry:
>
>I'm curious why you feel that capacitor safety gaps in an equidrive
>configuration would be any more appropriate?  If the concern is that an
>asymmetric DC bias might somehow develop between them, wouldn't bleeder
>resistors as MMC's use be better, as no adjustment is necessary?
>
>Regards, Gary Lau
>MA, USA
>
>
>>Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>>
>>Hi All,
>>
>>We are talking about two different safety gaps here.  The first is our old
>>safety gap across the NST.  Tried, true, and everyone should use them.
>>
>>In most cases (like 99%) that will protect the primary cap too.  With MMCs,
>>an occasional overvoltage is no problem at all.  No other safety gaps are
>>needed.
>>
>>The only time "another" safety gap across the primary cap is needed is if
>>it is a commercial cap that will fail if it is overvoltaged just once.
>>However, even then the normal safety gaps should protect it.  There are
>>some people that like to put a safety gap across these expensive caps just
>>as added insurance.  However, just a gap alone will short the cap directly
>>without any current limiting.  A 200,000+ amp pulse will destroy most caps
>>that are not rated for it.  Even pulse caps may be damaged...  So if you
>>put a big 100-1000 ohm resistor in series with the safety gap across the
>>capacitor, it will limit the short circuit current to say 20 amps which
>>will not destroy the cap but will discharge it.
>>
>>Commercial caps used in the "equidrive" configuration would be a good place
>>for an additional safety gap across the caps.
>>
>>I remember an off list mail about a person that was blowing commercial caps
>>often.  He noticed that it seemed to happen when the safety gap across his
>>caps fired.  Adding one of those big 225 watt power resistor in series with
>>the safety gap stopped the huge current pulse from tearing up his caps by
>>limiting the current when the safety gap fired to 20000/1000 = 20 amps.
>>The caps never failed again...  But again very few of use or need safety
>>gaps across the primary cap...
>>
>>Cheers,
>
>	Terry
>
>