RE: Safety gap resistor?

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

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