Re: Safety gap resistor?

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

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

At 08:51 AM 8/5/2001 -0500, you wrote:
>>Do you current limit your primary to keep the discharge current from
>damaging your cap?
>>I have NEVER seen a cap destroyed by a safety gap yet. I HAVE however
>seen many caps go to the great beyond for the lack of a safety gap!
>>The first question I ask someone who has just blown a capacitor is "were 
>>you running a safety gap on the cap?", and almost invariably the answer is 
>>no. I personally will use a safety gap every time!
>
>
>
>Someone please straighten me out on this! It makes since to use a current 
>limiting resistor for the MMC caps, but... WHERE should I put the resistor? 
>Are we talking about the resistors in the Terry filter (that I am using), or 
>should there be another resistor in series with the cap gap, or a resistor 
>in series with the primary coil? And if so, WHAT value and wattage resistor 
>should I use? Save my caps! Kevin
>
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