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Re: Bypass caps and Neons *NEEDED!* (fwd)



---------- Forwarded message ----------
Date: Tue, 16 Dec 1997 11:30:06 +1200
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Bypass caps and Neons *NEEDED!* (fwd)

Hi Ed,

> ---------- Forwarded message ----------
> Date: Mon, 15 Dec 1997 16:23:26 EST
> From: Esondrmn <Esondrmn-at-aol-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Bypass caps and Neons *NEEDED!*
> 
> In a message dated 97-12-13 20:55:16 EST, you write:
> 
> - snip - 
> 
> <<    I neglected to mention, safety gaps across the primary
>  cap IMHO is a BIG NO NO, safety gaps go from the
>  transformer end bell to ground, a gap across the primary
>  cap places a SMALL inductance directly across the
>  primary cap and when it fires results in a ring frequency
>  of 5-15 mhz or higher of very high current that is coupled
>  to NOTHING,  deadly for capacitors.
>  
>  Mark Graalman 
>   >>
> Mark,
> 
> You say a safety gap across the tank capacitor is a bad idea?  After blowing
> two C.P. caps, it was suggested by at least a few folks on this list that I
> add a safety gap across the cap to protect it.  I am using a 14.4 kv pole pig
> running at 5 to 8 kva.  The gap is currently set at 5/8" and fires like a
> rifle when I try to put too much power into the system.  The gap uses #10
> solid copper wire bolted directly to the capacitor terminals.  The gap ends
> are bent into 1.0" diameter circles and soldered and de-burred.
> 
> I would like to hear other folks opinion on this subject.  I believe both Mr.
> Cox and Malcolm suggested the addition of the safety gap across the cap.
> Malcolm, you have the same C.P. caps that I have.  What kind of safety gap do
> you use, what transformer voltage, and how far do you open the gap?
> 
> Ed Sonderman

The point Mark made about the high frequency ringing with the gap 
across the cap is well made IMO. I never use a gap across the cap.
I *do* think it is a good idea as a *last* resort to stop the cap 
dying from overvoltage until BPS etc. is optimized.
    
     I've never run a real high power supply like a pig. The 
only way a gap across the cap can fire is if the charging inductances 
allow the cap to ring up to that voltage in between breaks. It can't 
be energy coming back from the sec because you can't get more energy 
into the cap by that route than you started with. A gap across the 
transformer is a different matter entirely.
    
    I've been mostly confined to neons and other transformers in the 
2kW range at most. I think the Q of the charging circuit should be 
lowered if the cap safety gap fires. It would also help if you can 
continuously vary the break rate. You should be able to strike a 
point for a given set of charging conditions where the safety gap 
stops firing entirely. At the same time, coil output should also peak.

Malcolm