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Re: A new cap failure mode?




Hello Bob, all
  
-----Original Message-----
From: Tesla List <<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com>
To: <mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com
<<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com>
Date: Sunday, February 07, 1999 10:10 AM
Subject: Re: A new cap failure mode?

Original Poster: bob golding <<mailto:yubba-at-clara-dot-net>yubba-at-clara-dot-net> 

At 21:27 05/02/99 -0700, you wrote:
>The idea of a resistor in series with the cap gap posses the problem of
>needing a high voltage resistor.  However, they should be easy to make.
>Taking a few inches of nichrome wire and winding a few turns on a plastic
>former should do the trick. You could immerse this resistor in epoxy to hold
>the wire in place and increase the voltage capability. Cost
shouldn&acute;t be a
>problem, because you only need a small length of wire. The voltage drop
>(I^2*R losses) across the resistor should not be a problem either, because
>the neccessary resistance doesn&acute;t need to be very large (a few ohms or
>even less should be sufficient). As they gaps (are not supposed to) fire
>continuesly, the current capabilty of the wire in epoxy shouldn&acute;t be a
>problem either. You might not even need nichrome wire. Maybe a
>few (more) turns of thinner copper or steel wire could do the trick.

Bob wrote:

"Reinhard,
sorry if I sound a bit thick on this but could you explain how you would
connects this low value resistor? I can't see how it would work. Is the
resistor across the cap? between the cap and ground? if it is across the
cap wont it form a tuned circuit with the cap? I don't understand."
  
Hey, no problem.
  
Here is a schematic (use courier or similar font to view) of what I mean.
  
  
                                     O
                                      |
                                      |
        
x---|--#-----*------||-------*---P    S
x   |  #     *               *   P    S
x   |  #     *--IR--> <--IR--*   P    S
x   |  #                         P    S
x   |  #                         P    S
x   |  #                         P    |
x---|--#-------------------------P   ---
^   ^  ^        ^   ^ ^   ^      ^    ^
A   B  C        D   E F   D      G    H
  
  
"Parts List"
  
A: Secondary of your HV xformer.
B: Xformer safety gap.
C: The main gap.
D: The resistor / inductor I was talking about.
E: Your tank cap.
F: Cap saftey gap.
G: The primary coil.
H: The secondary with topload and ground.
  
I suggested using nichrome wire because of the following reasons:
  
1.) Nichrome wire has a high resistance value per length (ohms per foot).
2.) Any wire wound resistor acts as a:
                                        2a.) Resistor
                                        2b.) Inductor (which will further
limit the current)
  
Usually, you don&acute;t have part "D" in your coil setup. As Terry pointed
out, there is almost no resistance (R or L) present, when the cap&acute;s
safety gap fires. This means the peak pulse has very high currents that
might contribute to cap damage. The wire resistor will reduce this peak
current (the cap&acute;s safety gap will still fire and protect your cap)
to hopefully safe values.
  
The problem with the resistor:
  
It will have to withstand the full voltage across the capacitor (if/when
the gap fires). A carbon or metallic film resistor won&acute;t "cut it"
(unless you put a zillion in series or it is gigantic unit).
This is why I suggested encasing the homebrew wire resistor in a thick
layer of epoxy.(space winding the wire would help, too, but it requires a
larger former to keep the R & L up). Plus an "epoxy case" looks good, if
properly done.
  
  
I hope this helped.
  
Coiler greets from germany,
Reinhard