Re: Spark Gap (fwd)

To: tesla-at-pupman-dot-com
Subject: Re: Spark Gap (fwd)
From: Tesla List <mod1-at-pupman-dot-com>
Date: Sat, 23 May 1998 14:51:42 -0600 (MDT)
Approved: mod1-at-pupman-dot-com


---------- Forwarded message ----------
Date: Sat, 23 May 98 16:06:28 EDT
From: Gary Lau  23-May-1998 1549 <lau-at-hdecad.ENET.dec-dot-com>
To: tesla-at-pupman-dot-com
Subject: Re: Spark Gap

>From: Marco Denicolai <marco-at-vistacom.fi>
>To: tesla-at-pupman-dot-com
>Subject: Re: Spark Gap
>
>As a beginner, I have to ask this. When the main spark gap is supposed to
>trigger ?
>
>1 - When the tank capacitor is changed at full voltage (1.4 * transformer VAC)
>      OR
>2 - When the tank capacitor is changed a suitable voltage LOWER than that?
>
>If the correct answer is n.2, so what is the benefit of having a 14 kV
>transformer if the capacitor is discharged always at e.g. 8 kV?
>
>With my (supposed) 8kV tank I can get 0.37 inch sparks so that using a, say,
>0.15 inch gap would reduce my used voltage to about a 40% of the available...
>
>More, isn't that shorter gap much more difficult to quench than a wider gap?
>
>

Very good questions, I think I asked the same ones when I was starting
this.

Answer #1, when the cap is charged to full voltage, would be the best
thing if it were possible to _always_ achieve this.  However, gaps do not
always reliably fire at the same voltage, time after time.  If it misses
a firing on a half-cycle, then on the next half-cycle, the cap will
charge to an even higher voltage, due to resonance at the mains frequency.
This is not obvious and is probably responsible for many blown NST's.
Synchronous rotary spark gaps achieve the desired result by providing a
much smaller gap distance, only once per half-cycle, at precisely the
correct time when the cap is at full charge.

So for safety's (the NST's) sake, the gap must be less than the maximum.
Coil performance will be compromized with shorter total gap lengths,
true.  But your cap and NST will live longer.  Also, the gap will fire
proportionately more frequently.  Unfortunately, I've never seen an
authoratative set of guidelines for exactly how long a gap should be used
for any particular transformer voltage.  FWIW, I use 10 x .03" gaps on a
15KV NST.  I suppose I could even go another 1-2 gaps if I rebuilt it...

And actually, I believe multiple shorter sparks in series is easier to
quench than one long one (assuming the total gap length is the same).
That's why the "RQ" multi-short-gap static gap is so popular.  It may
also have lower resistance, but no one has actually proven this yet.

Regards, 
Gary Lau
Waltham, MA USA