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Re: Spark Gap (fwd)
---------- Forwarded message ----------
Date: Mon, 25 May 1998 09:15:48 +1200
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Spark Gap (fwd)
Hi Gary,
> 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.
I can't see a single reason why it should be lower. Besides, Rgap is
a dynamic impedance which varies with Ip. Multiple gaps most
certainly have a higher voltage drop when conducting as each has a
minimum saturation voltage, again dependent on Ip.
Malcolm