Re: Primary Q's and Spark Gaps
Subject: Re: Primary Q's and Spark Gaps
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Date: Fri, 8 Dec 1995 10:11:46 +1200
>Received: from rata.vuw.ac.nz (root-at-rata.vuw.ac.nz [22.214.171.124]) by uucp-1.csn-dot-net (8.6.12/8.6.12) with ESMTP id NAA00686 for <tesla-at-grendel.objinc-dot-com>; Thu, 7 Dec 1995 13:38:15 -0700
> The resonant frequency of the secondary (before discharge
> starts) is about 319 kHz, as measured with a signal generator.
> The "hump" phenomenon is indeed a "beating" between the two peaks
> in amplitude response of a double-tuned transformer when the
> coupling is well above critical.
It's times like this when I am glad I have the expertise and knowledge
of others to draw on.
> If you happen
> to have a copy of "Radiotron Designer's Handbook", Third edition,
> November, 1941 you will find appropriate equations on page 123.
I do indeed, but I'm not sure which edition. (My apologies Mark
Barton, I guess I haven't looked hard enough at what I have got).
It's about time I started reading from cover-to-cover instead of
picking some bits out. In fact I'm sure I must have at least glanced
at this part of it.
> secondary (assuming identical tuning and Q for both circuits). This
> equation is good for k*Q of 3 or greater, which I suspect is the
> case for a typical Tesla coil BEFORE the discharge starts and the
> secondary has become heavily loaded.
This is CERTAINLY true. Loading the secondary with either a decent
spark or Zo causes this phenomenon to cease - I have seen this on
the scope. Once again, thanks for the equations and reference.
> running the
> same SPICE simulation made it obvious that one would like much
> higher values of k than you might expect from critical coupling,
I not only reasoned this when I researched my article, but actually
varied the coupling to make the response single-humped (killed delta-
f, JUST) to make k = unloaded kc (if all that makes sense). I
realized then that k had to be >>kc in order to get any power through
the system. To make k = unloaded kc (without the spark gap
admittedly), I had to prop a secondary more than 2 feet above (clear
of) the primary. Not a whisker from the secondary under power in
> By the way, as of this evening I am getting 24" streamers
> (one or two at a time only) and white arcs to a grounded door knob
> when the streamers "come its way", with the coil I mentioned earlier.
This seems to be typical behaviour that I have observed. It supports
the "spark being far longer than real voltage idea".
> How do these
> results sound to you?
Whoops, I'm sorry - I snipped the results by accident. This sounds
like stirling performance. (this is about 300 - 360W/foot, yes?).
I'll check your previous posts to see how you've changed your
> One last thing. I took some pictures of the streamers
> with my camcorder this evening, and on playing them back slow
> motion got the impression (it might have been due to stroboscopic
> beats between spark frequency and camera frame rate) that I could
> streamers grow, disappear, and reappear in some other place. Any
> thoughts or comments?
Exactly what I've observed. Richard Quick's video was extremely
revealing in this regard and basically bore out what I'd been
thinking. The stretch per frame (per mains cycle) was almost
I'm sorry, my brief comments haven't done you justice.