Re: single shot
Subject: Re: single shot
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Date: Tue, 21 Nov 1995 16:41:21 +1200
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Hi all, Ed Harris wrote.....
> MW>Done it, and yes. The better the spark, the quicker the oscillation
> mw>dies. With a discharge rod positioned close enough to the terminal,
> MW>the "feathers" disappear as the current flashes back and forth in
> MW>a single discharge channel.
> MW> Loose pri-sec coupling means that the system cannot behave like
> MW>a voltage source. IMO Vo must drop with loading.
> Great! You would happen to have a digitizing scope would you? I'd love to
> see the waveforms.
O.K., I'm in a position to elaborate a little on the above incomplete
First, with no discharge rod near the top terminal and minimal
streamers coming off the top, the scope shows the slow-wave effect.
It looks very much like an A.M. signal modulated by a sinewave of
diminishing amplitude except that carrier amplitude diminishes to
zero. On my large coil (with the secondary and primary described
elsewhere), I get three A.M. "humps", each containing about 7 to 8
cycles of oscillation at fr with the normal sparkgap setting
(and no quench) before the oscillations die away exponentially. On
this coil, fr is about 145kHz which gives a "hump frequency" of
about 20kHz (i.e. about 50uS per hump).
On moving the discharge rod in to the point where visible
streamers are coming off it, the "humps" are reduced to one or two
(of about the same amplitude as previously) and a close examination
of the oscillatory waveform shows severe waveform degradation with
high frequency spikes corresponding to rapid discharge oscillations.
With the secondary discharge gap set to optimum (minimum primary
gap brilliance), only one hump appears with a slightly reduced
amplitude and severe waveform spiking after the first secondary oscil
lation. At this distance, a bright spark appears between the secondary
terminal and the discharge rod.
Finally, moving the rod closer causes progressive reduction of
the single hump amplitude to very low (relative) levels and the
primary gap once again becomes bright. In all cases, an exponential
oscillation decay occurs after the last distinct hump dies.
Points to note (and a query since I have not yet had the benefit
of reading the Corum's work on this) :
(a) The slow-wave humps obviously represent active energy flying
about in the system. I have been told that it represents a slow trans-
fer of energy from primary to secondary then back again (in the case
of my coil, over 7 cycles at fr). O.K., I have no problem with the
primary gradually ringing down while the secondary rings up, BUT,
does the secondary energy content end up being higher than the
primary even if there is no effective secondary load, and if so, how?
And what governs the slow wave time constant? M and Cp?
(b) Optimum energy loss from the secondary is clearly happening
when humps are reduced to one, pri. sparkgap brightness is minimum
and secondary spark is solid and bright.
(c) Given the number of cycles per hump, the number of humps that
can occur clearly corresponds with the number of primary rings before
the primary gap ceases to conduct with relation to the time constant
mentioned above. After gap firing has ceased, the secondary is
effectively left to ring down in isolation.
(d) It is clear from the above results that the ideal gap
conduction time (before quenching) should be about 25uS. BUT, it
does appear that when the secondary is optimally loaded, it is
(e) It appears that the individual streamers/feathers correspond to
a frequencies MUCH higher than the normal resonant frequency and are
IMO a reflection of the stray capacitance and inductance of the short
discharge path. The scope shows a plethora of spikes (that turn into
sinusoidal waveforms at high timebase speeds) corresponding to the
On reflection, measuring actual secondary voltages could be
something of a nightmare but I'll have a think about it. I was
basically observing a radiated waveform on the scope.
> Do you remember what kind of relative secondary voltages you got for
> pre and post arc? I just interested in the ratio....
Don't exactly know what you mean by pre and post sorry..