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: Mon, 11 Dec 1995 08:23:42 +1200
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Steve Roys writes in reply to MW....
MW> > Yep that's right. But you are tuning in each at a time doing
MW> > Until I finally decided to try operating the scope in the same
room > > as the coil (under power), that's all I was doing too.
MW> > (as an aside, you don't get the true primary Q doing this as of
MW> > course you know)
SR> How can you determine the Q of an operating coil? What is the
SR> "ping > and decrement" method that someone mentioned?
Do the slow cap charge and discharge (ping) into the primary and
capture the secondary output via loose capacitive coupling. I used
a digital storage scope to capture each discharge and hold so I
could observe the waveforms in detail. The probe was just a foot or
so of wire hanging over the edge of the bench. The coil was about
9 feet away. I looked at waveforms (a) with no load - i.e. no
discharge rod which doesn't allow the secondary to form a spark
of any subtance and therefore retain its high Q. Under this con-
dition, you can see the split frequencies beat in the time domain.
The output IS a DSBSC waveform. The spectrum consists of two discrete
frequencies. This is exactly what you would get at the output of such
a modulator. Having previously seen this in the time domain, it should
have come as no surprise the coil would do the same, knowing about the
spectral output. This waveform continues until the primary gap ceases
to fire after which remaining primary energy dissipates in the
transformer etc. and the secondary is left to ring down in effective
isolation. (b) with a discharge rod positioned away from the
secondary terminal to the point where the secondary spark is a
single bright, loud discharge. Under this condition, the beat
envelopes disappear and you can see the effects of secondary spark
generation. This is twofold- a massive drain of energy from the
secondary, and additional high frequency ( >> than fr) components
reflecting the characteristics of the discharge. The spray of
streamers you get under minimally loaded conditions obviously
reflects this spiking - on my coil, system energy has disappeared
in less than 5 cycles of fr when loading with Zo (which is what the
loud spark to rod does). (c) With the rod up close or touching the
terminal, what I see is an exponential decay of fr (no beats) with
a massive amount of spiking.
There is more info on this topic in previous posts.