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Re: [TCML] Spark gap comparisons
Hi John,
FutureT@xxxxxxx wrote:
Hi Bart,
I'm not surprised that the results were similar, but I was surprised
the results were the same because there is a view that a very wide
gap such as the triggered gap used, would cause noticeably larger
losses. Apparently the losses were similar in both cases, or are
not really that important in this coil. Both the rotary and the
triggered gaps had an adequate range of adjustable timing.
I personally would not expect larger losses in the greater spaced
"triggered" gap. I'm not sure why one would come to this conclusion. The
arc external trigger which forces the gap into conduction is completely
separate from the gap width. Once the trigger occurs - bang! I just
can't see the "length" of the arc as an indication of losses in this
particular situation.
Triggered gaps are also prone to the triac not turning off periodically
without adding some extra circuitry to ensure turn off. There are
probably some slight differences between the two gaps but the cap
voltage was probably very similar.
The firing seemed very steady, at least as steady as the rotary.
I agree the cap voltage was probably similar.
Yes, firing appears steady, but I think John Tebbs identified the triac
not turning off always as wanted. Not always, but periodically. This led
to the diac inclusion to the triac and then to the zero-crossing circuit.
My point was that despite any differences, the results (spark lengths)
were the same for both gap systems. This seems to suggest that
the exact design of a gap is not too critical provided that it's a good
design and suitable for the coil in question.
I agree with that totally.
To me it is significant that two such widely differing gap
systems gave the same spark lengths.
Their similarities are electrical. Their differences are really
mechanical. Not really surprising they performed similarly.
Regarding the static gaps
that I tried, none were able to equal the spark lengths and overall
performance of the rotary or the triggered gap, in the TT-42 coil.
Certainly for a coil that needed a higher break rate to perform,
neither this rotary nor this triggered gap would have performed well.
John, there were many static gaps I've built that performed lousy
compared to rotary gaps I've built. But, this last static gap I built
rivals their performance. That's why I'm starting to realize the
difficulty in comparing the gap types. Much of the comparison is based
on the construction, knowledge, and experience with static gaps. If I
were to make a comparison 4 years ago, it would be totally different
than a comparison today.
It has to do with the way the electrodes approach each other as the
voltage is rising across the gaps as the cap charges. Depending on
the electrode speed, the gaps may ionize and cause a premature
firing.
The ionization will occur of course, but it isn't significantly a large
field radius over the electrode. It is barely over the surface radius.
The corona ionization field is self limiting and is rather small. What
does affect the firing before alignment is mainly the cap voltage and
the arc distance of that voltage as based on the electrode geometry. For
rotary's, this geometry is small and approaches something barely beyond
a needle gap style of discharge. This makes the arc distance rather
substantial and agrees well why rotary gaps can fire before alignment.
Certainly rotaries fire before the gaps align because of the
excessive voltage, but this would be something beyond that effect.
Rotating the phase to try to delay the firing, may then prevent the
rotary from firing at all. This would depend very much on the
way the voltage rises. I've seen some NST systems where the
voltage rises, then levels off before firing, but in other systems it
fires right as it's rising, yet a delay in phase will stop the firing.
It seems to depend on both the particular NST and the cap value.
I certainly understand that. Cap size and power are absolutely
determinant with firing.
It's a little hard to explain the overall
concept in words. It was something I had considered in an attempt
to explain the results of equal spark lengths despite the very
wide gap (high losses?) of the triggered gap. Actually Mark
Reszotarski had done an analysis of spark gap losses due to
gap width, and found that wide gaps caused very large losses.
I would probably have to disagree based on the statements, but I don't
exactly know the variables Mark used as physics between small and large
gaps. There's likely some constraints we are not identifying.
It was around that time that I happened to do the comparisons,
and my thinking was along those lines. I do not know of any
other comparison tests between a rotary and a triggered
gap that were done, so I thought it might be of interest to those
who are thinking about various gap systems and their losses
and their advantages or disadvantages.
I think it's a worthy study but I don't think it can be substantiated as
a comparison of one type of gap against another except in your two gaps.
I think it's quite likely that others could do the same comparison and
come to different conclusions. And this is the nature of spark gap
comparisons I'm speaking of. I see spark gap comparisons as very
subjective to the gap itself, the coil being driven, and the way it's
being evaluated.
Take care,
Bart
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