[TCML] Spark gap comparisons
FutureT at aol.com
FutureT at aol.com
Sat Feb 23 14:32:36 MST 2008
In a message dated 2/23/2008 3:33:39 P.M. US Eastern Standard Time,
bartb at classictesla.com writes:
>It's not surprising to me they performed about the same. The sync rotary
>and the trigger gap both are controlled timed events of the mains
>frequency. So in that respect are similar. The rotary however does have
>the adjustment of where to fire along the waveform and I suspect the
>trigger was not adjustable (to fire after peak, maybe before peak).
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.
>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.
>I think gap comparisons are very difficult to make especially trying to
>compare different types of gaps, especially comparing a static gap to
>either a rotary or a triggered gap. They are different beasts and the
>static gap firing voltage is very dependent on thermal regulation,
>surface, size, etc. where a rotary is not (even a trigger gap is not as
>it's firing is independent of the electrode geometry or temperature).
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.
To me it is significant that two such widely differing gap
systems gave the same spark lengths. 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.
>I'm not sure I understand what you mean about rotary gaps not capable of
>firing at peak due to voltage rise upon electrodes approaching. Can you
>give more details to your hypothesis?
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. 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.
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.
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.
FutureT at aol.com wrote:
> In one experiment, I compared the performance of a 2 gap 120 bps sync
> to a single 120 bps triggered static gap. In both cases I got a 42" max
> spark length using my TT-42 TC. Using the rotary, the gap light and sound
> emitted was quite low. Using the triggered gap, the gap light and sound
> emitted was intense. The gap spacing when the rotary fired was small.
> The gap spacing of the triggered gap was large at 5/8". Unfortunately I
> remember if I measured the firing voltage in both cases, but input power
> draw was the same. The triggered gap
> may have been capable of firing at a higher voltage for some reason. I
> speculated in the past that rotaries may not be capable of truly firing at
> a peak voltage because of the way the voltage rises as the electrodes
> approach. I'm not sure if that's a valid idea. If both systems fired
> same voltage, then it's interesting that they both gave the same spark
> length despite the very wide triggered gap spacing. One might say that
> the 2 gaps of the rotary introduced extra losses. Yet in other
> I compared the performance of 2 gap and 4 gap rotaries, and didn't see
> a difference in spark length output. Perhaps some slight quenching
> differences occurred such that the negative and positive aspects
> canceled. Various multi-pipe gaps were tried on this coil and the sparks
> were considerably shorter and more feeble looking. I reported all these
> results in the past. I'm re-posting them for the benefit of new list
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