RE: [TCML] Spark gap

["Lau, Gary" <Gary.Lau@xxxxxx>]

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of bartb
> Sent: Sunday, February 24, 2008 4:41 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Spark gap
>
>> Measuring the actual BPS rate on a static gap has always been difficult.  The only
>> way I know to measure it is by using a storage scope and counting the bangs in
>> some time interval and doing the math.  Have you actually scoped and measured
>> your BPS?  I have, and this is how I arrived at the numbers I cited.  Particularly
>> when using LTR caps with a static gap, non-linear ferro-resonance effects cause
>> totally unpredictable secondary currents far in excess of the faceplate rating to
>> occur, and I don't believe that any method exists to reliably calculate BPS.

> I have scoped the gap but have not gone as far as to store the data and
> run the numbers for bps. It certainly would be difficult to measure and
> your method is perfectly acceptable. I have a new data acquisition tool
> that can capture 14,400 samples per second. I should attempt a method
> for recording my gap using it. I understand the gap is chaotic, but we
> have to look at the average over a decent time frame. I realize the bps
> will be up and down from event to event. However, I expect bps to be
> different from one coil to the next and it sounded like a blanket
> statement of 150 to 300 bps for static gaps, and that just doesn't sit
> well with me. Maybe I misinterpreted that as well.

Of course the BPS will be different from coil to coil, but I believe the 150-300BPS range would include most coils built to reasonable parameters.  My measurements were over, I think, a 1/2 sec interval.  Mostly I wanted to emphasize that static gaps will never operate at a constant 120BPS, as many newbies (including myself once) might think.


>> Let's first recall that in the tables which recommend the optimal capacitor size
>> based on the transformer, there are separate columns for static gap and for sync
>> RSG, the sync RSG using a much larger cap.  With my 15/60 NST and a static  gap,
>> I found that a .02uF cap worked best.  Anything larger caused excessive current
>> draw and ran poorly.  But the same NST with a 120BPS sync RSG ran best with a
>> .04uF cap.  I wish that I could explain why that a SRSG allows a larger cap to be
>> used without a lot of hand waving, but it is a well documented fact.  Do you
>> disagree?
>
> It's not that I disagree, it's maybe that I view the cause differently?
> With your 15/60 and static gap, the .02uF cap worked best and larger
> sizes caused excessive current draw and ran poorly. I would certainly
> think so. Even .02uF is pushing it. The time to get to peak firing
> voltage to arc the gap is getting excessive at 18ms! Try a .03uF and you
> are nearing half a second! That's the problem I see and it makes perfect
> sense to me. The rotary has the option at firing at a much lower voltage
> (about 14kVp with the .04uF cap). This is "why" the rotary can use the
> larger cap size. Now the static gap could also be made to work, but the
> total gap is going to have to get really narrow. Static gaps are fixed
> and usually the only way to narrow the gap is to tap inward on pipes
> (which increases thermal losses).

I can say with certainty that when I used .02uF and the static gap, it charged to >20kV (what the static gap was always set to) in far less than 18ms.  I don't know that I had measured the BPS with the storage scope at that point, but I'm certain it was a great deal more than 55 BPS.  And when I tried using a .03uF cap, the gap was still set to at least 20kV, and while it ran rough, it surely fired more than at 2BPS!  I think your analysis fails to take into account the super-charging that occurs due to ferro-resonance.

> However, you say larger cap sizes allow a larger bang size. In general,
> yes, but not always. Your 15/60 you might calc at 12 joules if you
> didn't consider the voltage at the cap at each firing of the RSG at
> 120bps. Energy drops down to 3.92j! The .02uF cap is up at 4.03j. So,
> this is part of my problem. To state there's a higher bang energy is not
> correct (and I've heard that stated many times but without any data to
> support it). I think the larger cap size performing well is for another
> reason such as arc dynamics and transfer functions.

With my static gap, I left the gap the same, so increasing the cap size would always directly increase the bang size.

When I made the move from static to SRSG, I experimented with cap size to see which resulted in the largest bang size with the 120BPS SRSG.  Unlike in the case of the static gap, the bang voltage now varies with cap size, so here it IS correct to say that a larger cap size will not necessarily result in a larger bang size.  Here is the table of measurements, showing cap size vs. Vbang and bang size:
.02uF   24.4kV  5.95J
.03uF   21.9kV  7.19J
.04uF   19.6kV  7.68J <- Biggest bang size!
.05uF   17.1kV  7.31J

>> Spark length is all about using the biggest bang possible.  And it guarantees
>> that each bang is identically and maximally size
>> I assume you're questioning the part about spark length being correlated with
>> bang size?  I think it was one of the folks (maybe Finn?) with a DRSSTC who had
>> the ability to vary bang parameters independently, found that varying just the bang
>> BPS varied the streamer brightness but the length was relatively unaffected.  But
>> varying the bang size directly affected the streamer length.  So, optimizing a TC to
>> have the largest bang size at a relatively slow BPS will yield the longest, thinnest
>> sparks, while choosing a small bang size at a very high BPS will yield shorter, but
>> brighter sparks.

> Using a larger cap doesn't always correlate to a larger bang size.

In all of the instances I have discussed in the context of static gaps, it does, as the bang voltage is assumed to (and does) remain the same.

> BPS
> "must" be slowed in order to accomplish that and the transformer "must"
> be able to supply the current, if not voltage drops and bang size is not
> increased. I agree that a larger bang size will yield longer sparks in
> most cases. I don't agree that adding a larger cap will do it unless the
> transformer is beefy enough to supply the current. In your case with the
> NST, it doesn't increase bang size, however the RSG allows the firing
> anyway, so something else is the cause.
>
> Am I making any sense?
> Take care,
> Bart

I think we just need to be clear on whether we're talking about static or RSG gaps.  For a static gap, leaving the gap unchanged, a larger cap will result in a larger bang size, and likely longer but thinner sparks at a lower BPS.  That was the extent of my point.

Regards, Gary Lau
MA, USA
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