Re: Re[2]: Capacitor Size (long)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FIFTYGUY@xxxxxxx

In a message dated 10/8/05 12:41:04 AM Eastern 
Daylight Time, tesla@xxxxxxxxxx writes:

Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>

> I disagree on two points.

>1) The width of a static gap should NOT be set to
>achieve 120BPS.  While I agree that adjusting the
>static gap width does permit one to adjust the
>BPS to almost any desired value, it ignores the
>most important consideration in setting gap
>width.  The width must be such that the breakdown
>voltage does not exceed what's safe for the NST.


    I think we would all agree that this would 
be the most prudent setting without a safety gap. 
However, I think it can't hurt to open up the 
main gap to where it works best, so long as the 
safety gap is adjusted properly.


> In my earlier post today where I cited
>measured BPS measurements for my mini coil, the
>BPS was between 157 and 257, depending on the
>tank cap size.  The gap width was fixed and had
>been set by connecting only the NST to the gap
>and adjusting it just wider than where it began
>to arc.  Had I adjusted the gap wider still to
>where the BPS measured 120, it would have been
>operating at a higher and unsafe bang voltage.


    Well, Gary, you scoped the bangs - but did 
you measure the actual voltages at the main gap 
and the transformer terminals? I'm not being 
facetious - I'm the last guy you want trying to 
do any meaningful measurements. Better someone 
with good technique and equipment!


>It's not clear what voltage you consider the plus
>and minus peaks to be.  Depending upon how close
>to mains-resonant the cap is and if you're
>saturating the NST core, mains-resonance will
>occur to varying degrees.  If it didn't, nothing
>would happen when the Variac was at 50%.  The
>point is, the NST & cap will achieve 60 Hz
>resonant rise to the point of the gap breakdown
>voltage, and if the gap is set to 30kV, a 15kV
>NST can easily ring up to that.  So it's not like
>a 15kV NST can't charge a cap to greater than
>SQRT(2)*15kV and that's what the gap must be set for.

    Granted, but that takes more than AC 
half-cycle to resonate up to higher-than-normal voltage.


> 2) A static gap firing more than 120 times per
> second is not uncommon, it's the norm.  I base
> this opinion on scope measurements of my coils.


    So assuming folks "properly" set their main 
gap by using just the NST output alone (no prim 
cap), how come the break rate is *higher* than 
120? How is resonant charging occurring to drive 
the cap charge voltage up, if the bps is higher 
than 120 and the cap value is properly LTR?

>Think of it another way.  For any given NST size,
>the recommended cap size for a 120BPS sync gap is
>about twice the value as for an LTR static
>gap.  The goal for both cases is to achieve the
>largest bang as is safe, so the bang voltage must be comparable.


    I'm inclined to believe that larger bangs 
energies make longer sparks, all other things 
equal. And it seems we need a minimum of 100 bps, for some reason.


> Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>
>
> It's done with the setting (distance) of the
> spark gap itself.  It is adjusted such that the
> gap can only fire when the AC waveform hits its
> plus and minus peaks, thus insuring it fires at 120 pps.


    If this was true, then what's the point of 
running at 240 bps? Are the bangs at the 
zero-crossing smaller ones, where the cap isn't 
charged fully, yet any additional bang gives a contribution?


> If the gap is set too close together it could
> re-fire more than 120 times per sec but this is uncommon.
>
> Dr. Resonance

    And so Gary Lau disagrees...


>
> Thursday, October 6, 2005, 10:08:55 PM, you wrote:
>
>
>  > Original poster: "Lau, Gary" <<mailto:Gary.Lau@xxxxxx>Gary.Lau@xxxxxx>
>
>  > In my early naïve days of coiling I thought that
>  > 120 BPS was the natural and normal mode of
>  > operation for a static gap.  I know better now
>  > and don't wish to perpetuate that misunderstanding.

    I'm glad you brought it up, because it got me thinking....


>   >> Original poster: "D.C. Cox"
> <<mailto:resonance@xxxxxxxxxx>resonance@xxxxxxxxxx>
>
>
>   >> Erms resonant value times 1.57 for best effective C value.  With 30
>   >> mA its near .008 and doubles to 0.[0]16 at 60 mA.  This works best with
>   >> a static copper tube sparkgap firing 120 pps.


    Well, at least one thing we're certain of 
with proper *rotary* gaps: the bangs aren't 
occurring any *faster* than the design break rate.
    So here's what I'm thinking, how Gary Lau and D.C. can *both* be right:

    We know we need a minimum break rate to 
propagate nice long streamers. This means there's 
some time constant in the air that allows 
streamer channels to stay open/charged/whatever. 
I'm still waiting for somebody to give a detailed 
explanation of why that is - which ion species it 
is that we have to worry about, how many need to 
be excited to what energy, how the energy 
dissipates, and how to model the streamer up from 
the small to the full-size. It may seem academic, 
because we're stuck with sparking through air - but see below.
    There seems to be a wide performance in 
spark gaps, depending on the design. Folks have 
tried all sorts of things, and I'm not going to 
try to reinvent the wheel. Especially when I 
don't have a meaningful way to accurately measure 
the many important parameters at once. But it 
seems like the things we try to quench the gap 
would be the *opposite* of what we want in order 
to better propagate the streamers off the secondary.
    Likewise, if the secondary streamers appear 
to "like" 100+ bps, why wouldn't the main gap 
"like" that is well? After all, they're both 
high-voltage, high-frequency, pulsed discharges 
in air. And on a SGTC, the two rates are the same.
    So what if - by picking a break rate over 
minimum, and by discharging repetitively through 
the same air space, we are doing the same thing 
on the primary gap that we're trying to do at the 
streamer: decreasing the voltage necessary to propagate the next spark.
    I think this would explain a few things:
    With a properly set gap (barely over 
breakdown for X-fmr), the break rate goes up past 
the nominal 120 bps. This could be because the 
cap does not have to charge to full voltage to 
break down the gap. Since it doesn't have to 
charge to full voltage, it can be charged quicker 
by the same X-fmr. That's why similar performance 
from a LTR cap - it's not being charged to full 
voltage anyway, so the increased size doesn't matter much.
    A way to fix this would be to open up the 
main gap - as D.C. advocates (with an adequate 
safety gap that doesn't fire prematurely or 
frequently, of course!). This would move the main 
gap's necessary firing voltage back up, and increase performance.
    Force-ventilated gaps, and gaps with serious 
cooling surfaces take the heat/charge/whatever 
out of the gap region, or replace 
energized/charged air with fresh. This would 
bring the gap firing voltage back up, and increase performance.
    However, we have a double-edged sword. 
Whatever we do to increase the breakdown voltage 
for the gap, by replacing the air or increasing 
the gap distance, also hurts us by increasing the 
on-state resistance of the gap, and therefore the 
gap losses (more gap losses also make it harder 
to keep the gap voltage up!). And so it spirals, with marginal gains.
    We need a way to keep the excited/energized 
air in the primary gap, so we don't waste the 
energy we've already put into it. Keeping the gap 
width/number low would keep the excited air in 
the gap where we could use its low resistance for 
the next bang. Keeping this gap as short as 
possible might keep our losses low as well. But 
we also need a way to make the gap fire at only 
the proper high voltage. Perhaps an interesting 
case with the magnetically-quenched and/or triggered gaps?
    Since RSGTC's can run even bigger caps with 
better performance, there must be some way the 
humble NST is actually getting more energy into 
the cap than the resonant charging numbers say it 
can. And if we can improve the static gaps, then 
we can do the same to the rotary gaps.
    And thus gleaning what goes on in the 
secondary streamer helps us at both ends.
    Must be nice using solid-state switching, 
where everything in the primary resets for the next bang.

-Phil LaBudde