Re: On sparks

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Kennan C Herrick by way of Terry Fritz <twftesla-at-uswest-dot-net>" <kcha1-at-juno-dot-com>

John (& all)-

Comments from Ken Herrick interspersed:

On Mon, 26 Mar 2001 12:03:37 -0700 "Tesla list" <tesla-at-pupman-dot-com>
writes:
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" 
> <FutureT-at-aol-dot-com>
> 
> In a message dated 3/26/01 5:07:04 AM Eastern Standard Time, 
> tesla-at-pupman-dot-com 
> writes:
> 
> > 
> >  Just to see what would happen, I thought to extend the function 
> of my
> >  128-cycle counter so as to interrupt the primary's excitation in 
> bursts
> >  of 128 cycles over the entire 7 ms duration.  400 us on, 400 us 
> off, etc.  [Not 128 but rather, 64--kch]
> >  
> >  The sparks appear identical in form--jagged & branched--but they 
> are, of
> >  course, less "fat" when interrupted.  But that they appear 
> otherwise
> >  identical tells me that each burst of sparks travels essentially 
> the same
> >  path during the entire series of 400 us bursts.  I suspect that 
> it's the
> >  heated air, along the path, that induces all the repeating sparks 
> to
> >  follow along. 
> 
> Ken,
> 
> That's an interesting test.  You seem to be implying that the 
> sparks
> are still the same length as before. This would suggest to me that
> the duration of the application of current to the arcs does not 
> seem
> to make them shorter, but just dimmer or thinner.

Well, I'll try to be a little more accurate, as I might have been in the
first place:  I've just taken a more careful look, in the dark, at the
sparks when in the 3 modes of operation--~400 us duration, ~7 ms but
periodically interrupted, and ~7 ms steadily on.  The character of the
sparks is the same: branched, often if not usually with 2 or 3 branches. 
But the general length does seem to increase, perhaps 30%, overall, from
the shortest- to longest-duration modes.  And, of course, the apparent
thickness increases along with that.

> By reversing
> this thought, it seems to suggest that applying current for a 
> longer
> duration would not make the spark longer either.

It makes it longer, but not a whole lot longer--nowhere near
proportionally to the duration.

> I'm assuming
> that since the coil is off for half the time during a burst now, 
> that
> the total average current supplied to the arc is much less overall.
> Are you thinking the same way?  Am I missing some point?

It is definitely less; I haven't measured it exactly but it is surely, in
my system, going to be directly proportional to the overall duty cycle of
sparking.  When I make no sparks, I have close to zero line-input current
since everything drawing static current in my system, with the exception
of a few pull-up resistors, is CMOS or MOSFETs, & their static current is
negligible.  Essentially all the line power goes into charging the
storage capacitors, and all of their output currents go into the primary.
 The main losses are in the switching supplies' MOSFETs and the
primary-loop MOSFETs, and those are modest.
> 
> Maybe it's the peak currents, rather than the average currents
> that are important in determining spark length?  Any thoughts
> about this in light of your experiment?

That's worth some thinking about--or experimenting.  I haven't looked
into it; my primary current is only slowly variable, dependent upon the
partial discharge during the pulse-burst of the electrolytic storage
capacitors.  I don't have any handy way of varying it abruptly.  I am
reminded that I do notice a definite reduction in length when I increase
the rep. rate so much that the capacitor voltages sag pretty far down. 
So there's a clue.

> 
> For instance if you can channel the power you "saved" by
> having the coil off during half the burst time, into higher peak
> powers instead, that may make the sparks longer.  Since
> the MOSFETS are only "on" half of the time now, they should
> be able to handle higher peak currents without burning up.

I tend to think not for this reason:  That 400 us spark is definitely 30%
or so shorter than the 7 ms one and yet it emanates from the toroid at
the instant when the toroid voltage is upwards of 8x the voltage there
while the 7 ms spark is occurring.  Surely (correct me if I'm wrong) the
peak power at that initial instant is much greater than the power being
expended at comparable instants during the 7 ms.  So if the conjecture
were correct, one might expect that initial spark to be, although
thinner, longer and not shorter.  But it isn't.

What I'm going to need is more ampere-turns in the primary.  I'm not
analytical enough, unfortunately, so far, to be able to dope out ahead of
time what is the optimum mix of switches/capacitor-banks vs. quantity of
primary turns in my current-loop primary system.  Right now I have 4
switches, 4 capacitor banks and 3 turns.  I happen to have 1 more set of
circuit boards from which I could make up a 5th set of
switches/capacitors.  If I can dope out how best to wire that 5th section
into a 3-turn primary configuration, I may take a shot at that; I think
the MOSFETs would stand the current.  That will raise the ampere-turns by
~5/4 and surely make the spark longer--but to what extent, who knows?


> 
> John Freau
> 

Ken Herrick
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