Re: More on spark delay

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Kchdlh-at-aol-dot-com>

John (& all)-

Comments below...


>
> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <FutureT-at-aol-dot-com>
>
> In a message dated 4/23/02 10:29:52 AM Eastern Daylight Time,
> tesla-at-pupman-dot-com
> writes:
>
>
> >
> > >So my supposition remains:  It is the capability of spark-gap systems to
> > >deliver higher power during the (at least first part of) 100 us or so that
> > >allows for the longer sparks.  And it is the physical/thermal inertia of
> the
> > >air in the path of the spark that causes the 100-us phenomenon to exist.
> > >
> > >But perhaps this is old-hat to spark experts.  Comments?...
> > >
> > >Ken Herrick
>
>
>
> Ken,
>
> I would think that a first step would be to compare the spark length
> for a spark gap TC running at 16 bps, to match the 16 bps of your
> system.  Maybe such operation would show equal spark lengths
> for each system? (assuming equal peak toroid voltages, with 
> equal toroid sizes.)  I still think that most of the power in your system
> is being "wasted" in the sense of feeding power for a too-long
> duration (7mS), at a too slow pulse-rate.  This seems to make
> the spark bright and thick,
> rather than long.  In a spark gap TC, the spark pulse duration is short,
> so it has no time to grow bright and thick.  There's no time for the spark
> impedance to drop very much.  Instead, the streamer
> path cools some, then reignites and extends on the next "bang".
> These bangs come close enough together to give proper spark
> length growth.  For best growth, it seems the break-rate must
> be neither too fast nor too slow.  I have no idea if my ideas are
> correct here.  It's possible that what I mention is only a part of the
> answer.  
>
> In any case, it's definitely a most interesting subject, and your
> analyses are most appreciated.  It would be good to prove the
> issue one way or another.
>
> Cheers,
> John




Tough to make such a comparison for obvious reasons...  As I remarked
yesterday, I'll try to modify my system to give a more-controllable spark
length.  

I'll explain my current problem in that regard:  There are two reasons why I've
confined my rep. rate to sub-multiples of 120 Hz.  Firstly, I have, currently,
60 power-MOSFETs (absurd, no?) that I have to turn on & off efficiently during
each 3.5 us half-cycle of excitation.  I could probably do with fewer of them
but until then, I need large pops of ~18V power for that purpose.  Rather that
depend on overly-humungous electrolytic capacitors for that, I draw that power
largely from the l.v. supply-transformer itself because I synchronize with the
120 Hz peaks.  Secondly, during development I had exasperating problems with
getting R-C timing circuits, of numerous kinds, to work properly while the coil
was firing.  So I finally had to develop a scheme that employs such a circuit
only to initiate the start of a spark event, with the ending of the event
controlled by the decline of the 120 Hz mains waveform-peak (which is not to be
budged signif! icantly by piddly little Tesla-coil sparks).  Or, later, by the
counting of excitation cycles where an IC counter is used and not an R-C timing
circuit.

Thus, when I do manage to change my IC-counter circuit so as to allow shut-off
after chosen quantities of excitation cycles, I'll also want to change the
timing circuits to allow rep. rates in excess of 120 Hz.  To do that, I'll need
to try again to get an R-C circuit to work. I face more exasperation, I fear,
and at my age I don't need that.  I suspect I can always put the timing circuit
in the next room--but I remain determined to keep all the apparatus within my
original 2 ft. square footprint.

So on we go...

Ken Herrick