Re: Certain s.s. phenomena; to J.F. et al

["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 interspersed from Ken Herrick:


> 
> Ken from J. F.,
> 
> Maybe the coupling is not as tight using just one turn?  Did you
> measure and adjust the couplings when using various numbers of
> primary turns?  Maybe impedance matching becomes an issue?
> Impedance will be very different before breakout than after, so I
> guess some sort of compromise is at work.

The coupling is as tight as I can make it: the primary tw. pairs bunched
together, formed to the same diameter as the secondary, and with the
secondary just plunked down on top of them.

The primary impedance seem definitely a factor: with the 1-turn
configuration, I get marked heating of the MOSFETs whereas with 3 turns
the heating is quite moderate.  3 turns yields, presumably, 9x the
impedance but on the other hand I drive it with 3/2 x the voltage, and no
doubt actually more due to the reduced MOSFET and capacitor drops.

> >  
> >  1.1  With the 3-turn configuration, spark appearance is 
> essentially
> >  identical while mains current, for the same spark duty-cycle, is 
> reduced
> >  by ~20%.  
> 
> Maybe the 3 turn arrangement gives tighter coupling, and/or a 
> better
> Z match?

See above.  I'd love to be able to devise a means of dynamically changing
the turns-ratio at the moment of spark-breakout.  That might well take
care of the Z-match problem!

> 
> >  2.4  The conclusion might be that if one wants to use a smooth 6" 
> x 24"
> >  toroid without a break-out point, one would definitely want a 
> space-wound
> >  primary, and with that spacing greater than mine--which averages 
> ~.03"
> >  betwen turns.  Either that or liberally slather on epoxy or the 
> like and
> >  hope that that substance withstands the voltage.  Is that what 
> people do?
> >   I'd hesitate to do it since a breakdown within that coating 
> could be a
> >  pain to repair.

See my "...revisited" posting of today.  Even though I seem--at present,
anyway--no longer to have the coil-arcing problem, I'll likely make a new
secondary using a scheme I've come up with that will allow me to utilize
20 ga bare copper wire spaced, very uniformly, at about 0.075" and wound
mostly in free air.  As long as I can get break-out with that, on a 3' or
so coil, let it spark!

> 
> I think CW coils have more of a problem with racing sparks for a 
> given
> toroid size and spark length than spark gap TC's.  I know that many
> folks get long sparks using huge toroids with relatively small 
> secondaries,
> and do not get racing sparks.  This is little or no coating on the 
> coil.
> 
> I noticed on my 36" spark tube coil, there was a lot of corona on
> the primary, and I was using a breakout point.  With a big toroid,
> the corona would have been much worse.  Yet, on my spark gap
> coils, with even longer spark outputs, from the same coil, I didn't
> have the corona problem.
> 
> In another test, I used a 7" dia by 9" tall secondary (28awg).  The
> spark was reduced slightly, but i still got 34" sparks or so.  So
> these were tube coil sparks that were almost 4 times the length
> of the secondary.  Of course I use a breakout point.  This would
> not have worked with breakout from a toroid.  (I may have posted
> this before, I'm not sure if it was lost with the list-server 
> crash.)

It may have been lost; I didn't see that comment.  You are saying, I take
it, that such a short coil, vs. the spark-length, would not tolerate an
attempt to establish a horizontal spark.  The spark would just jump the
coil to ground.

> 
> In any case, it would appear that tube coils of the typical design,
> have to have a breakout point for longest spark output.  This seems
> to be because of the double-tuned design, the resultant frequency
> splitting, the changes in Q and bandwidth with breakout, and 
> probably related Z matching factors also.

Well, perhaps...but a) spark-gap systems are also double-tuned, b) I do
not know what "frequency splitting" is, and c) the secondary's Q should
diminish essentially the same amount regardless of the manner in which
the spark is induced to initially break out.  I'd think that the
differing relative impedances during the build-up and sparking events
(Z-matching, as you say) would likely constitute a major factor in
determining the differences in performance of tube, s.s. and spark-gap
coils.

> >  
> >  3.  Finally, a parenthetical observation of interest:  Since I 
> can make
> >  just 1 spark-event occur at a time if I wish, I can see that, 
> often, more
> >  than 1 spark will appear at the same time, i.e. during a given
> >  pulse-burst, from the surface of the toroid.  As best I can tell 
> by
> >  closely watching the coil's E-field as displayed on the nearby
> >  oscilloscope, such event is accompanied by a very sharp drop in 
> the
> >  E-field, at the instant of break-out and only on a negative 
> half-cycle of
> >  that field; never on a positive one.  Subsequent to that very 
> sharp,
> >  apparently negative-half-cycle, reduction, the E-field 
> diminishes
> >  uniformly, as expected, and is flat for the remaining duration of 
> the
> >  pulse-burst.  Anyone care to explain that?
> 
> The large drop in e-field with two simultaneous breakouts may 
> just be due to the extra resistive streamer loading of two 
> streamers.

It seems that that's what it is.  Also, when I examine the very first
cycle of the E-field, as expanded on the scope, I see that it is always
the negative one that diminishes first.  Trying to recall my 1950's
Physics 1A, I conclude that that is a direct indication that it's the
electrons (those little >negative< rascals), crowding too much onto the
toroid and as a result jumping off, that are the reason for that.

> I have no idea about the negative half-cycle thing, but it's 
> interesting.
> I notice that my pulsed coil with a point on top also sometimes
> creates two sword-like sparks instead of one.  Maybe this is also
> caused at a negative half-cycle breakout?
> 
> Regarding what you said about the spark being basically unchanged
> with the 3 turn secondary.  I'm not sure what you mean.  You also
> say above that the spark was the same, but mains current was
> 20% lower, which suggests better efficiency at least overall.  Can
> you raise the mains current to what it was with the 2 turn coil, 
> and
> see how the spark behaves?

I raise the mains current merely by increasing the rep-rate--up to what
my current-limited power supplies are set for.  I've tweaked it up to
40/second now, producing a very satisfying buzz, but it's a lot of effort
to change back to the 1- or 2- turn configuration in order to make any
kind of a close comparison.  Have to unplug a whole lot of connectors,
removing the entire primary bundle, and substitute the other one.  Since
I don't >have< to, I just may not.
> 
> I wonder how long the spark would grow from a sharp point sticking
> out from the toroid's edge?  The spark would not be as thick, but
> may grow longer?

The spark is as thick, or even thicker (from a 3" dia ball), but not
appreciably differing in length.  It seems as if, regardless of the
actual break-out voltage (from 3" vs. 1 1/2" radii, in my case), I don't
reach longer than 2 1/2 to 3 ft., just eyeballing it. As to a "sharp"
point...but wait: 

About to finish this email, I became intrigued & can now offer something
>quantitative< for a change:

Spark rep-rate & duty cycle, power input: the same.  

Emitting radius:                   3"      1.5"      a sharp awl-point
E-field deflection on the scope:   8 cm    5 cm      4 cm

Spark length, appearance:  Essentially unchanged: relatively fat,
branched.

Now that's interesting!!  The sharp point cuts the peak voltage in half
(surely reducing stress on the coil) but no more than that!  I'd never
have believed it.  At least as observed visually, no sparks reach any
conductor to ground during this test; they just end in air.  But perhaps
it is the presence of nearby conductors, nevertheless, that accounts for
the only 2:1 difference in attained voltage between the toroid and a
sharp emitter.  I've not tried the coil yet out in the open, which is a
bit awkward in my circumstance since I have to schlep it and the scope
upstairs to a deck off the living room.  I'll try to do that before too
long. 

Marco DeNicolai had a relevant posting, on 1/31, on the subject.  

Very roughly, since I am unable precisely to measure my input power, I
measured around 1.5 mains KW going into 13/second sparks drawing power 6%
of the time.  Each spark, therefor, would contains 1.5/6% or 25 KW--over
about 5 ms duration. That's what gets me 2.5-3 ft. sparks.  I'd like to
know how to "massage" that set of criteria so as to push that spark out
further!

Ken Herrick 

> 
> John Freau
> 
> >  
> >  Ken Herrick
> 
> 
> 
> 
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