Re: S.s. info...re: differing secondaries

["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>

 
On Thu, 15 Feb 2001 11:45:54 -0700 
 
[snipped]
 
> > Original poster: "Kennan C Herrick by way of Terry Fritz 
> <<mailto:twftesla-at-uswest-dot-net>twftesla-at-uswest-dot-net>
> > " <<mailto:kcha1-at-juno-dot-com>kcha1-at-juno-dot-com>
> >  
> >  S.s. coilers may be interested in the following:
> >   
> >  As I've previously reported, my s.s. system utilizes an untuned 
> primary and
> >  also a means for making the excitation frequency always exactly 
> that of the
> >  secondary's resonant frequency.  I have now made two secondaries, 
> both
> >  according to "HERRICK'S RECIPE..." posted in 8/00, one 52" high 
> and the 
> > second,
> >  36" high.  Fr's are ~100 KHz and ~125 KHz respectively.  I use 
> the 
> Landgren 
> > 6"
> >  x 24" toroid with no added breakout-point.
> 
> Ken, all,
[snipped]
 
Have you tried using a breakout point, or smaller 
> toroids,
> to see what effect it has?  I seem to remember that you mentioned
> in the past that the sparks looked somewhat disruptive in 
> appearance?
> They must be very fierce and thick though.  Are there any photos
> available?  I seem to remember you saying you were going to offer
> these at a price?  (or that may have been someone else.)
 
Breakout point?--no: I want to achieve the maximum voltage the 6 x 24 toroid
will accomodate.  "Disruptive"?--not sure I know what that means.  Photos?--no,
I need to Get a Round TUIT, to borrow the old witticism.  A price?--yeah, I
have
had that in mind.  But I may relent since there are seemingly so few s.s.
aficionados around.  Also, my scheme ends up being pretty complex, & thus
expensive, & so would not be everyone's cuppa tea.  Plus, I am finding that the
IRFP460LC MOSFETs, while good parts (the IR ones, anyway; not STs!), get a tad
too warm for my liking at the higher rep rates--with ~225A pulse-burst current
thru every 6 of them in parallel.  To preclude more extensive heat-sinking
and/or forced air cooling, I may look into better ones so that may change
things.
 
> >  I find that, while the 52" coil yields very satisfactory sparks, 
> those 
> from 
> > the
> >  36" coil are markedly less so.  It might be that the driving 
> impedance for 
> > the
> >  spark is diminished, away from optimum, by going to the lower 
> turns-ratio
> >  (~2:760 vs. ~2:1100).  But then, the operating frequency 
> increases with the
> >  shorter coil, so one might expect the driving impedance to be 
> higher due to
> >  that.
> 
> The impedance may be shifting more than the frequency, when all is
> considered?
> You say the sparks from the shorter coil are less impressive, but 
> can
> you give us some idea of what's different about them, and their 
> sizes?
> Are the sparks mostly shorter, or thinner, or branched differently, 
> etc.?
 
As to the impedance vs. frequency, I'd think they'd be pretty much directly
proportional.  As to the sparks, they were shorter & thinner; altogether
wimpy.  I'll make a more thorough evaluation when I make the close-wound coil.
> >   
> >  Manually sweeping the two coils with a signal generator shows 
> that their 
> Qs 
> > are
> >  essentially the same--around 100.  So with a Q of 100 and with 
> only 760 
> > turns
> >  in the secondary, I am finding that spark breakout still occurs 
> from the 
> 6" 
> > x  24" toroid.
> 
> I assume your generator is low enough impedance to give a true Q 
> reading?
> I would have thought the Q would be higher, but that's a guess 
> because
> I don't know the details of the secondary.
 
I had measured the Q of the 52" coil some time ago, driving it at the bottom
end from a sig gen via a several-hundred ohm isolating resistor, and then
eyeballing the E-field picked up by a nearby scope probe and utilizing the
usual formula f2/(f3-f1) where f3&f1 are at -3db and f2 is at max.  For the 36"
coil, I merely (& much more roughly) eyeballed the coil responses while
tweaking the sig gen's knob to & fro; they looked much the same.
> >   
> >  I also notice the following:  1.  The electric field, as measured 
> by a
> >  partially-shielded scope probe placed ~4' away from the coil 
> c.l., is
> >  essentially the same with both coils--both at the peak of the 
> envelope 
> when 
> > the
> >  spark breaks out (as expected since it's the same toroid) and 
> also during 
> > the
> >  remaining ~5 ms spark duration.  And 2:  The input line (mains) 
> current,
> >  directly equal to average primary current in my case, is 
> essentially the 
> > same. 
> >  It should be noted that, during the entire pulse-burst time, the 
> secondary 
> > is
> >  driven at its instantaneous self-resonant frequency.
> >   
> >  So I tentatively reach this conclusion:  The factor that mostly 
> caused the
> >  decrease in spark energy was the increased operating frequency 
> since a) Qs 
> > were
> >  the same, b) primary power was the same and c) the turns ratios 
> in the two
> >  cases were sufficiently high to allow spark break-out from the 
> same 
> toroid. 
> >  Thus I would conclude that one wants to employ a secondary 
> construction
> >  technique that minimizes Fr while at the same time providing a Q 
> vs.
> >  turns-ratio condition such that spark break-out reliably occurs 
> from the 
> > toroid of choice.
> 
> I'm very surprised that such a small difference in frequency would 
> make
> much difference.  In work I've done, I've run at double the 
> frequency
> or more, and not seen very much difference, although my tests were
> somewhat poorly controlled, and may not be valid.  I wouldn't be 
> surprised 
> if something else is the cause.  Then again, I have no idea how 
> much
> shorter or less impressive the spark is from the 36" secondary.  
> It can be a real bear to track down the causes of spark differences 
> in
> these systems because of the many variables.  I suspect something
> is the area of impedances perhaps.  It will be interesting to see 
> what
> others have to say about this.
 
Yeah, I was surprised myself.  As I say, I'll take another look at it when I
have the close-wound coil to test as well.  Also, I need to get the whole
assembly out from under my relatively low (conducting) ceiling.  It happens to
be awkward to do that so I'll wait until I have the 3 coils to play around with
so I don't have to lug it all to & fro more than once.
> >   
> >  My next task will therefore be to construct another 36"-high 
> secondary, 
> also
> >  with 20 ga. wire but close-wound instead of spaced ~.05" 
> center-center.  
> > That
> >  will bring the frequency down substantially while maintaining 
> ~1100 turns,
> >  although Q will diminish due to the lack of spacing.  But the 
> lower Q will 
> > be
> >  compensated for by the higher turns-ratio, in bringing the toroid 
> voltage 
> to
> >  break-out potential.  So, we'll see...
> 
> That does seem like a good test.
> >   
[snipped]
> 
> I'm glad to see the interest lately in VTTC and SSTC work.  These
> coils are often shunned by coilers, I doubt if even 1 out of 10 
> coilers
> have built one.
 
> John Freau
 
I'm sure you're right, there!  It's dozens upon dozens of parts, plus the
requirement to well-understand s.s. electronics, vs. half a dozen parts or so,
that no doubt puts most people off.  And rightly so!--I can't begin to tell you
how many episodes of discouragement I've experienced.  At 73 I need to begin
thinking of retiring from all this some day.
 
Ken Herrick