Sweet (and sour) spots, was 120bps vs 240bps tests

Malcolm, Richie, all,

I did some more tests to clarify (cloud?) the issue   :)  I of course
used my standard 42" spark TC for which I've posted the specs
numerous times in the past.

In these tests, I ran the TC at various power levels, and with two
different toroid sizes, to see how far the spark length would vary
from the square law predicted lengths.  Spark length shortfalls from
predicted lengths might mean that the coil is becoming inefficient in
some cases, or in other cases it could mean that the square law 
is not precise for predicting TC spark lengths as the power input

All tests at 120 bps.  42" is a reference length.  (This table can
be more easily analyzed if it's printed out on paper, so the headings
don't scroll out of view as you scroll down.)

The first test series used a 4" by 17" toroid, ballast at # 1, pri
tap = 23T.  I didn't retune as power was reduced:

Watts    cap W     length pre    length actual    shortfall%
620         498             44.6"            43"                3.6
570         440              42               42                  0
500          ---               39                38                  3
480    needs retune     38                34                 11
450    needs retune     37                30                 19

Here I retuned as needed at 21 turns, and changed ballast to #2
(most of improvement due to retuning)
400         318              35                30                 14

Changed the toroid to 4" by 13", tuned at 20 turns:
400         318              35                 32                 9
(this shows that the 4" by 17" toroid was becoming too big for
the reduced spark length.)

Retuned as needed:
620         498               44.6              43                 3.6
580         440               42                 42                  0

Retuned at 19 turns as needed:
380         ----                 34                 30                 12
320         ----                 31                 27                 13
280         210                29                 24                 17   

Possible causes of spark length shortfall as power is reduced:
1.  relative gap losses
2.  sub-optimal toroid sizing as spark length decreases
3.  relative Ctop decrease, relative to Cs + Ctop total
4.  Generally a doubling of the input power
     should demand a 41% larger toroid, to keep the output impedance
     the same (same balance of voltage vs. current).  This makes 
     sense based on the unvarying 212k ohm arc impedance found by
     Terry and Greg Leyh for different sized sparks.  This should also
     result in a 41% longer spark.  But other unknown factors may 
     cause the TC output to vary slightly from the the square
     law predictions.  Or the capacitance of the arc may be having some
     effect here?  I'm not sure how that effects it.

In any case, the results above show that:
1.  Just ramping up the power (or maybe the break rate too) may
     give false results; the coil has to retuned for each power level
     (nothing new here).

2.  The toroid size is very important.  Too small or large will throw
     the TC "off the curve" and give misleading results.  This is nothing
     new either.

3.  When comparing efficiency at different break rates, power levels,
     etc., sweet spots as shown above must be taken into account.

4.  Not shown in the table, but I found that the charging efficiency
     stays quite constant at all the different power levels, assuming
     the ballast is properly adjusted for best results at each power
     level.  (Note:  I did not check the charging efficiency at higher
     than 240 bps.)

5.  Thus, when making comparisons of 120 and 240bps, it may be
     best to keep the output spark the same length, and let the bang
     size vary.  But there's still the gap loss variable to consider in 
     this case.  

A question:  Has anyone determined how much the gap losses
might change when the power is ramped up by 41%, in a particular
coil?  How much of the results seen might be caused by relative 
gap loss % changes?

In any case, sweet spots seem to follow a predictable trend, and seem
to depend mostly on proper toroid sizing, proper tuning, and proper
input power for a particular TC perhaps.  For instance, if your coil has
a small cap, and a big toroid, it will need a high break rate to have any
hope of a good output.  But if it has a big cap, and big toroid, then it
will perform well at a low break rate, but will give an even longer spark
at a higher break rate, but it might not be as efficient overall.  Etc, etc.

John Freau