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TT-32 TC results and specs
Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>
Someone asked me some time ago if a smaller coil than my
TT-42 TC would obey my empirical efficiency equation. I figured
it would, but I wanted to make sure. Now I built a new smaller
coil, and tested it at various power levels. The results were
excellent, and closely obeyed the equation. I'll list here the
TT-42 and TT-32 specifications:
toroid 4" x 13" 3" x 10"
secondary 4.2" x 18.75" 3.25" x 16.25"
sec. wire dia. 28awg 28awg
primary (flat) 14.5 turns 10.7 turns
pri. wire dia. 10awg 12awg
capacitance 0.0147uF 0.0164uF
sync rotary gap 120bps 120 bps
variac (7.5A) 0 - 140V 0 - 140V
NST 12/30 Jefferson 9/30 Jefferson
spark length * 38" 33"
NST 12/30 Jefferson 6/30 Jefferson
spark length * 38" 26"
The original equation is:
spark length (inches) = 1.7*sqrt input watts (wallplug).
If the rated VA for the NST is used in
the equation, it doesn't work out right. This is because a NST
can draw more than its rated power when used in a TC,
especially when supplied with 140VAC. Nevertheless, it can
be seen that the output of the TT-42 and TT-32 coils obeys the
equation within 1" of spark output, from a proportional point of view.
Here's a new equation which may be useful for NST users who
build efficient coils:
spark length (inches) = 2*sqrt VA of NST (rated)
Examples: (be sure to use the NST rated VA)
2*sqrt 180VA = 27" 6/30 NST
2*sqrt 220VA = 30" 7.5/30 NST or 9/25 NST
2*sqrt 270VA = 33" 9/30 NST
2*sqrt 360VA = 38" 12/30 NST
2*sqrt 450VA = 42" 15/30 NST
2*sqrt 900VA = 60" 15/60 NST
Note: A NST with built-in PFC may show a lower than normal
rated VA on the nameplate. In this case, multiply the rated kV,
times the rated mA. Don't use the nameplate VA. Coils should
be built larger as the VA is increased. Truly optimal coils may
slightly outperform the figures shown above. Many less optimal
coils may severely underperform the figures shown above.
I also tested an unlabeled NST which I think is a 9/25, and it
gave 29" sparks which is within 1" of the expected length.
Misc. TT-32 notes:
The primary gets somewhat warm, so the coil may benefit
from using a thicker primary wire, although I suspect the
benefit would be slight. The capacitor is a Maxwell pulse cap,
rated at 45kV. The sync rotary is an old one from previous work.
I used my remote electrical phase adjuster to adjust the rotary gap
phase. Spark length measurements were made using unmodified
NST's. The secondary was coated with 3 coats of water based
latex polyurethane. The form is grey PVC drain pipe. There's about
1/2" laterally between the primary and the secondary. Also, the
secondary is raised about 1/2" above the primary. I can't easily
lower it, so I didn't try closer coupling. I didn't measure the
coupling. No racing sparks occured during my tests. I would have
used 30awg wire to make the secondary shorter, but I didn't have
any. I didn't use any kind of safety gap or protective networks.
Safety gaps never fire with a sync gap, unless something fails.
So far the coil is just cobbled up, not built onto a nice base
or anything. The toroids are smooth. The TT-32 toroid is 1" above
the secondary winding. Pictures are not available, but it should be
possible to build the coil from the information I give in this posting.
I may tidy it up to make it portable like the TT-42. *I list the TT-42
spark length as 38" because this is the spark length it gives using
a normal NST. I usually use an old robust 12/30 NST on the coil
which gives 42". My old research coil gave 42" using a normal
I suspect that the coil can be scaled down even more while
staying rather true to the equations, although at some point the
performance will fall off.
Twin coils produce sparks that are about 1.4 times longer than
what I show above.