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Re: Getting to the limit with an NST powered SGTC
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- Subject: Re: Getting to the limit with an NST powered SGTC
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- Date: Wed, 23 Mar 2005 17:37:07 -0700
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Original poster: FutureT@xxxxxxx
In a message dated 3/23/05 2:11:05 PM Eastern Standard Time,
G'day everyone! It's been a while. Still coiling :) Just finished my
second coil, a table top coil.
It can be difficult to analyze without seeing the coil in action but
I'll make some comments below interspersed. Many of the
comments will be of a general nature.
2 1/4 " x 11 1/4" secondary wound with 32awg, 1150 turns, ~367kHz
primary is flat spiral 10 turns of 0.1" solid copper, 1/4" spacing,
currently tapped at just over 8 turns
about 7/8" clearance between primary and secondary, coupling is level
primary to base secondary
I don't usually make my secondaries that narrow, so I don't
have much experience to draw upon. I have noticed that when
the wire is too thin, it can hurt the performance. I like to use
at least number 28 wire unless I'm building a tiny coil. In one
experiment I built the coil using 36awg wire, then tried a
different secondary using 28awg wire, and it outperformed
strongly. It also had less tendency to give racing sparks.
The insulation on the thin wire may be inadequate for
best results. I also like to use at least 14 to 20 turns in the
primary which usually gives lower gap losses. I'd probably use
a taller secondary also. For example my TT-32 coil used a 3.2"
dia x 17" tall secondary if I remember correctly and this gave
24" sparks or so using a 9/30 NST. This used 28awg wire, and
a synchronous rotary gap. I think I used 0.016uF or so. The
toroid was a 3" x 10" smooth spun aluminum toroid. It gave
only one output streamer. To force a corregated (ducting) toroid
to give one streamer, put a breakout bump on the side of the
toroid. If this doesn't work you'll need a larger toroid. For a given
spark length, ducting toroids have to be larger than smooth ones.
topload is 1x6 ACR, and 3x10 toroid (3" ducting)
power is 10/30 NST, Terry filter protected
cap is 15nF (10 x 942C)
gap is sucker style single static gap, .203" spacing (currently), with
reduced air flow (about 1/3)
The gap spacing may be too narrow? I'm not sure.
First light last weekend and tuning is producing consistent 12-15"
streamers. Looks good. Behaves well. I am happy. Much louder than I
expected. More streamers (at a time) than I expected.
Are they blue/white or purple? Blue/white is good, purple is not so good.
I am not getting near the performance expected from what I have gleaned
from this list and from programs like JavaTC. I should be able to push
25". I know this must be partly due to the fact I am getting multiple
streamers, but I dont know exactly how to counteract this.
Yes, multiple streamers hurts the spark length by maybe 30% but
it varies with the coil, tuning, toroid, etc.
I have tried playing with topload and tuning. At one point with a loaded
toroid (scrap 6" ducting) and out to 9 turns, output noticably brighter
(but not really longer), and less than perfect e-field resulted in strikes
to strike rail, and with reduced air flow I got racing arcs. Increasing
air flow to increase quenching stopped the racing arcs. I feel I am
pushing the current configuration to its limit, so something must be wrong
with my configuration.
I notice that Dr. R. usually advocates tuning with a 20-30 turn primary and
tuning at much more turns than I currently am. However I cant see how to
do this without making the topload much larger. And also I know that he
would say allow at least an inch height between base of secondary and
Some of my coils which worked well had the base of the secondary
below the plane of the primary, so it depends on various factors.
Basically I tighten the coupling until racing sparks form, then I raise
the secondary until they stop.
Are these the secret ingredients to squeezing the extra bit
of performance I am not getting? Increase topload (therefore decreased
frequency and increased primary inductance), and decrease coupling ?
Sometimes the decreased coupling helps some, other times it makes
The reason I ask is that I just cannot see how to get the stated 25" I am
supposed to to be able to get at this power level.
Many factors affect the results. I don't see any glaring flaws
with your design, so it may be just a combination of minor
factors adding up to a reduced performance. Thus you have
plenty of room for experimentation.