Re: LTR vs. more primary turns

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 12/5/02 1:32:09 PM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

Dave,

The TT-42 coil only gives about 38" sparks with a typical 12/30
NST.  I'm using a old "robust" NST on that coil to get the 42".
My old research coil which had a taller secondary gave 42" with
a normal 12/30 NST.

I do have a rule of thumb for matching the secondary to a
particular NST.  It's based on physical proportions and number
of turns.  My first assumption is that it's good to use about 1300
to 1500 turns on a small coil.  I don't like to use wire that's too
thin, or which has insulation that's too thin.  Especially with a
120 bps srsg coil, it's good to have a tall enough coil.  I've had
good results powering my old research coil with smaller NST's
as small as a 7.5/30.  I kept the 0.0147uF cap for each NST;
7.5/30, 9/30, 12/30, and 15/30.  Theoretically, one could use
thinner than 28 awg for the smaller NST size coils.  I posted the results
some time ago for a smaller coil that used a 7.5/30 NST.  This coil
gave about 23" to 25" sparks.  This used a 3" x 16" secondary I
think.  This is the one that needed a certain value MMC cap to
stop the "reversal mode" firing, which heated up the NST too much.
This coil had fewer secondary turns than I would have liked.

The basic way I determine the secondary size is:

1)  use the equation:

   spark length inches = 1.7*sqrt input power in watts

2) divide the spark length by 2.6.  This is the minimum secondary
length for 120 bps.

3) use a wire gauge which gives 1300 to 1500 turns.  Use magnet
wire with heavy insulation.  Use an aspect ratio of 4.5 to 5.5.

4) use an LTR cap which is large enough to prevent "reversal mode"
firing with a 120 bps srsg.

5) use a large enough toroid so that 1 or 2 simultaneous streamers
is produced.  This toroid diameter will be 1/2 to 1/3 the spark length as
calc'd in step one above.

6) finally, use enough turns on the primary to tune the coil based
on the above design parameters.  Bart's JavaTC or a similar program
works well for this.

I got carried away and gave more information than you asked for,
but it may perhaps benefit someone else.

John


>coil was fashioned after John Freau's tt 42 as far as turns, height,
>diameter. I still think that he uses some kind of Voodoo magic to get 42"
>sparks from a 12/30 neon, (grumble grumble).
>      If you have a way to determine the optimal secondary for a given
>transformer, I am all ears!
>Thanks, Dave
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