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More Questions!
* Carbons Sent to: tesla-at-poodle.pupman-dot-com
Quoting DavidF4797-at-aol-dot-com:
> Thanks heaps for your response! :)
No problem.
> Another question for you: If cost is the only consideration, I
> would seem to remember that the equivalent of lets say ten
> turns of typical 14 gage house electrical wire would hardly be
> more expensive than ten truns of 1/4 inch hollow refridgeration
> tubing (or would larger diameter solid wire [say 1/4 in thick
> solid] be better because of its larger diameter even if it is
> more expensive than 14 gage)
Cost is not the only consideration. No. 14 AWG copper is not
really a suitable primary coil conductor because it has too
little surface area and the radius of curvature is too small.
Yes, 1/4 inch OD solid would work better than No. 14 AWG, but
then you are paying for a lot of unused conductor.
Basically you want to use a conductor with a lot of smooth
surface area for the primary coil. The two best materials
are either thin strap or hollow tube. They both work very
well, and they are cost effective.
> Also, if one is using ten turns of 1/4 tube, spaced 1/2 inch
> apart for the primary, one would end up with a coil more than 7
> inches high, while if one is using 14 gage wire, one would end
> up with a coil not much more than 5 inches high. Would this
> difference in overall coil lenght affect the coupling or
> resonant frequency of the coil pair and which is better and
> why? I was also wondering about the shape of the primary. You
> seem to prefer the inverted conical shape or the flat "pancake"
> for thier coupling characteristics, but when I put the specks
> for the coil I'm building into Noon's computer program (which I
> seem to recall you reviewed rather highly) the different coil
> geometries don't seem to make much difference in anything...
> what gives?... Is there a minimum primary spacing you would
> recomend and why... You also recomend that more than the
> standard few number of turns of the primary would be better
> (perhaps 15) why 15 and not say 20...would not this give you
> even better coupling characteristics?
Wow, I would almost need to write a short paper to answer these
questions thoroughly...
First off I would have to say I have not been happy with
vertical helix primaries when used with 1/4 wave Tesla coils.
The vertical helix is preferred for primaries when building Tesla
Magnifiers however, so the design does have a place. But I
digress...
You noted in your design calculations above that when using a 1/4
inch OD conductor spaced at 1/2 inch you end up with a coil that
is 7 inches tall. What becomes a problem with tall vertical helix
primary coils is when you discover later that the secondary coil
must be mounted above the last turn of the primary in order to
couple properly when the system is peaked out. This means the
coil set gets physically tall. If you are like me and you fire
indoors, ceiling heights become a restriction. Anything you can
do to reduce the overall height of the system is a plus.
The inductance and operating frequency of two coils wound in
different ways (say vertical helix vs. flat pancake) is going to
be about the same if the same conductor length and spacing is
used; this is not really an issue, as any variation can be tuned
for by simply moving the tap. What makes a difference is that the
flatter the primary coil, the lower the secondary is going to
sit, and the shorter the system. Unless you are building a small
table top system, or you fire in a commercial type structure
with 20' ceiling heights, the actual height of discharge terminal
and distance from the ceiling will probably end up being critical
in the system.
As far as the minimum "standard" spacing between primary turns I
would have to say not to space less than 1/4 inch. I prefer 3/8
spacing for neon powered coils, and 1/2 inch spacing for the
higher powered work. The reasoning here is that you are trading
off inductance to flashover voltage. The closer spaced turns
produce higher inducance in a small area (good), but turns spaced
too close will arc (flash-over) which is bad. On small to medium
neon powered Tesla coils where input voltages do not get much
over 15KVAC you will find 3/8 inch spacing between turns works
pretty well.
As far as number of turns on the primary goes you will want to
access at least 14-15 primary turns in order to have tuning and
experimental flexibility. If you build yourself a 6-8 turn
primary you will find yourself trapped with a limited amount of
flexibility. Building a primary with 20 or more turns is going to
run up the cost and difficulty of the project, and you will
rarely (if ever) find yourself using those last turns. From a
practical point of view 15 turns is plenty.
Richard Quick
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12