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[Fwd: Re: Just a brush]



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In a message dated 97-03-06 10:46:51 EST, you write:

<< Taylor, Chris CC wrote:
 > 
 > I have just recently completed building my first coil and fired it up
 > last night. To my utter disappointment all I got was a big racket from
 > the spark gap and little else.
 > 
 > I have a 15/30 neon with a six element spark gap and a plate poly
 > capacitor, (20 plates 10x8" with an 80 mil poly dielectric, theoretical
 > C=0.0085 uF). I haven't got around to placing the cap in an oil bath yet
 > and it doesn't arc unless the gap separation is very large, the cap then
 > arcs around the edges and I shut down and reduce the gap separation.
 > Would the cap be providing 0.0085 uF without the oil bath, I can't see
 > why not, it is my understanding that the oil is only used to provide a
 > better insulator than air around the cap.
 > 
 > The coil also has a 1/4" copper tubing primary with 15 turns spaced at
 > 1/4" in a saucer configuration, (30 degree). The secondary is 22 ga
 > transformer wire on a 2.8" PVC pipe with 840 turns, (22.5" long).
 > 
 > All I seem to be able to get from this baby is a small brush discharge
 > from the tip of the secondary wire, (primary optimally tapped at turn
 > 5). I have also built an 11 x 3.5" toroid which when attached does not
 > produce any discharge.
 > 
 > My questions to the lads are;
 > 
 > What sort of diagnostics can I perform to point me in the right
 > direction for larger discharges.
 > 
 > Have I missed anything, I have used a number of the circulating formulas
 > to roughly size various components.
 > 
 > The coil is working but just doesn't seem the be very efficient. The
 > lowest turn on the secondary is in line with the inner turn of the
 > primary, do I need to lower the primary to get better coupling.
 > 
 > Is my secondary too long for efficient coupling, (H/W ~ 8.5) I have read
 > this may be a bit large.
 > 
 > The addition of the torus increases the resonant frequency, do I
 > increase or decrease my tank capacitance as the secondary frequency
 > increases and how do I adjust the primary tap as the secondary resonant
 > frequency increases.
 > 
 > Any help would be greatly appreciated and will speed me on my way to
 > coilers nirvana.
 > 
 > Chris Taylor
 > Perth, AUSTRALIA
  >>
Chris,

I've done some calculations on your coil.  Secondary is 5.6 mh, self
capacitance is 6.0, resonant at 865 khz unloaded and 557 khz with the 11"
toroid.  The primary is 66 microheneries, and I agree your capacitor should
be .0085 mfd - with or without the oil.  These calculations say it should
tune at about turn 4 with no toroid and turn 6.25 with the toroid in place.
 You didn't say what your gap spacing was.  You should be able to open it up
to about .150" total without damaging your neon.  You said you get no
discharge with the toroid in place, which is an indication that the voltage
is not high enough - which means the gap spacing is too small.  You should
put the cap in oil.

You are right, this coil won't perform very well with the 8.4 H/D ratio, but
I think we should be able to get it to work.  For best performance, you want
to be using most of your primary - i.e. taped out near the last couple of
turns.  The way your coil is set up right now, to tune at turn 13.75 (80% of
total primary inductance) it needs a .0016 mfd cap.  You might try reducing
your cap to 5 plates total (about .0018 mfd) and see if it will perform
better.

You have a good primary and a good capacitor.  The weak link is the
secondary.  You might consider building another one maybe using a 4" form.
 Even if you had to remove the inside turn of the primary to get it to fit. A
4" form wound with #28 wire, 18" winding length, with your existing toroid
would work very well with your existing primary and cap.

Good luck,

Ed Sonderman 






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