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Re: Just a brush]



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Hi all,
         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).

Ls = 5.8mH  (approx)
Cself = 4pF (approx) 
I'm guessing the toroid will add around 9pF or so which makes Fr about
580kHz.
To resonate with your cap based on its "theoretical value" Lp needs 
to be around 9uH. I think you coil is probably way out of tune and 
needs only a fraction of the available primary inductance. Try 
tapping in at just a few turns. 
Assuming Cp is charging to the peak voltage the transformer can 
deliver, primary energy is about 1.9Joules which should be more than 
enough to spit some healthy sparks from the top.
I have to assume your cap value is what you say it is because I don't 
have the formulae handy.

> 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.

Possible, but get the primary tuned to the secondary first.
 
> Is my secondary too long for efficient coupling, (H/W ~ 8.5) I have read
> this may be a bit large.

It is not the most lossless H/D ratio but it should work.
 
> 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.

It decreases Fr because it is adding capacitance to the secondary. 
The formula Fr = 1/2*PI*SQRT(L*C) can be used for both coils. You can 
see that if C goes up, F must come down.

Best of luck.
Malcolm 


> Any help would be greatly appreciated and will speed me on my way to
> coilers nirvana.
> 
> Chris Taylor
> Perth, AUSTRALIA






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