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Re: overcoupling
Original poster: "harry" <harry-at-prcn-dot-org>
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: September 15, 2003 11:25 AM
Subject: Re: overcoupling
> Original poster: FutureT-at-aol-dot-com
>
> Lance,
>
> I get the impression that this is your first coil. Overcoupling can cause
> racing sparks along the secondary, causing burned spots on the wire,
> or even destruction of the secondary coil and form.
> The degree of coupling that can be tolerated by any coil depends on the
> input power and break-rate. A conical primary does not automatically
> cause overcoupling. Rather it depends on how close the primary is to
> the secondary and other factors. Consider a 30 degree primary that is
> 14" I.D., and 28" O.D.. Such a primary is likely to overcouple to your
> 12" dia secondary. Now consider a 30 degree primary that is 22" I.D.,
> and 36" O.D.. Such a primary is unlikely to overcouple to your 12"
> secondary. In fact the coupling may be a little loose and the secondary
> may need to be lowered below the primary plane a little. And you are
> correct, you can always raise the secondary
> to reduce the coupling to an acceptable amount. I generally use
> flat primaries for my coils these days. Awhile back, I used to use 15
> degree conical coils. Generally the best degree of coupling is found
> by trial and error by raising or lowering the secondary until the best
> spark output is obtained free of racing sparks and primary to secondary
> flash-overs.
>
> John
Thank you for the reply
This is actualy my second coil (I had most of the parts already),the first
coil was a 4" secondary powered with 2 12/30 nst's ,homemade rolled poly
caps , static spark gap with a couple MOV fans, 1/4" copper tube conical
primary, after I put it all together I had it tuned to 20+" sparks in less
than an hour and a couple of weeks later had them out to 32", I believe it
can do an inch or two more if I push it (spark gap is at .22 and I was
thinking of going to .3)but I don't want to fry anything till I get the big
one going.I have run this coil fairly steady for almost an hour with just a
warming of the nst's (I imagine it was getting hot inside :) Kids love it
especialy with the 3" toroid instead of the "boring" 4".
Since I fotunately/unfortunately did not have too much trouble with the
first coil I decided to build the second one,I have made asmuch of it as
modular as I can ,It will be easy to flatten the primary and either raise
/lower the secondary or primary.Initial power will be 4 15/30 nst's with
PFC,mains filter and Terry filter all on a HEAVY but removable Powerpak
,spark gap is a washing machine motor that I modified to work as synchronous
at 1800 rpm with a propellor gap using 1/8 tungsten electrodes (4 stationary
posts for 120bps).
Main Cap will be .045 uF (3 strings of 10 .015uF Geek caps) Is this a good
size for a SRSG ?
Primary is 16 turns on 5/8" spacing 14" ID. 38" OD.
Secondary is 12.5" PVC sewerpipe turned down to 12.25" ....Could the bell
end be used as a sheild between the Pri. and Sec. to prevent Pri. to Sec.
arcing ?........winding is 22Ga. 36" legnth with multiple heavy coats of
poyurethane.
Any suggestions for a toroid size ?
Lance
PS there is a safety gap too.