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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.