Re: New Coil Design

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 9/3/01 11:48:23 PM Eastern Daylight Time, tesla-at-pupman-dot-com 
writes:

>  All,
>  Could You advise me on how to better
>  > this coil? I am open to any suggestions, excepting the Magnet wire that I
>  > already have.
>  >
>  > Thanks,
>  > Ben

Ben,

Well,  since you're using a 9/30 NST, let's plug that
into my spark length equation with a twist.  The twist
is needed because an NST can draw about 1.7 times
it's rated power if you use a 0 - 140 step up type variac
to power the coil, along with an LTR cap.  Take 270VA,
multiply it by 1.7 to give 459VA.  Now take the sqrt of 
459 = 21.4 x 1.7 = 36.4" = the expected spark length
from an optimal design.  Now I like to use about 1600
turns on a secondary, so this may give a secondary 
length of about 20" or so.  But 1200 turns is OK also,
and this would permit the secondary to be maybe 17"
tall.  Note that the spark length will be more than twice
as long as the secondary which is nice.  Of course if the
spark is too much longer than the secondary, the coil
will destroyed by voltage breakdown.  Now if the spark
is going to be 36" long (or even 30" long), you'll want
a toroid that is something like 3.5" x 12", or 4" x 13".
If the toroid is corregated, it can be even a little larger.
The size you mentioned may be fine also, since you 
have a large ROC.  If you don't use a step up type
variac on the input, then the sparks will be shorter at
28" or so.  Many folks get only about a 12" to 18" spark
from a 9/30 NST due to design compromises.

Let's jump to the cap now.  I assume the size you
show is the resonant or matched size.  That will work
if you don't make your gaps too wide, but then you
won't get 36" sparks.  You may want to consider an LTR 
value cap which would be maybe 0.022uF or so for use
with a sync rotary gap or a triggered gap.  If you use a 
static gap, 0.017uf may be a good LTR value.  I've never 
gotten very good results with static gaps at low breakrates.  
However Gary Lau's single vortex gap works very well, so 
that's something you might consider.  Your primary looks
good except you should leave about a 1" spacing between
the primary and secondary to prevent flashover.  Also, 
I wouldn't bother with the 15 degree angle, although it will
work fine, but it may promote strikes to the primary.

You may wish to try many different gaps, caps, toroids,
coupling positions, etc. to experiment.

John Freau