Re: Primary > secondary distance?

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

Original poster: "Bert Hickman" <bert.hickman-at-aquila-dot-net> 

Tesla list wrote:
> 
> Original poster: "Simon Yorkston" <quantumx-at-mail.ozemail-dot-com.au>
> 
> Hey all [again]
> I get the feeling that I should concatenate all my frequent posts into
> one.. :)
> 
> Anyway, I've been looking at pics of other people's coils, and noticed that
> the distance from the primary [inner turn] to the secondary is only about
> one to one and a half inches. On my coil, this distance is about 2 or 2 1/4
> inches.. this won't affect the performace [too much?] will it - or is it
> not that important?
> 
> The only way to fix this is to rewind the primary- it wouldn't take THAT
> long, but I really want to have to do that..
> 
> Thanks again
> Simon

Simon,

Congratulations! Most first-time coilers start out with insufficient
clearance between their primary and secondary - the first time they fire
the coil, the resulting arcs often destroy a portion of the lower
secondary. 1 - 1.5" is about right for most small to medium sized coils,
but 2 - 2.25" should also work OK. Increasing the primary:secondary
spacing reduces the degree of magnetic coupling (called the coupling
coefficient, or k) between the primary and secondary. Most Tesla coils
operate best when k is in the range of 0.15 - 0.22. Because of the
somewhat wider primary:secondary spacing, your system will operate best
with the secondary positioned a bit "lower" relative to the primary. 

BTW, the coupling adjustment is one of the LAST adjustments you want to
make on your system. Starting out with too much initial coupling is
often the OTHER mistake that first time coilers make...  :^) 

When you get to the point of "first light" on your coil, the proper
system setup sequence should be as follows:

1. Start out with the bottom of the secondary winding raised at least
1/2 - 1" ABOVE the plane of the primary, and tune the system for maximum
spark at low to medium input power. Start with the primary set at the
nominal tap point predicted by your design support tool (such as
WinTesla, etc...). Force the system to break out with a nail taped to
the toroid if necessary. 

2. Once the system tuned for best spark, slowly increase power until the
system can run at full power with no abnormal behavior. Be sure to
observe the system while running in the dark, looking for any signs of
corona where it shouldn't be, or any abnormal flashovers.

3. Once you can run the system at full power with no problems, try
removing the nail to see if the system breaks out from the bare toroid.
If not, leave the nail taped in place. Slowly increase the coupling by
lowerering the secondary versus the primary about 1/4" at a time between
runs. Again observe the behavior of the coil in the dark to make sure
that you aren't getting any thin "racing sparks" from one portion of the
secondary to another. Verify proper operation at full power. Continue
increasing the coupling until until you begin to get racing sparks at
full power, and raise it 1/4" to stop them. 

Remove the nail (if still present), and verify that you can consistently
break out and that you're not getting any racing sparks. If your system
still won't break out, the toroid is too large, the tank cap too small,
or you're not getting sufficient voltage (or current) from your NST. If
you are beginning to get racing sparks (the voltage stress will be
higher with no breakout point), decrease the coupling a bit more.  

4. Finally, try increasing the primary inductance a bit by increasing
the number of turns by 1/8 turn at a time. Most systems tend to run best
at a slightly lower frequency (1/4 - 1/2 primary turn) under full power
due to the loading from ion cloud and streamer capacitance.

Hope this helped, and good luck on your coil!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com