Re: Newbie with questions...

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

Original poster: Bart Anderson <classi6-at-classictesla-dot-com> 

Hi MCP (hey, we like first names here),

Your coil is pretty small and outside the norm. Some of these norms are:
Your h/d aspect ratio is 14:1 (norm is about 4:1 or 5:1).
Fres is 2166 kHz (norm is about 60kHz to 300 kHz).

But, your coil is small and with that comes high frequency's (thus, the 2nF 
cap you had to use). I don't really see anything grossly off on the small 
coil. The helical primary was a good idea, but your coupling coefficient is 
still low with it's current position (k=0.082). It would be nice to see 
this around 0.13. However, because of the relatively large diameter of the 
primary, even with the primary centered with the center of the secondary 
(max coupling position possible), this will only get you to 0.1. The 
primary would need to be rewound to a smaller diameter to tighten up the 
coupling.

Given your tank cap size and Fres (I used a 2" x 2" topload in the shape of 
a cylinder), I would use #8 solid copper wire keeping a 0.25" spacing 
between turns and wind 6.9 turns with a diameter of 3" and height of 2.6". 
This will put the coil in tune with the topload mentioned. With these 
changes and with the bottom pri turn even with the bottom sec turn, 
coupling is 0.12 which is about right for the position. Thus, it is then 
easy enough to raise the sec to loosen up coupling or raise the pri to 
tighten up coupling.

I think your coiled wire with it's shape was acting as a topload similar to 
a cage topload configuration. With this coil, changing topload sizes or 
types will affect the tuning of the coil significantly. The tuning is 
probably rather sharp. Adjusting the topload size and position is probably 
a prefered method of tuning this particular coil given it's 
characteristics. One idea might be to stack discs (coins?) and adjust the 
sec to the pri by changing topload C. I was actually thinking of this when 
I decided to model the 2" x 2" cylinder mentioned above. One of those small 
tomato paste cans or similar would be about right.

I'm not even sure a topload would gain much considering this coil is 
outside the norm. You could tune the primary by changing the turns and 
simply use the end of the sec winding as the discharge point. Compare and 
see which works best. BTW, without a topload, Fres goes up and requires 
less pri turns (thus, coupling goes down again). I would use a slightly 
smaller pri diameter at 2.5", same spacing between turns, and 5.53 turns 
(2.1" height). This readjusts coupling to 0.13 and keeps the coil tuned 
with your cap size (Fres hits about 2947 kHz). Quite a change in Fres when 
using or not using toploads.

Take care,
Bart



Tesla list wrote:

>Original poster: The MCP <ejkeever-at-comcast-dot-net>
>Hello, everyone. I just signed up and have some questions to ask about some
>aspects of coiling.
>
>I've built a small (tabletop-size) tesla coil for fun, and I am observing some
>very odd behavior from it. But first, I should provide some basic info.
>
>The secondary is a .75" PVC pipe (I got excited and built it before doing
>enough research), with about 10.5 inches of 32 gauge magnet wire on it, about
>1000 turns. The helical primary is 5 inches in diameter, ~3.5 inches tall,
>with 5 turns.
>
>The main tank circuit has a smallish 5000/20 NST and currently a single
>40KV/2nF cap from TDK that I picked up for $30.
>
>The spark gap is a simple static gap, currently separated by about .125
>inches, using the tips of a pair of screws. When these wear out, I can just
>replace them with fresh ones. The gap can be set for up to 1/2 or so inch
>separation.
>
>Currently, the whole thing is in a highly experimental state. By that I mean
>that very little is set in stone; Almost all the major connections are either
>twisted together or clipped. So you've got a lot of latitude to change
>things.
>
>Anyway, on to my questions. First of all, I'm wondering if I've got any of the
>components grossly off-key. Comment away.
>
>Second, I've found something odd( to me at least). One day, just for the heck
>of it, I took off the topload and stretched the wire straight out. Nothing
>much happened. But when I took the 6 inches or so of wire and wound them into
>a small spiral (about 7 turns, 3/8 inch diameter, 1 inch long), the coil
>worked almost as well as with the big copper topload so I abandoned it. Can
>someone tell me why?
>
>Thanks for taking the time to slog through all this and answer my probably
>silly questions!
>
>
>
>