Re: Toroid size selection

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

Original poster: FutureT-at-aol-dot-com 

In a message dated 1/11/04 3:41:31 PM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

>Original poster: "Luke" <Bluu-at-cox-dot-net>
>
>My question is what is the best method / formula for selecting a toroid
>size?  And why?
>Various opinions and methods would be appreciated so I can get an over
>all view of what everyone thinks.


Luke,

The main things to consider are:

1)  power input and coil "efficiency".
2)  breakrate
3)  desired spark appearance
4)  actual toroid size vs. spark length relationships

I'll discuss each of these.  Power input is directly related to potential
spark length.  So a direct relationship between power input and toroid
size makes sense.  For example my TT-42 coil uses about 750 watts
from a robust 12/30 NST and uses a 4" x 13" spun toroid, and gives
a 42" spark length.  This toroid size gives at least two simultaneous
streamers which gives a nice look.  A smaller toroid would give
shorter but more numerous streamers.  A larger toroid would give
fewer but longer streamers up to a point.  I tested a 6"x 24" toroid
on the coil and it gave a single 44" long streamer.  The spark was
barely able to break out of the toroid.  The coil runs at 120 bps.
Many folks build coils that are not as efficient, and they then have
to use a smaller toroid to permit breakout.

A coil that runs at a higher breakrate can use a smaller toroid for
a given spark length because more spark growth occurs, but the
overall results may be less "efficient" due to the higher breakrate.
This seems to be an inherent effect of breakrate.

I usually aim for a major toroid diameter that is about 1/3 the expected
spark length.  The minor diameter of the toroid should be about 1/10th
or 12th the expected spark length.

It should be noted that for a given coil, a corregated (ducting) toroid
will need to be larger than a smooth toroid for a given spark length.
For example I obtained the same spark length from the TT-42 coil
using a 4" x 13" smooth toroid as I did using a 5" x 20" corregated
toroid.


>I will be using 2 NST's in parallel each rated at 15KV 60mA.  For a
>total
>of 1800 watts.
>The capacitance I have chosen is 0.0214 micro farads.


That is a resonant value capacitor for those NSTs.  This will work OK
for a coil using a static spark gap if the gap is not made too wide.
Usually it is preferable to use an LTR value cap which is about
60% larger than the resonant value for a static gap coil.  Sometimes
an even larger cap value will work better.  Also, coils can often draw
more than the rated NST power, especially using SRSGs, or when
using a 140V step-up type variac to power the coil.

>I will be winding a coil with a diameter of 8 inches and a 36 inch coil
>length.
>How do I go about figuring out the best size toroid with the above
>information?


Figure that it takes about 4 times the input power to double the
spark length.  I use the formula:

   spark length (inches) = 1.7*sqrt input power (wallplug watts)

to calculate the expected spark length for an efficient coil.

So if your coil actually draws 1800 watts, this would suggest a
spark length of 72" for a well designed efficient coil.  Based on
this, a 6" x 24" toroid should be excellent for this coil.  Some
coils using two such NSTs can draw a lot more than 1800 watts
under certain conditions, so in such a case a larger toroid can
be used.

John


>Thanx.
>
>Luke Galyan