Re: [TCML] Re: Toroid Size

[mddeming@xxxxxxx]

Hi Greg, Brandon, John, All,


It seems not unreasonable that for good spark length you would want both (1) a high total charge (q) and (2) a high charge density (q/A) at the toroid's surface. This would imply that for (1) you should aim for C(toroid) > C(self) This would give a way to calculate a lower limit on toroid size.
 As a toroid becomes larger for a given power level, then (2) q/A becomes the controlling factor for as q/A begins to drop off rapidly for a given voltage, spark formation is reduced, possibly to zero. If q/A is too high, then breakout occurs at too low a voltage and the result is many small sparks.


If you want to crunch some numbers, these are the basic equations:


The area of a standard torus is simply  A = ( 2 pi R) ( 2 pi r) 
while its capacitance is given approximately as C = 1.4(1.2781+r/R) 2 sqrt(pi r R)
and Q=CV. 


where R is the major radius to the center-line , r is the cross section radius, C is capacitance in pF and V is voltage drop across secondary.


A first approximation to self-capacitance is:


C(self) = 0.24L+ 0.41R + 1.94sqrt(R^3/L)


Where R is the diameter of the secondary and L is its length (in inches) and C is in picofarads.


For your coil, the C(self) is therefore approximately 10.26 pF


The area of the proposed toroid is approx. A = 473.7 sq.in.
and its capacitance is approx. C(toroid) = 27.7 pF, well above the suggested minimum conditions. 


I have not yet found a simple way of calculating minimum charge density needed for breakout. but it's obviously tied to q/A, which is a function of applied power.


For the upper limit, one needs to consider weight and mechanical stability as well as charge density.


 Many people tend to throw in a "aesthetic factor" of minor diam = secondary diam. and major diam. = winding length,. This also provides reasonable mechanical stability at modest weight. Some others use the approach of "if it gives me an inch more spark, I'll prop it up with clothes poles and it can look like a limp blimp." Others are somewhere in between.


Hope this helps somewhat.


Matt D.


PS Better check the arithmetic as my calculator battery is very low.


-----Original Message-----
From: Greg Leyh <lod@xxxxxxxxxxx>
To: tesla <tesla@xxxxxxxxxx>
Sent: Tue, Aug 13, 2013 8:35 am
Subject: [TCML] Re: Toroid Size


Hi John,

Is there a good rule-of-thumb for sizing a toroid to a given secondary 
dimension?  I'm planning on using a 4x16 toroid with a 5x32 secondary 
winding.  Does that seem like a reasonable match?  I'm about to wind the 
secondary and thought I'd ask first.

Cheers,
Greg


> Hi Brandon,
>
> Yes, it's definitely possible to have a toroid that's too big.  If the toroid
> is too big, sparks will only break out when using a breakout point or
> bump.
>
> The proper size of toroid depends on how many sparks you want to
> see coming off the toroid.  If the toroid is large enough to produce
> only one spark streamer... well then you'll get one streamer.  If the
> toroid is smaller, then you'll get two streamers, and if smaller yet,
> then you'll get 3 streamers, etc.  The smaller the toroid, the more
> streamers, but they'll be shorter.
>
> I happen to like the look when the streamers oscillate from side
> to side of the toroid and occasionally show 2 full streamers.  This
> will give pretty long sparks and good spark action.  To me this is
> the most exciting type of look.  Some folks prefer a full head of
> streamers, etc.
>
> Low break rate coils probably need a little larger toroid to get just
> one streamer.
>
> On my TT-42 TC which gives 42" sparks using a robust 12/30 NST,
> I use a 4" x 13" spun toroid and I get about 3 simultaneous sparks
> I guess.  I tested a 6" x 24" toroid on the coil and it gave a single
> streamer but needed a breakout bump to break out reliably.
> Clearly a 6" x 24" toroid was too large in my view.  However the
> sparks were a little longer, about 44" or so.  I didn't try a 5" x 16"
> toroid, but that would have been interesting to try.  Folks often use
> a 5" x 16" spun toroid for a 15/60 NST powered coil.  I used to offer
> a 5" x 16" size but it was just too time consuming to make.
> I do offer a 4" x 13" spun toroid ($165, free shipping in USA,
> as-spun finish).  Amazing Science ( www.amazing1.com/toroids.htm )
> offers a 4.5" x 18" toroid for about $240 I think plus shipping.  I have
> no connection with them.
>
>    My website is:
>     http://futuret.110mb.com
>
> Cheers,
> John
>
>
> -----Original Message-----
> From: Brandon Hendershot <brandonhendershot@xxxxxxxxx>
> To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
> Sent: Sun, Feb 19, 2012 5:55 pm
> Subject: [TCML] Toroid Size
>
>
> Hi List,
>
> It's been quite a while since I hung up coiling, looks like the bug's back
> though.
>
> So I finally got myself a job and some stable income (:D) and can finally
> afford to buy a spun aluminum toroid for my coil.
>
> First question; Can you have a toroid that's simply TOO big? I'm suspecting
> that my 4"x16" Al Ducting/Pie Pan do-dad is a bit too small. Would a 6"x24"
> Spun Al toroid be too big of a jump?
> I suppose it would be helpful to plug it into JavaTC, but It's not working
> on my browser right now, and I simply don't want to measure my up whole
> coil again (old PC died).
> Before I go through that errand, does anybody have any wisdom for me
> regarding using bigger toroids?
>
> You can see how my 4"x16" looks here: teslacoilers.blogspot.com
>
> Thanks Everyone,
> Brandon H.


 
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