Re: [TCML] toroids instead of spheres - why?

[Scott Bogard <sdbogard@xxxxxxxxx>]

Hi Jason
     If you are getting a dozen or so breakouts with a breakout point your
top load is WAY to small.  Use this formula to size a toroid (figured out
by John Freu) Topload diameter in inches = (0.5 through 0.9)*(SQRT power in
watts).  Since toroid capacitance in pF is very nearly equivalent to
diameter in inches of a toroid, you could use that formula to find pF, and
calculate sphere size also (you would find the sphere would need to be too
large to be practical).  I like to use closer to 0.9 as opposed to 0.5, as
that tends to give only one spark.  You should see a substantial
performance improvement after you retune for a larger top load.  Also, I've
found a good breakout point to be a piece of foil bent into a loop and
stuck to the surface, I think I picked that trick up from Ed Wingate's
awesome magnifier when I went to one of his teslathons a few years back.
 Anyway, best of luck!

Scott Bogard.


On Thu, Aug 22, 2013 at 8:13 AM, Jason <jason@xxxxxxxxxxxxxxxxxxx> wrote:

> While we are talking about toroids and spheres,
>
> Whats the best way to work out size? and is there a way to only get the
> break out at one point with a traditional spark gap coil? I am using a
> break out point but still getting a dozen or so break outs
>
> Cheers
> Jason
>
> On 22/08/2013 10:05 PM, Jim Lux wrote:
>
>> On 8/22/13 4:48 AM, Scott Bogard wrote:
>>
>>> I'm not sure which document you are referencing, but if I am not
>>> mistaken,
>>> a toroid typically has a larger capacitance for a given breakdown voltage
>>> does it not?  IT also has far superior top turns shielding
>>> characteristics
>>> which is why we use them so much; I believe this has to do with the
>>> E-field
>>> shape though and not the capacitance.
>>>
>>>
>> Yes, toroids will give a more "uniform" field along the secondary.  An
>> infinite flat plate would be ideal, of course, but a toroid is a nice way
>> to get an edge on the flat plate while keeping the field at the edge high
>> enough that breakdown doesn't occur too soon, allowing more charge to be
>> stored.  You could have a Rogowski type profile on the bottom of a sort of
>> hemispherical electrode.  That's a flat plate that has a rolled edge, where
>> the field is greatest in the flat plate part and gradually decreases.
>>
>> http://home.earthlink.net/~**jimlux/hv/rogowski.htm<http://home.earthlink.net/~jimlux/hv/rogowski.htm>
>>
>>
>> However, for a given diameter, a sphere has larger capacitance than a
>> toroid.
>> C(pF) for sphere is 4*pi*8.85*radius in meters.
>> An approximate formula from Bert Pool for a toroid (sorry for the change
>> in units to inches) is
>> C(pf) = (1 + (.2781 - d2 / d1)) x 2.8 x sqrt ( pi( ( d1 - d2 ) ( d2 / 2
>> )) / 2)
>>
>> d1 = outside diameter of toroid in inches
>> d2 = diameter of cross section (cord) in inches
>>
>> Some practical numbers.. a 36" diameter sphere has a C of about 50pF, a
>> 36" diameter toroid with 18" "tubes" has 35 pF capacitance. With 8" tubes
>> (and a 20" disk in the middle) about 39 pF
>>
>>
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