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Re: Top Toroid
Tesla List wrote:
>
> > Subject: Re: Top Toroid
<Big Snip>
>
> Richard, Bert, Anyone Else,
> What do you actually mean when you say a toroid is 7.8pf anyway? In a
> classic capacitor the capacitance is a function of the Area of the
> plate(s), the Distance that separates the plates, and the dielectric
> Konstant of the insulating material that separates the two plates.
>
> i.e. C= (KA)/(D^2)
>
> When you say a toroid has a 7.8 pf capacitance, is this based on some
> standard distance away from earth in dry air, or what? The reason I ask
> this, is twofold:
>
> 1) If measuring the capacitance of a toroid using a sensitive capacitance
> meter, what are the actual test conditions. For example, is it measured
> between earth and ground at a distance of one meter in dry air at Standard
> Temperature and Pressure (STP)? I would definitely think that the distance
> from ground would influence the value tremendously, but also any
> surrounding objects (especially if THEY are connected to ground or are
> simply very large in comparison to the toroid). For example, a person might
> not THINK about it, but a ROOF and the WALLS would probably contribute a
> significant amount to any actually measured capacitance. It also seems to
> me that to make any valid repeatable measurement you would have to also
> define the orientation of the toroid with respect to earth.
> Any object up close (such as a human being!) would add a significant error.
> The closer the object/person, the greater the error.
>
> 2) If Toroid capacitance IS subject to such large variations in value due
> to surroundings, then any value derived by formula would only give proper
> results if the formula accounted for all the extraneous forces, and if the
> experimenter ENTERED all the required data into the equation. A formidable
> task, even if you decide to remove all such variable elements by doing it
> inside a Huge building. Outdoors you have OTHER things to contend with that
> we probably don't want to get into! (Outside, an insulated body will pick
> up a charge from several sources. Tesla actually filed a patent based on
> this!)
>
> It seems to me that what should be done is measure real-world toroids under
> real-world conditions, and then use the results to come up with a corrected
> formula that would generate an answer that is reasonably close to the
> observed values. Then the result from the formula would be more useful in
> practice. I think you would also have to caution people that the calculated
> value could actually vary from the real by up to plus or minus so much
> percent.
>
> Since we coilers are generally interested in the toroid as it applies to
> our secondary coil, what we need is a method for measuring toroid
> capacitance AS A TOP LOAD on the secondary. A first approach might be to
> measure the self resonant point of a secondary coil with and without the
> toroid. Or even better yet, with several different toroids! The more the
> better!! Then do the same thing with every one of your coils, each time
> also trying every one of your toroids (even ones that you think are *the
> same*). The results might be something other than what you suspect. And
> then the Hunt is ON! If results are consistent and reproducible by others,
> then one might be able to gather enough data to come up with a reasonable
> formula applicable to coiling. If not, the results in themselves would be
> useful as-is as a kind of sampling of What To Expect.
>
> My reason for proposing using the self-resonant frequency of a secondary
> with and without toroid as the meausrement means is simple: Almost all of
> us can do THAT with very simple equipment. I might not have a picoammeter,
> but I do have a sine wave generator, and two LED's. An oscilloscope and
> frequency counter are NICE, but not NECESSARY for getting some working
> data. I am still concerned by the distance factor of the toroid to earth or
> its nearest component. A reasonable isolation of the toroid from
> surrounding objects may be enough to get useable results.
>
> Just a thought.
>
> Fr. Tom McGahee
The capacitance of the toroid is, indeed, a function of its
surroundings. Most capacitance formulas calculate the "isotropic"
capacitance which is the capacitance one would get if the other "plate"
were moved infinately far away. You've made some excellent points - in
the real world of coiling, the toroid is only inches away from the top
of the secondary coil, a few feet away from the primary, maybe a foot or
two more from a ground plane, and surrounded by other conductive objects
at various distances all around.
Some of these nearby objeccts will tend to increase the toroid's
capacitance, while the electrically-connected secondary coil below tends
to "shield" part of the toroid, reducing its capacitance from the
theoretical calculated value. Although one could try to solve for all of
these contributions to the toroid's effective capacitance, derated
estimating and actual measurement are easier and more accurate.
First, from a design standpoint, use the theoretically calculated value
and derate it by 25-35% (due mostly to the effects of the secondary coil
below) as Richard Hull has indicated. The secondary self-C can be
estimated using Medhurst's equations. The sum of these is what to use
for the secondary capacitance - by then calculating the secondary
inductance, you can estimate the coil's operating frequency. This is
usefule in initally "sizing" your coil's primary circuit L and C.
However, once you've actually built the secondary and toroid, simply
measure the actual 1/4 wave resonant frequencies of the coil alone, and
then the coil plus the toroid as you've described above. Back-calculate
the coil self-C, and the coil plus toroid capacitance. Using a sine-wave
signal generator and back-to-back LED's is easy, and is also very
accurate. Note that all of these measurements should be done with the
coil in its normal operating location, centered inside the primary. High
power tune should be within 1/2 to one turn outwards from here.
Safe coilin' to you, Tom!
-- Bert H. --