RE: First Light for 10" Coil.

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

Original poster: "John H. Couture" <couturejh-at-mgte-dot-com> 


Bart -

You can forget the reduction factors and the 47.4pf isolated toroid
capacitance for Peter's Tesla coil. The reason is because the surrounding
conditions that determine the toroid capacitance on his TC are completely
different. In fact there is no reason to call this a reduced toroid
capacitance because the toroid capacitance on the TC has nothing to do with
the capacitance that Inca, etc, calculates. The conditions around a Tesla
coil have nothing to do with a toroid in isolated space. I don't intend to
use "toroid capacity reduction" in the future.

So lets start over and also use real numbers of a real TC like 145.49 and
84.43. For Peter's TC if you use a computer program or manual calcs (brave
coilers) these two frequencies will not be available to you. The computer or
calcs will give you some fictitious resonant frequencies based upon your
inputs. What you do with these frequencies is up to you. However, if you
want to know the real world toroid capacitance of Peter's TC proceed as
follows. The accuracy will depend upon the  accuracy of the real world
frequency tests.

For Peter's Tesla coil he found that the test resonant frequency without the
toroid was 145.49 KHz. The secondary coil capacitance per the Wheeler
equation was 69.33 mh. The real world self capacitance of the secondary coil
without the toroid is

   Cc = 1/(39.5 F^2 L) = 1/((39.478)(145490^2)(.06933)) = 17.26pf

The test resonant frequency was 84.43 KHz with the toroid on the secondary.
The toroid + coil capacity is

   Cs  = 1/((39.5)(84430^2)(.06933)) = 51.26pf

The real world toroid capacity is

    Ct = (Cs - Cc) = 51.26 - 17.26 = 34.00pf

What could be easier?

What do you believe is incorrect with the above calcs?

Note that the above calcs are  straight forward and not a back-calculation
as you mention.

Note that this also includes the minor change in the secondary coil self
capacitance that you mentioned when the toroid is on top. It also takes care
of the ground plane, reduction factors, etc.

You mentioned that the secondary capacitance changes when operating at Fres.
Can you explain this in more detail including the frequency equations that
shows this effect other than the equation shown above?

What equations show the capacitance changes when the currents are displaced?

I agree it would be silly to think only the top load capacitance is
affected. The above calcs show the secondary self capacitance also is
affected.

I agree it is "misleading" to state the top load capacitance is reduced,
etc. This refers to a reduction from the INCA capacitance. As I said above
the INCA capacitance has nothing to do with the toroid capacitance when on
Tesla coils. The INCA capacitance conditions never match the conditions
around Tesla coils.

I agree with your equation for solving for C. Why can't the Wheeler
inductance L be used in this equation? The L and C inputs you are using give
you frequencies that do not agree with the test resonant frequencies. Why
bother using these inputs?

If you have the two real world resonant test frequencies and a computer
program why not use this combination to find the real world secondary coil
and toroid capacities as I show above? A relatively simple job. Note that
this takes care of the distributed current effects, the toroid reduction
factors, etc.

Bart - My Ctop calcs are not recent. I have been trying to point out to
coilers for many years how the real world TC coil self capacitance and
toroid capacitance   can be found by using the real world resonant test
frequencies. The problem is that most coilers are not interested.

John Couture

--------------------------------


----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Thursday, December 25, 2003 10:26 PM
To: tesla-at-pupman-dot-com
Subject: Re: First Light for 10" Coil.


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

John,

Measurement with and without topload is one method to obtain the affects of
the effective topload capacitance each situation, but not the "only"
method. There are a few "highlights" I would like to note on your reduction
factor so that coilers are not mislead.

1) The freespace or exact capacitance of 47pF assumes no ground plane. If
we were to look at the topload without the secondary, but at the same
position above ground (about 84"), the 12" x 42.5" toroid would have a
capacitance of about 50.4pF.

2) The capacitance of 34pF you showed (topload installed and based off
Fres), is a back-calculation "assuming" the secondary's self capacitance is
100% unchanged (the DC capacitance or uniform current). We know the
secondary capacitance changes when operating at Fres. Once the coil is
operational with the topload installed, "all external and internal
capacitance" is affected as the currents are displaced. Also, the proximity
of toroid to coil affects one another. Not only does the topload
capacitance change, but also the secondary capacitance changes (it would be
silly to think only the topload capacitance is affected).

3) Calculating for topload with and without is fine to show that the
topload changes. I have no problem with that. And for any coilers own
calculations, via program or pencil n' paper, reducing the topload by some
factor (reduction %, whatever) is fine to arrive at a loaded Fres value.

But I think it is "misleading" to state "the toploads capacitance is
reduced "this much" when placed on the coil. It does change, but not by the
factor stated (the factor is a change from the DC capacitance and is what
should be made clear to coilers).

Solving for C with say C=1/(4pi^2*F^2*L)

F is the resonant frequency
L is the inductance "at the resonant frequency" (=Les, not Ldc)
C is the capacitance "at the resonant frequency" (=Ces, not Cdc)

Imagine if you used F with Ldc to solve for C. You are using a DC value (L)
with a frequency that does not represent DC values. Would you find Cdc or
Ces? The answer is "neither" capacitance. You would calculate some value
"between" Cdc and Ces. This is what your using with your topload reduction.

The factor your using for the effects of the topload is a factor from "an
exact topload capacitance without ground plane" (which doesn't exist on
this planet) to a capacitance value that is somewhere between Cdc and Ces.
As mentioned, I don't have an issue with using a factor in this way to
arrive at a correct loaded Fres value (that is the goal). But I think  it
should be clearly stated that the factor involves this method and does not
represent "actual" topload change in C.

In the old Javatc, I also used the reduction factor for the same purpose.
With the new Javatc now using Paul's Geotc code, I no longer need to use a
factor. Now the topload represents the "true capacitance" of the topload at
it's position and with the effects of the entire system to the degree of
accuracy of the program.

For a check, you could do this.

1) Install the 12" x 42.5" topload at a height of 84". Run the program.
You'll find Javatc will show the "exact capacitance" without groundplane
and will agree with Inca at 47.35pF. Note, the height is really unnecessary
since there isn't a groundplane involved.

2) Now install a ground plane, say 50 inch radius. Run the program. You'll
find that the topload capacitance will change to 50.38pF.

3) Install the coil (see below). Run the program. You'll find the topload
capacitance will change to 42.5pF. Note, this is "not" 34pF as you have
shown. The difference is Javatc is using Les, Ces, etc.. and "not" the DC
values.

coil inputs for above: [ r1=5, r2=5, h1=31, h2=72, turns=1150, wire size
(not awg)=0.035433 ]

In this light, you'll find the reduction factor is:

10.24% : based on exact Ctop [47.35pF] to Ces Topload [42.5pF] at Fres
28.19% : based on exact Ctop [47.35pF] to Cdc Topload [34pF] at Fres
or
15.64% : based on Ctop w/groundplane [50.38%] to Ces [42.5pF] at Fres
32.51% : based on Ctop w/groundplane [50.38%] to Cdc [34pF] at Fres

I sure hope this helps with the understanding of "where" the reduction
factors are coming from and "how" they are being derived.

I would like to add that I feel strongly that the "true" C of toroid at
Fres is the difference of the topload above a groundplane at it's position
with and without the coil running at the resonant frequency. In this case,
that would be a reduction of 15.64%. But, that is personal preference. For
any program that cannot yet pull in the distributed current affects at RF,
then a reduction factor is required to make up for the "unknowns", and in
this case would be 28.19% as John has shown.

Why? Because Ctop above ground is not known and the programmer must resort
to C exact without groundplane. Also, because Ces is not known, the
programmer must also resort to Cdc. There's nothing wrong that. The only
thing required at that point is finding (preferably by imperical data)
factors to approximate so that Fres loaded is as close as possible to
reality. This is what John has done recently with his Ctop calcs.

Take care,
Bart


Tesla list wrote:

 >Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com>
 >
 >John:
 >
 >I think it's simple physics.  The top closed turns of the coil act to
shield
 >the electrostatic field from the underside of the toroid.  This will cause
a
 >10-20% reduction in the toroid's measured isotropic value.
 >
 >D.C. Cox
 >
 >
 >
 >Dr. Resonance
 >
 >Resonance Research Corporation
 >E11870 Shadylane Rd.
 >Baraboo   WI   53913
 > > toroid capacitance when placed on the TC secondary. This capacitance is
 >much
 > > less than the toroid capacitance calculated by the standard equations.
 >Why??
 > >
 > > For example with your coil the 12"x42.5" toroid capacitance using the
 > > standard toroid equations is about 47.4 pf. This will not agree with the
 > > actual toroid capacitance when it is located on the secondary. The
actual
 > > toroid capacitance is 34 pf which gives the 84.8 KHz (you measured). The
 > > only way to find this actual toroid capacitance when it is on the
 >secondary
 > > is by measuring the operating frequency with and without the toroid
after
 > > the coil is built. You then calculate the resonant frequencies with and
 > > without the toroid. All of the surrounding conditions that affect the
 >toroid
 > > and secondary capacitances will then be accounted for.
 > >
 >
 >
 >
 >