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Re: Helical cap
Tesla List wrote:>
> Original Poster: Bryan Work <bryan-at-apexrad-dot-com>
> Wells,
>
> This is almost exactly the design we were talking about a few weeks ago,
> I take it your tabs on the cap are at the center of the roll on one plate
> and at the edge of the roll on the other plate. You get the best separation
> of connections by putting them on opposite ends of the roll, as well;
> plus this has the added effect of creating a conical-helical net current
> path.
> The vertical component of the net vectors on the two plates cancels if
they>
> are on the same end of the cap roll. Bottom-center and top-edge tabs gives
> a typical inverted cone net current path. How may turns did you end up with?
> (I calculate 13.7 ) The inside diameter is 7", what is the outside
> diameter?
Actually, I was only approximating diameter of my coil form, so that
people could get an idea of the physical dimensions (oops, sorry) To put
this one through the math, use these numbers:
Diameter of coil form: 7.75 in.
length of plates: 96 in.
spacing: .032 in/turn (4 sheets of 4mil poly x
2plates)
Also, I believe your result adds the plate lengths, for 16 feet total.
This
would be correct if both plates were conducting the full current load
from
one end to the other. However, when you think about how a plate
capacitor
charges, with charge being distributed evenly over the surface of the
plate, the last inch of the plate on the end away from the tap will pass
nearly zero ccurrent as it charges, because no charge needs to pass
through that point to get to points on down the line. The first inch
after the tap, however, would pass nearly all of the current it would
take to charge the cap. If this reasoning is correct, the current along
the length of the plate would decrease in linear fashion from capacitor
input current at the tap end to zero at the far end. Since the plates
are, as you surmised, arranged so that one tap is at the center and the
other is on the outside, the sum of the two plates' current-paths should
add to resemble the length of the plates when stacked, for inductance
calculations, and be uniform over the length of the plates, if their
widths are uniform. Also, since I was rolling foil and PE, my coil width
was
very low, about 1/4 inch total when rolled tightly.
So:
Dia=7.75 in.
Circumference=24.3 In.
Turns= 3.95 calculated, 3.75 actual.
> center axis
> _ |
> Height of Coil H_ | | | | | | | | | | | <---cross section
> | of flat spiral
> |---A----| |---W---| coil.
> |
>
> L = inductance in microhenries.
> a = average radius in inches as measured from the central axis to
> the middle of the winding.
> h = height of the cap/winding
> n = number of turns in the winding.
> w = width of the coil in inches.
> Note: Make sure you measure "a" from center axis - the very middle
> of your secondary sitting inside of your primary
>
> Does anybody remember the conical primary equation?
> I suspect that it will look sometning like:
> 2 2
> a x n
> L = ___________________
> ________________
> *(8a+11w)(9a+10h)
The most influential variables, then, are a and n. The small w, due to
the close
spacing and thin insulation, effectively drops that variable from the
equation, and since both of my taps are at the bottom, the shape is
really a flat spiral, unless the width of the plates have some effect on
the inductance. This drops the h (?). The a's in the denominator are
multiplied, and we can remove the square root to give a-root-72 or
8.49a.
Further cleaning gives the following:
2
a x n
L= ___________
8.49
Please keep in mind that aside from formulas designed to calculate how
much alcohol was in those 7 beers last friday night, I have been fairly
light on the math in the past year or so. Please feel free to bring to
my
attention any glaring errors, and pardon me for my off-the-cuff
approach. I'll crunch the rest of the numbers in cleaner fashion this
weekend, from inductance and capacitance through resonant frequency.
Then on to tuning mechanisms, then throw copper at the Q! :~)
>
> Keep us posted on the success of this technique.
>
> Bryan Kaufman
Thanks, I will.
Wells
>
> > Original Poster: Wells Campbell <wellscampbell-at-erols-dot-com>
<<Self-Snip>>
> > My new design idea came about when I read a post a few months
> > ago about parasitic oscillations occuring between the capacitance and
> > self-inductance in a rolled-PE cap. if these caps have inductances, can
> > they be designed for a specific frequency? then it hit--maybe I can
> > build a combo cap/primary by rolling thinner strips of foil around a
> > large form (just large enough to fit over a secondary) and connecting
> > them on opposite ends (as the opposite charges move toward opposite ends
> > of the plate, their inductances add). Just last week, I saw a post by
> > Terry Fritz musing about the same thing. I decided to try it. I set the
> > plate area and no. of turns based on my current coil, trying to match
> > the capacitance and no. of turns of my coil at resonance. I wound 2 8'
> > lengths of 6" wide foil separated by 4 sheets of 4mil PE on a 7" dia.
> > form. I used a 9kv 30ma neon with primary in series with another to
> > keep the power low (nothing in oil, yet) and placed my grounded
> > secondary in the center of the pri/cap. The sparkgap (a mini-RQ-style
> > gap) was in parallel with the power supply and cap/primary unit. to my
> > surprise and enjoyment, I got about 1" to ground off the top of my
> > secondary. (to keep perspective, I tried the same power supply
> > arrangement on my conventional setup, and got about 3" at tune) so I'm
> > seeing 1/3 the usual output on a blind guess--I don't even have a way to
> > tune it yet. I plan to build a model to withstand usual TC voltages, and
> > with a few design improvements: a few taps on the outer end of the roll,
> > to vary the inductance, and a sturdier design. I will probably add a
> > single turn of copper pipe around the outside, for continuous tuning.
> > And of course the whole thing will have to be put into oil and pumped
> > out.> > I think the major advantages for such a system would be high Q
for the
> > tank circuit- the sparkgap could be situated very close to the leads to
> > the plates, and with no caps to charge away from the primary with leads,
> > etc. The current would be simply kicking from plate to plate through the
> > gap (while it is conducting, of course) I also think that for the
> > dimensions of such a coil/cap, that it would lend itself well to
> > magnifier design, ie wrapped on top of a driver secondary. Thus my next
> > project....
<<Self-snip>>