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Re: helical cap
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?
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)
Keep us posted on the success of this technique.
Bryan Kaufman
> Original Poster: Wells Campbell <wellscampbell-at-erols-dot-com>
>
> I'm writing as a new member to the list... I started coiling about two
> years ago, building a small "table-top" coil with a 15000/30 NST and
> 2.125" x 12" secondary w/ #28 ga. wire. Anyway, I started reading the
> archives a few months ago, beginning with Jan. '98, to get a little
> perspective. I finally caught up, and have nothing to do now but join.
>
> Reading the list has been informative, but now that I'm current, I
> figured the best course of action is to start experimenting so that I
> will have something to say. I have a design element to share, and some
> questions. 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, and a few questions...
> I am planning a magnifier design with the pri/cap unit, a rotary gap,
> and a DC power supply. Due to my living situation (one-bedroom apt.--I
> can feel the whole list wince) I am limited to table top coils. It gives
> me the opportunity, however, to concentrate on compactness, efficiency,
> adjustability, and innovation, as well as pulling max. spark length
> from a fixed power source: Aforementioned 15,000/30 NST. Anyway, I have
> 20 square feet of copper foil I picked up at a metal surplus place, and
> some diodes on order (sourced from the list--thanks) but I haven't built
> a rectified power source before, and could use some hints, and things to
> watch. While I was in the surplus place, I checked out an odd-looking
> transformer--the plate says it is a GE Current transformer with a 200:5
> ratio. Judging from the shape-- like a thick cylinder with a hole in the
> center-- it goes onto power lines, perhaps to measure current? This
> would mean it is just one coil within the rubber housing--could this
> have any TC use? could I insert a primary and core into the tube for a
> homebrew trannie? could I use it as an adjustable ballast with a movable
> core?
> But I ramble on...anyway, back to work.
>
> Wells Campbell