[Prev][Next][Index][Thread]
Re: Helical cap
Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
Hello Wells, Bryan, Malcom, all
Now, I AM thoroughly confused. I thought Wells was looking
for the CONICAL inductance equation, which would be:
(please use courier font to view picture)
/\
-- o / o
| o / o
| o N turns / o
o Z / o
h o / o /
o / o /
| o / o / Angle = X
| o \ o /
-- o o ------------
|
| w | R |
|
|<-- W -->|
Center | Line
Z = Coil Width (hypotenuse length)
X = Angle of Cone
h = Z*sin(X) Effective vertical Height
w = Z*cos(X) Effective horizontal Width
W = R + w/2 Average horizontal Radius
L1 = W^2*N^2/(9*W+10*h) (Vertical Inductance Component)
L2 = W^2*N^2/(8*W+11*w) (Horizontal Inductance Component)
L = SQRT[(L1*Sin(X))^2 + (L2*cos(X))^2]
And for the guys, who work it backwards (like me), here is
the equation that gives the number of turns for a specific
inductance value. Very special thanks go to JIM MONTE, who
re-wired the equation for me (I had tried to re-wire it, but it only
resulted in "garbage in, garbage out") (:o))
N = sqrt(L) /(W * ((sin(X)/(9*W+10*h))^2 + (cos(X)/(8*W+11*w))^2 )^0.25)
<<Snip>>
Hope this helped and was what your were looking for.
?????????????????????
Coiler greets from germany,
Reinhard
Thanks for the equations, they will serve as useful guides, anyway.
Here's the application, in a nutshell: I have built a helical cap of
large diameter and few turns to serve both as cap and primary. In
construction, the two plates (10' long x 6.5") are sanwiched in PE, with
one tap at one end of the stack and the other tap, on the other plate,
at the other end of the stack. It is then rolled onto a 7" diameter
form, which puts one tap at the center and the other on the outside. As
each plate is charged (during resonant oscillation--NOT 60Hz charging,
because the inductive component would not matter then) The oppositely
charged current travels in opposite directions around the form, such
that their inductances add. (taps on the same side would have the
inductances cancel)
The current also decreases in linear fashion from the tap-side of the
plate (max current) to the end (Zero current), and the sum of the
plates' current-paths equal the length of one plate.
Bryan had suggested that if the outer tap was coming off the top of the
cylinder, and the inner tap off the bottom, then the net current-path
would resemble a conical primary. My foil prototype had both taps on the
bottom, so the net current path was largely horizontal. I feel that it
would behave as a flat-spiral primary, because of the shape of the
plates and the fact that the charge would be distributed evenly over the
surface of the plates. If the vertical vectors would largely cancel, as
Bryan pointed out, then the coil would be just an extension of it's
cross-section. Think of 6 wire flat spirals wound close and stacked on
top of each other, one every inch, and connected in parallel.
The field would be shaped differently than just one wire spiral, would
this change the inductance? Would the same inductance be spread over a
larger area and decrease in density? Is there a function for "plate
height" in this configruation that affects inductance?
Anyway, I decided to go ahead with a full-strength although small model,
and just to make it interesting, I built it as a magnifier. I used the
secondary from my old coil (1st) a 2.25" diameter x 10.5" wound w/ 28
awg for the bottom 1/3, and 30 awg for the top 2/3, for about 800 turns.
The driver secondary started at 250 turns of #20 magnet wire, close
wound for 8 in. on a 5" dia. plexi form (I have since peeled the driver
down to a 4.5" winding length w/ 135 turns), which fits neatly inside
the 7" form on which I have wrapped my helical cap. I used 8 layers of
4mil PE to separate each of 2 copper foil plates, 6.5" x 10'. This forms
5.2 turns, with taps at 4.5 and 3.8 turns on one plate. The net radius
of the primary/cap is 3.625" and the inner to outer winding width is
0.5".
Preliminary tests with close gapping and a 9,000 / 30 nst on a variac to
protect the as of yet dry pri/cap shows promising results: Nearly 5"
point to point from the resonator toroid, and a very hot and snappy 1.5"
from my driver. A question: should the apparent driver voltage be lower?
Could it indicate still too many turns on my driver? When I still had
250 turns on, the primary spark was almost as long as the secondary, and
depending on the tune, sometimes longer. Anyway, my next step is to add
some length on the plates of my primary, and pot the whole thing in
mineral oil.
I can't tell you how hard it was to keep from cranking the variac up,
and widening the gaps. With two gaps and very close electrodes, I was
getting clear "harmonics" of the 60hz line- at certian points on the
variac's scale, the bang rate would seem to double--several clear steps
from about 25% (just sparking) to 75% (the highest I would allow myself
to go) and at the higher rates, the arc had already shown some brush
characteristics when near the ground rod. I can't believe I'm actually
running a maggey!
I'll post my progress and maybe a pic or two if the whole scrap heap
actually comes to something...Also I plan to come by some inductance and
capacitance measuring equipment...Anyone have any reccomendations for
reasonable but accurate?
Wells