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Another "My First Coil" story.
Subject: Another "My First Coil" story.
Date: Mon, 26 May 1997 22:30:54 +1000
From: Phil Chalk <philoc-at-ozemail-dot-com.au>
Organization: Secure_Logic
To: tesla-at-pupman-dot-com
Greetings all,
I've finally joined the ranks of those that've actually built one,
throwing sparks ! But........
Here's the details :
(1) 12kV 30mA Neon Tx that's a bit 'sus' It was given to me by the neon
sign shop, so I figured it's not perfect. The secondary measures
(to CT, case) 8k from one end & 11k from the other. However it makes an
OK Jacob's Ladder & seems pretty much OK.
Chokes in each leg are about 400 turns #23, in 3 layers, well
insulated, on a 10mm x 100mm (3/8" x 4") ferrite rod. This was
completely arbitrary - how are they ??
(2) Cap is stacked plate; Plates are 22cm x 17cm x .015mm (~ 8 5/8" x 6
3/4" x .006") Aluminium 'Catering Foil'. Dielectric is 20 x 100um
(~ .004") sheets LDPE, i.e. .080" 26 plates (25 dielectrics) & approx
8000pF. Drowned in 20W50 motor oil, & I don't have a vacuum pump, but so
far so good.
(3) Primary is 1/4" copper refrig. tubing in approx 30 degree spiral. 12
1/2 turns till I ran out of Cu. Inner turn is ~5" dia. linear spacing is
15mm (~ 5/8") Built on 5mm acrylic sheet base & supports.
(4) Secondary is ~1020 turns of #24 SWG( ~#23AWG .56mm Cu., .63mm with
insulation) Winding length is 25 1/2" close wound (~230m/250yd) on a
70mm OD (2 3/4") acrylic tube, 3mm wall, sealed at both ends with
acrylic disks, & coated with several coats of polyurethane. Brass plate
as ground terminal, 1/4" bolt thru CB antenna base mount conical
insulator, on top.
I can adjust the (vertical) position of the secondary such that the
bottom turn can be between about 2" above to 6" below the plane of the
bottom primary turn.
(5) Spark gap is merely 2 x 3/8" bolts thru 2 bits of threaded 1/4"
plate steel mounted on some 5mm acrylic sheet. Wires soldered to
the steel blocks.
With nothing on the top terminal bolt it tunes at around turn 6, and
throws several branching, violet, lightning-like sparks from the top of
the bolt into the air, of maximum length perhaps 9-10". (and LOTS of
NOISE !) It seems best with the secondary level or lower down inside the
primary, than above. Seems best with gap = roughly .1"
With a 6" copper sphere (toilet cistern float) on top, it tunes at about
turn 7, and throws the odd spark from the centre rib - sometimes. If I
put a pin on the sphere it throws sparks from there, but no longer -
perhaps shorter - than without the sphere. A roughly hemispherical spun
aluminium 'wok lid' about 12" diameter throws many short (<= 1") sparks
from the rim.
I know the gap is bad - it was my first roughie from when I was dying to
get it going. I feel the secondary is probably too long. I'd like more
current than my 30mA (actively hunting transformers tomorrow!). I now
know that reducing the spacing of my primary turns will help - Thanx
Gary Weaver. I felt the coupling was inadequate, and was thinking of
trying a cylindrical (solenoid) primary when I can, for comparison. I
think the cap is OK.
I welcome anyone's comments on all the above, but the thing that puzzles
me is that it seems to tune at relatively few turns for such a
relatively long primary. It is of course a thin primary. Based purely
on wire length, the secondary is a quarter wave at about 325kHz
I haven't done any maths about it yet, except to choose .008uF as the
cap value, as its Xc at 50Hz is abot 400k, and I figure the effective
impedance of a 12kV/30mA neon is about 400k. Is this an uncommonly
large value cap to use on this tranny ??
Also why won't the sphere throw sparks into the air - too small ?,
insufficient drive current ? A torroid is on the "soon" list.
So thanx for listening. Any comments most welcome.
Phil Chalk.