Original poster: "K. C. Herrick" <mailto:kchdlh@xxxxxxx><kchdlh@xxxxxxx>
Herrick is still at it, albeit fitfully. Ref.
My postings of some time ago, regarding the s.s.t.c. primary I'd
been building inside an 18-gallon bucket, may be remembered. I
threw out the scheme featuring an internal primary: too much
electrical stress. Also threw out my paper-design for a variable-
inductance primary coil and have -- finally! -- almost completed
the new design. I post this, well-prior to firing it up again
since I'm facing other reworking tasks, thinking that a) it may
offer an interesting idea or two and/or b) someone may be pleased
to tell me in what ways it's a bum design.
If you will ignore the clutter in the photo... At the left are 2
knurled 1/4-20 nuts for the H-bridge input leads. The series-
paralleled capacitors are split into two strings, with the coil
connected to those strings at the right side, underneath. The coil
is made from two stacked layers of 1/4" Cu tubing, formed in
McMaster-Carr "multi-slot routing mounts" (their no. 7659A11 and if
you didn't know what to call those you'd play hell finding them in
their catalog without the no!). The routing mounts, actually
multi- position plastic tubing-clips, are arrayed on a Lexan disk and the
upper coil is fastened to the lower one with pop-rivets from
underneath so its top surface is smooth. The disk, in turn, is
mounted against a flange of the bucket.
Nine sliding contacts are made from 0.04"-diameter phosphor-bronze
spring-wire. They are clamped between aluminum guide-plates along
with the end of a piece of 3/4"-wide copper braid. The braid is
led down through the inside of the hollow driving-shaft at the center.
Rotated by an offset pin fastened to the shaft is another, smaller,
Lexan disk which not only rotates but also slides on the top coil
-- back and forth along the axis of the phenolic stiffener that is
fastened to it. Its lateral position is determined by a pair of
nylon guide-screws to be seen near the right end of the stiffener.
The nylon screws project down between turns of the upper coil so
that, when the disk is rotated (via a plastic timing-belt sprocket
underneath), the ends of the contacts accurately track the turns of
The brass-tubing shaft/braid assembly is stiffened by the insertion
of a solid metal rod inside the braid. Underneath, a 4-turn coil
of the same copper braid -- having minimal diameter when it is
wound the tightest -- connects the shaft/braid to the appropriate
end of a capacitor-array. (The outer end of the copper tubing
coil- pair connects similarly but directly since it does not move.) I've
wound the braid-coil in the direction that will cause it to
contribute to inductance as it expands, for the rotation-direction
that increases coil-inductance (and vice versa, of course). I
measure, for the entire coil assembly, ~8 m-ohms dc + 7 uH at one
end and ~13 m-ohms + 22 uH at the other -- for a travel of a little
over 3 (inside) coil-turns. I want to ultimately accommodate 2
secondaries, resonating at ~100 and ~130 KHz.
The 12"-diameter secondary has fastened inside its lower end an
identically-shaped bucket, with its top section cut off down to the
(identical) flange. Such buckets are designed to stack, so this
yields a convenient way of holding the secondary firmly upright
atop the primary--kind of like a bayonet-mount. It serendipitously
happens that the bucket is a perfect fit inside the secondary's
I may expect corona from some of those capacitor-screws, or the
copper straps; but when & if that happy event should occur I could
likely add some small hollow aluminum balls to smooth out the
fields. McMaster-Carr stocks them.