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Re: Continuously variable primary

Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

Gary (& all)-

You make a good point about the contact; that's clearly the weak point. But absent that, I'd think that the mechanism for moving the contact(s) and a mechanism for moving a 2nd coil would be more or less comparable in complexity.

My objection to a 2nd Coil is two-fold: First, I'm loath to introduce more resistance into the circuit--both dc resistance and skin-effect resistance. Already, I'm mounting the capacitor-string as close as possible to the primary coil; in fact, it constitutes 1 turn, in effect, of that coil. And also, I intend to make the two fixed (exterior) turns of the primary out of paralleled 1/4" tubing, to reduce the resistance there.

Secondly, I would like to be able to tune the primary over a ~1.5:1 frequency range, requiring a 1.5^2 or 2.25:1 inductance range. That's to accomodate my two secondaries. I'd like not to have to re-adjust a tap if I can avoid it and perhaps it's unlikely that a variometer by itself would accomplish that.

I'm going to partially put together the adjustable portion of the apparatus and then test the contact scheme to find out whether it's adequate. If so, I'll proceed and if not, I'll try to adapt what I have to a variometer scheme. There's plenty of room within the assembly for such a coil, and I could live, of course, with having to move an external tap.

I've thought of a simpler contact-rotor so I will go with that.

So onward, as long as I hold out; absent misfortune, in 10 years or so I leave coiling and the saving of the world to the rest of you. God help you, say I who don't believe it.


Tesla list wrote:
Original poster: "Lau, Gary" <mailto:Gary.Lau@xxxxxx><Gary.Lau@xxxxxx>

Hi Ken:

I'm sure your scheme could be made to work, but it seems very complex.
I think the challenge of building a rotor contact capable of supporting
multi-hundred Amp currents is significant.

I don't think that it's necessary to be able to continuously vary the
inductance from 0-100%.  A far simpler approach would be to construct
two conventional flat primary coils, joined in the center with a short
length of flexible cable, and have a mechanism to vary the separation
between them.  This will vary the mutual inductance, and consequently,
the net inductance of the two coils in series.  The achievable
adjustment range is probably equivalent to a couple of turns, and that
should be sufficient for fine tuning.  Rough tuning would be achieved by
tapping the upper and lower coils in the traditional sense, with power

Regards, Gary Lau

> Original poster: "K. C. Herrick" <mailto:kchdlh@xxxxxxx><kchdlh@xxxxxxx>
> I'll post on hot-streamer/temp a mostly-complete drawing of what I
> have in mind for a c.v. primary.  File TCHTUNE.JPG.  Comments
> welcomed, of course.
> <http://hot-streamer.com/temp/TCHTUNE.jpg>http://hot-streamer.com/temp/TCHTUNE.jpg
> The whole thing is to fit into one of my 18 gal. plastic
> food-buckets.  In the drawing, the "near disk" fits against the
> bucket's inner ridge.  The view is looking into the bottom of the
> bucket, which will be "down" when it's all put together.  The primary
> capacitor-string is to be wrapped around the outside of the bucket
> (to constitute an additional turn), and the secondary coil is then to
> be plunked down on top of the assembly.  The spiral coil shown in the
> drawing ends up close to in-plane with the bottom of the secondary.
> The coil is mounted on the "far disk" and it spirals toward the
> viewer (which is ultimately downward when it's all turned over, of
> course), spaced gradually by increasing quantities of 0.02"-thick
> washers under the fastening points.  As a consequence, as the rotor
> assembly turns, it always clears inner coil turns by at least 0.16
> inch.  The rotor assembly slides back & forth on the drive disk and
> at the same time the contact-carrier part of it hinges up & down as
> required, to maintain contact with the coil.
> To wind the coil in the first place, I'll mount flat onto the "far
> disk" a set of the little plastic tubing-clips marketed by
> McMaster-Carr; that's what the starred holes are for.  The rest of
> the clip-holes are drilled to clear 4-40. Then, I bend the tubing as
> required & snap it into the clips.  Then, using a marking pen, I'll
> put a dot on the under-side of the tubing thru each of those 4-40
> holes.  Then carefully remove the coil and remove & put aside the
> clips.  Then drill & tap for 4-40 at all the marked places on the
> coil.  Then re-mount the coil on 4-40 machine screws, spiraling it
> upward (in the drawing) with the 0.02"-thick washers.
> The contacts are 9 segments of phosphor-bronze spring-wire (each
> handily already-curved as received in a large coil).  Each will have
> a tang bent up, to pass through matching holes in the support plate,
> and all are to be  clamped in place with 2 screws + a small
> cross-plate (shown dashed).  The soldered connection to them is made
> at the tangs, with 3/4"-wide copper braid,  The braid is to pass
> through a hole (not shown) in the drive disk and thence to coil
> around the driving shaft (also not shown) to allow for the 3 or so
> turns of rotation.
> The business-ends of the contacts are kept in place by two nylon
> screws, which also serve to guide the in & out motion of the rotor
> assembly, by virtue of their being positioned between coil turns.
> In addition to the variable turns, I'll have 3 fixed external turns,
> also of 1/4" tubing.  I'm shooting for tuning over a range of ~70-110
> KHz, to accomodate the two 12"-diameter secondaries that I have,
> which resonate at ~100 and ~125 KHz.
> So, everyone...what have I overlooked?