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RE: Remote Tuned Primary (RTP) ideas...
Hi Terry:
I have no suggestions for your cap switching idea, but wanted to throw out
something that's been rolling about in my head, also on the remote-tuning
theme.
I was thinking about tuning the secondary topload. Consider two toroids,
one atop the other, with a metallic cylinder between them. The cylinder is
actually two nested cylinders with some sort of inflatable bellows within
it. An (insulated) air hose would inflate or deflate the bellows to vary
the distance between the toroids. It may also work out if there's an
overhead beam above the coil that a string and pulley could do the same
thing.
It's unclear to what degree the topload capacitance would vary by changing
the toroid spacing. Certainly not the same latitude as a binary switched
primary cap, but smoother and perhaps cheaper.
Regards, Gary Lau
Waltham, MA USA
Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
Hi All,
One of my new projects (one of a zillion I have
planned for the cold
months ;-)) is to make a remote primary tuning device. It
has two parts:
The first is a way to adjust gap timing remotely. This one
is pretty
simple. Just a DC gear motor attached to a lead screw to
rotate the motor
on my rotary sync gap. There is a nut on the handle so when
the screw
turns, it turns the handle allowing about 40 degrees of
movement. I will
run the motor from a 12 volt gel cell battery and control it
through two
fiber optic cables for total safety (I hate having a wire
hooked from
primary stuff to a box I am holding :o) and to eliminate
RFI. The fiber
optic circuit is cheap and easy (low tech plastic fiber) and
actually
simpler than trying to safely bring out a regular control
cable to a hand
held box.
The second part is more interesting. Having thought of many
"great ideas"
and having them all disintegrate for various reasons, I have
settled on the
following: I will have an "extra" MMC with four stings.
They will have a
value of 0.5, 1.0, 2.0, and 4.0nF (could be other values
too). This device
will be parallel with the regular primary cap. By selecting
the strings in
a binary fashion, I can select from 0nF to 7.5nF of
additional capacitance
in 0.5nF steps. Four fiber optic cables and simple
circuits, as above,
will select the capacitance from a simple binary thumb wheel
switch. Piece
of cake! Except for the four actual switching devices.
My "pals" at Jennings could sell me vacuum contactors at
$2000 a piece but
that is kind of rough even for me. I have taken regular
relays, pulled the
covers off, and submerged them in oil. It works "sorta
good" but the ~1mm
distance between the contacts of most relays is a bit too
small. I would
probably have to use two in series which is not big deal (or
series a DPDT
relay's contacts - I just thought of that ;-)). The
contacts still make
good continuity and the oil and surfaces are not
contaminated to any degree
by the switching. A fairly typical relay (of high quality)
will handle the
voltage and current of just this small additional
capacitance so that is
not a problem either. I would have to vacuum impregnate the
relay with oil
to get it thoroughly saturated but that is easy. Soooo... I
have three
great questions for the collective minds of the great Tesla
list to help me
over some hard spots:
1. Is mineral oil the best thing to use for insulation?
Or is there some
other substance (like silicone oil) that is really better?
2. There is one serious problem I have found. Most
small relays have
little isolation distance between the switch elements and
the coil. An arc
to the coil will probably blow up the control electronics
(fiber optics
prevents such a thing from blowing the operator ;-)). Some
small "safety
relays" have a large distance between the coil and the
switches that should
work good. They are usually pricey but they have cheap
cousins. However,
perhaps someone knows of a perfect relay for such a use??
Big power
contactors would be great but they are really too big for
such a use.
3. I would think there would be a cheap high voltage
oil (or whatever)
relay out there somewhere??? Kilovac makes HV reed relays
(vacuum) but I
think they would be over voltage and under current for this.
Is there a
better commercial relay I could get for not too much money
that would solve
the problem straight away??? I would think a small high
voltage oil relay
would be cheap and easy to make commercially??? I have
thought of putting
the relays in a super high vacuum but that seems far too
hard for me. The
specs are: 30KV isolation and switching, 100kHz, 2 amps RMS,
and 70 amps
peak. The fiber optic cable isolation allows these specs to
be abused
wildly ;-)
Any help, ideas, etc. are welcome. It would really be cool
to adjust the
gap and primary tuning remotely from simple switches. One
could really
dial in a coil while at full power very easily with such a
simple
arrangement. It would be the envy of all your friends :-)
It would really
be useful for people that mount coils 40 feet in the air in
large arenas
and find the tuning is not the same as it was on the ground
;-)) The same
controls would probably work with variacs and such too.
Just needs a
little research and development work...
Cheers,
Terry - on the bleeding edge of Tesla coil
technology...
terrellf-at-uswest-dot-net