FW: Remote Tuned Primary (RTP) ideas...
1.) How about a 1000/2000pF motor driven vacuum variable, +- 1000pF should
provide good range around a viable tap.
encl: pict of representative motor driven unit (higher C & V's available)
The TRW swapmeet always has a selection, priced anywhere from ~$30 to about
(last Sat of every month, next one is tomorrow -
want me to look for you ?
2.) or How about making an oil emersed motor driven variable Cap - start
with a big transmitting variable cap, add poly to multiply capacitance ?
(in place of oil emersed relay switch matrix - advantage of "analog"
Xmtg variables also frequently avail at TRW radio club/electronics swapmeet.
From: Tesla List [mailto:tesla-at-pupman-dot-com]
Sent: Tuesday, October 26, 1999 8:32 PM
Subject: Remote Tuned Primary (RTP) ideas...
Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
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
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
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
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...
Terry - on the bleeding edge of Tesla coil technology...