Re: Remote Tuned Primary (RTP) ideas...

>Tesla List wrote:
>> 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
>> months ;-)) is to make a remote primary tuning device.  It has two parts:

A few days ago, Gary Lau suggested tuning the secondary by adjusting the
between two torroids, so I thought I would mention the results I have been
doing just that...

My tesla coil (a small 15 kv, 30 ma neon-powered, 4" diameter secondary unit)
is outfitted to allow continuous adjustment of the secondary frequency and
the coupling while the unit is in operation.  I have been using it this way
for several months now, and have learned much from the system.  Since the list
has been discussing realtime adjustment of TCs, I thought I would tell you
how mine
works and see what people think.  Here's the setup...

Secondary tuning adjustment
My secondary coil has two torroids on top of it.  A large torroid is
electrically and structrually attached to the top of the secondary, 
and a smaller torroid rests on top of the larger one, but is not 
attached to it, except with a loose coil of wire which electrically connects 
the two torroids.  When the smaller torroid is resting directly on top of the 
larger one, the smaller one is almost completely within the "electrostatic
of the larger one, so it changes the overall capacitance very little.
When the smaller torroid is lifted above the surface of the larger one, the
capacitance of the pair increases.  The secondary unit can thereby be
smoothly tuned by adjusting the distance between the upper and lower
torroids.  I have implemented this system by attaching a pully to a beam
high above the tesla coil and running a string from the upper torroid,
through the pully, and down to the operator, who sits some distance
away from the tesla coil.  By raising and lowering the upper torroid,
the frequency of the secondary can be decreased and increased while the
coil is in operation.

Coupling adjustment
My seconary coil / lower torroid unit is also suspended from an overhead
beam with a 
string and pully setup.  Several pullies are used in a miniature block-and-
tackle arrangement which allows for very small changes in coupling to be
made (by moving the secondary/ lower torroid combination up and down).  The
block and tackle also makes operation of the system easier (since the
secondary / 
lower torroid combination is a bit heavy).  This string runs up through the 
centers of the both torroids and is attached to the secondary and the lower
This string is NOT attached to the upper torroid, though.  

To change the coupling of the system while it is operating I simultaniously
pull on BOTH strings, which raises and lowers the secondary and both torroids,
without changing the distance between the two torroids (so the tuning does not

To change the tuning of the system, I only pull on the string that is attached
to the upper torroid, which changes the distance between the two torroids but
does not affect the coupling.

Do the arcs track along the strings?
I use pure cotton string instead of nylon because I am guessing that it's
constant is closer to that of air then nylon's is, which results in a smaller
"dielectric disparity" between the air and the string, which, theoretically,
makes the sparks less likely to track along the surface of the strings.

Sparks seem to like to track along the boundary between two substances
of very different dielectric constants - like along the surface of a glass 
plate in air, so the idea was to keep the "dielectric disparity" between
my string and the air as small as possible.  At least that's the theory...

The proof of the pudding is in the eating thereof, and, well, the sparks 
have not chosen to track along my strings, so I continue to use cotton.
(Note that I live in non-humid Utah.  If you live in a humid area, your
results could be very different...)

Is it safe?
Originally, I had planned on ending my two strings with about
6 feet of glass optical fiber, teflon rod, nylong string, clear rubber
air hoze, or some other really good insulator, so that if the sparks did
decide to track along the strings, they would be stopped in their
tracks (as it were) by the glass fiber, etc.  I could then hold onto the
ends of the glass fibers, and I would be safely isolated from the HV
of the coil.  However, the sparks ignore the strings, so I never implemented
this additional safety measure.  I *do* use nice, long strings, though.

An additional safety measure would be to make the upper pullies metal, and
then ground them.  That way, any sparks that tracked up the strings from the
torroids would be grounded long before they started down the strings towards
the human at the other end....

The seconary/coil assembly DOES tend to swing back and forth a bit, since it
is a free-hanging unit, but that has not been a problem.  There has been no 
arcing between the base of the seconary and the primary, etc.  Using the 
coil on a windy day would probably be problematic, though...

The length of the wire that connects my two torroids is about 1.5 feet long.
This means that the maximum distance I can achieve between my two torroids
is about a foot or so.  I therefore have the primary tap adjusted so the
is more or less in tune with the upper torroid about 6" above the lower
This "centers" the tuning range of my secondary, insuring that the optimal
point is within the movement range of my torroids.

To operate the system, I sit in a chair placed well away from the coil with
the variac on the floor at my feet.  In my right hand I hold the tuning
in my left hand I hold the coupling string, and my foot controls the variac.
This give me real-time control of 3 variables of TC operation.

Why go to all the trouble?
Good point.  Statically tuned and coupled coils have been working great for
The purpose of this project was to give me a better understanding of how
and tuning effect the operation of a TC.  I, like many of us, have spent hours
turning off the coil, moving the primary tap, turning on the coil, watching
sparks, turning off the coil, adjusting the height of the secondary,
turning on 
the coil, etc.  While this method has allowed me to "dial in" a TC, I still do
not have the "feel" for coupling and tuning that I would like to have.
why I implemented the current system.  

It has been a great experience for me to be able to turn on the TC, and watch 
the arcs get longer and longer and shorter and shorter as I change the tuning 
and coupling and power input to the system.  By changing these things on
the fly,
with a running coil, and watching what it does to the spark length, spark 
brightness, spark color, amount of spark "branchyness", "smoothness"
of operation, sound from the spark gap, etc. in real time, I feel that I have 
learned more in 5 minutes of TC operation than I have learned in years of
previous TC work.
All of the parameters of TC operation are amazingly inter-related.  Just
what the coupling and tuning do to how often the safety gaps fire has been
worth the 

Watching what the coupling and tuning (and power input) do to the operation
of a tesla coil in real time simply does not compare to turning the coil
off, making
an adjustment, turning it back on again, etc.  By variying the parameters
in real
time, the changes of the parameters become a dynamic thing, and it is much,
easier to get a "feel" for what they do.  If nothing else, being able to
these things in realtime has been an amazing learning experience for me.

What's next?
Since I have both hands busy with the tuning and coupling, and my feet control
the power input, perhaps I could use my noze to adjust the width of the spark
gap.  :-)  Seriously, though, I think the next thing to do would be to come
up with a decent method of continuously adjusting the breakdown voltage of the
spark gap while the coil is in operation.  Somebody suggested a while ago
a bunch of parallel copper tubes on a flexible strip of plastic.  By
flexing the
strip in one direction or the other, the total gap distance could be increased
and decreased.  This sounds like a good thing to try.  This would allow the 
interactions between "bang size" and coupling to be explored in real time...

That's all I have for now!

Suggestions?   Comments?    Generous donations?

Thanks for listening!

- Kevin Christiansen

     Kevin D Christiansen  -  Crystal Canyon Interactive
"The universe is composed of space, galaxies, and intergalactic
 dust.  Galaxies themselves are composed of space, stars, and 
interstellar dust.  From the omnipresence of dust, we conclude
that nature abhors a vacuum and won't pick up a broom, either."