Tesla coil information

Lindsay's offers primarily reprints of books and other information concerning
technology and techniques no longer in general use.
Recently I noticed the book "Inventions, Researches, and Writings of Nikola
Tesla" by Thomas Commerford Martin at a local bookstore.  Lindsay's offers
this book, but the one at the store was published by someone else, so this
book is currently available from two publishers.  Note: this book was
originally published in 1894 and deals mostly with Tesla's work with
polyphase currents and his lectures and demonstrations up to that time.
There is some information on his early work with "Tesla coils", but he was
actually just beginning his more famous work in this area at the time.
(Tesla's Colorado Springs experiments took place in 1899-1900.)
Regarding the specific questions on Tesla coil construction:  There are a 
number of factors that need to be taken into account to get the best
performance (longest sparks).  One of them, the "Q", has already been
mentioned.  Basic rule of thumb is you want the length of the secondary
coil to be 2 to 2-1/2 times the diameter.  The next is a lot more tricky to
work out.  The length of wire used in the secondary should be equal to a
quarter wavelength of the resonant frequency.  This way you have a voltage
maximum at the free end of the coil.  From what I can tell (I've never
actually built a Tesla coil due to lack of money and facilities to do the
job right) this is no easy task.  You have to figure out where the secondary
is self-resonant, and tune the primary accordingly.  Alternately, or in
combination, you can tune the secondary by means of what Tesla called an
"elevated capacity", those globular capacitors one often sees on the end
of Tesla coil secondaries.  The disadvantage of the elevated capacity is
that it accumulates a charge.  This should make for shorter, less continuous,
less impressive discharges as opposed to a point.  Also, it's quite a bit
more dangerous due to the accumulated charge.
The most common alternative to the single spark gap in the primary, which
offers improved performance, is the rotary spark gap.  This is a set of
rotating contacts to increase the number of sparks and get more energy
into the system.  I have heard of the vacuum tube driven primary technique
but I haven't heard of many successes using it.  I'm figuring it's more
difficult because the tube oscillator must be tuned to the frequency of
the rest of the system, and this is usually not a known quantity until
after everything has been tuned up and operating.
I hope this information is of some use, and again I apologize for taking
up so much space on a (somewhat) unrelated topic.  Is there a Tesla
mailing list, or enough interest to start a newsgroup?  :)
Paul Prescott

From: mac@cis.ksu.edu (Myron A. Calhoun)
Newsgroups: rec.radio.amateur.misc
Subject: Here is a TV flyback Tesla design
While cleaning up my workroom I found a schematic for a "TESLA COIL KIT"
sold (many years ago) by B & F Enterprises, 119 Foster Street, Peabody,
Massachusetts 01960 (617) 531-5774:
                                         O CAP terminal (VERY HIGH VOLTAGE!)
   +--------------------------------+    |
   |           +------------+       |    |
   |        | /             |       |    |
   |        |/              |       | || $         ******** DANGER *******
   +--------|               |  L2 = $ || $         *                     *
     2N3055 |\       L1 = 4 $   10  $ || $ L3      * I TAKE ABSOLUTELY   *
            | v       turns $  turns$ || $ is a    *                     *
              |             $       $ || $ fly-    *       ** NO **      *
              +---+      +--$    +--$ || $ back    *                     *
              |   |      |  $    |  $ || $ xfrmr   * RESPONSIBILITY FOR  *
            | ^   |      |  $    |  $ || $         * YOUR USE OR ABUSE   *
     2N3055 |/    |      |  $    |  $ || $         * OF THIS CIRCUIT!!!  *
   +--------|     |      |  |    |  $ || $         *                     *
   |        |\    |      |  |    |  | || $         ******** DANGER *******
   |        | \   |      |  |    |  |    |
   |           +--|------|--+    |  |    +------o 1  8  -  5  Pin connections
   +--------------|------|-------|--+    +-----o  8  7  5  4  to the following
                  |      |       |       +----o   5  6  6  2  flyback 
                  |      |       |       +---o    7  4  8  1  transformers:
                  |      |       |           |    |  |  |  +-- 738192
                  +------|-------|-----------|    |  |  +----- 32-10008-4
                  |27 ohm|240 ohm|           |    |  +-------- 32-10014-1
                  +--R1--+---R2--+           |    +----------- 32-8859
                  |              |         -----
                  |              |         /////
                  -   12 volts   +
Quoting from the instructions:
   The new windings must be added to the flyback transformer to allow
   adequate transistor drive capability.  These windings are hand wound
   on the bottom leg of the ferrite core where the original filament
   winding was located.  Remove and discard the original filament winding.
   In its place, wind first a ten turn, center-tapped, winding using
   approximately 30 inches of #20 or larger hookup wire.  This is easily
   accomplished by winding five turns at one end of the core and then
   twisting a loop in the free end before adding the second five turns.
   The complete ten-turn winding should then be held in place with a turn
   or two of electrical tape with the two ends and the center loop all
   protruding.  Connection can be made to the center loop when the
   insulation has been carefully removed.  If it becomes necessary to cut
   the center loop, be sure that the two ends are scraped and joined to
   form a mechanical as well as an electrical center-tap to the winding.
   [L2 in my drawing]
  "The second winding should be wound directly on top of the first but
   it should only have a total of four turns--two each side of the
   center-tap.  Again, wind two turns of #20 or larger hookup wire, pull
   and twist a center loop, and wind the other two turns.  Tape this
   winding in place on top of the first.  Do not let the center taps of
   two windings touch each other but they should not be more than one
   quarter of an inch apart.  [L1 in my drawing]

  "Wire the modified transformer to the transistors and the bias circuit
   resistors as indicated on the schematic circuit diagram.  Be sure to
   mount the transistors on a suitable heat sink with the insulating
   hardware provided to protect them from over heating.  Since the
   collector connection of these transistors is made to the case, it is
   necessary to keep them electrically isolated from one another and
   from the heat sink and insulators are provided to allow this.  Heat
   sink area of approximately 25 square inches should be adequate.  If,
   however, the power transistors become too hot to hold a finger on,
   more heat sink area should be provided.  Heat sink material of one-
   eighth inch aluminum is desirable.
  "Note:  If Oscillation does not start Immediately Interchange the two
          Transistor Base connections [capitalization as in original!]
There were other instructions about how much damage might accrue to a
human if contact were made to the high-voltage output, so BE CAREFUL!
I used one-half watt resistors for R1 (27 ohms) and R2 (240 ohms) and
got a nice Jacob's ladder effect.  It also wiped out most of the
computer terminals in the entire building, so I couldn't run it very
much!  I later tried 18 volts and one resistor zapped immediately!

Date: Tue, 27 Sep 1994 18:00:34 EDT
To: mtracy@arrl.org
Subject: Error in TESLA.TXT file and more Information

There is an error in the schematic diagram for the transistorized Tesla
coil.  In the configuration shown, the positive supply voltage goes
through L2 to the Bases of Q1 and Q2, with the emitter grounded.  This
would tend to draw a very heavy current from the power supply, only
limited by the resistance in L2 and the Base-Emitter junction in the

R1 and R2 are the base biasing resistors.  The connection between these
two resistors is shown going through L1 to the collectors of the

For proper operation of this circuit, the respective ends of L2 should
be connected to the Collectors of the transistors, and the ends of L2
should be connected to the bases.  This will allow proper biasing and
oscillation of the circuit.

Also, for the modern builder of Tesla Coils, the following are three
books I feel are essential reading:

1)  "Modern Tesla Coil Theory" by Duane A. Bylund, Tesla Book Company,
P. O. Box 121873 Chula Vista CA 91912.  This is the best modern book I
have read on the subject of Tesla Coil Building and Theory.  Covered
are aspects Capacitive Discharge, Vacuum Tube, and Transistor Tesla
Coils as well as basic theory.  It cost about 25 dollars two years ago
and is 140 pages, 8 and a half by 11 format.  I recommend any potential
builder obtain this book!

2)  "The Tesla Coil Compendium" by Randall S. Peterson, 16671 T. R. 51,
Mt. Cory OH 45868.  Phone 419-358-1222.  A complete description of
several successful Tesla Coil projects.  Very Good Info.  Approx. 20
dollars 2 years ago.  About 100 pages.  May be available at the Tesla
Book Company See 1 above)

3)  "Tesla Handbook", by J. H. Couture, JHC Engineering, 10823 New
Salem Point, San Diego, CA, 92126.  Very Good Information with a
systematic step-by-step method for construction of Tesla Coils of all
sizes.  Approx. 20 dollars two years ago.    May be available at the
Tesla Book Company See 1 above)

I hope the above information is helpful.  73 de WB4GHZ, --- George

Subject: Tesla.txt
To: mtracy@arrl.org
Date: Thu, 13 Oct 1994 19:13:22 +0100 (MEZ)

As to tesla coils there is a good scientific article:

"A Tesla transformer high-voltage generator", C.R.J. Hoffmann
Reviev of Scientific Instrumentation, Vol. 46, 1.Jan. 1975, pp1-4.

73, Moritz DL5UH

The members and HQ staff would like to thank the following people for
their contributions to this information file:


Send any additional information or changes to mtracy@arrl.org. 

73 from ARRL HQ.