tesla.faq [15k]

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       T      EEEEEE  SSSSSSSSS  L       AAAAAAAAA         FAQ
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              version 1.0    28.01.1996

 " And from the peace of darkness the neighbours were cast into a firey
   hell of writhing flame, as the raised coil turned night into day! "



  0.      Warnings
  1.      Tesla himself
  1.1      personal background
  1.2      general inventions
  1.3      what he means to us
  2.      Tesla-coil, types: 
  2.1      1/4 wave
  2.2      bipolar
  2.3      tesla magnifier 
  2.4      other configs
  3.      individuals components
  3.1      primary system
  3.1.1     primary coil
  3.1.2     capacitor
  3.1.3     spark-gap
  3.1.4     protective components
  3.1.5     wiring
  3.2      secondary 
  3.2.1     secondary coil
  3.2.2     capasitive hat
  3.2.2     ground connection
  3.3      tertiary coil
  3.4      control circuitry
  3.4.1     variac
  3.4.2     current limiting
  3.4.3     contactors
  3.4.4     fuses/circuit-breakers
  3.5      test-equipment
  4.      sources for more information
  5.      acknowledgements


0.  Warnings

The equipment (tesla-coils) described in this text-file are potentially
lethal and should be handled with respect. The voltages present are very
high, even megavolts are common, and especially at primary circuit have
such high currents that death can be imminent in case of an accident.
Without proper design large amounts of RFI or radio-frequency
interference can result.


1.  Nikola Tesla

1.1     Personal background

Tesla was born in Smilja, Kroatia 1856. He arrived to New York in 1884
and founded a lab soon. His Colorado Springs experimental station was
founded in 1889 and there he made his most interesting experiments with
resonators of many kinds running up to gigawatts of power. His biggest
project was the global transmitter of electricity, Wardenclyffe, at Long
Island in Manhattan area of New York - he never finished this project
because of lack of funds.

1.2      general inventions

Tesla invented the AC-engine and multi-phase power distribution
systems still use today, is the official inventor of radio, has
his name as the unit of field flux density etc.

1.3      what he means to us

< to be added - someone? >

2.      Tesla-coil system

A Tesla Coil is an air core, RF resonate, transformer. It is a very
efficient source of very high voltage RF energy. A simple coil setup is
capable of producing more or less continous spark discharges several
feet in length, or longer.

  2.1      1/4 wave

A normal coil system operates by having a VSWR, or voltage standing-wave
resonance, -effect on the secondary coil and thus the peak of voltage
appears on the top of the secondary. One can visualize the 1/4-wave
nature by thinking about the normal sine wave starting from origo and
continuing on the x-axis. The sine-wave has it's peak value at 90
degrees or pi/2 which is one fourth of full cycle or 2*pi and the
1/4-wave system has this very peak of voltage on the top of secondary.
The formal name for the secondary is a slow-wave helical resonator.

  2.2      bipolar

On a bipolar system the 1/4-wave peak appears at both ends of the
secondary so this could be called half-wave system. One can visualize
this kind of a set-up by thinking that the bipolar secondary is in fact
two 1/4-wave secondaries with grounds connected together at the middle
of the coil. In fact, some set-ups do use separated coils to get more
practical systems.

  2.3      tesla magnifier

A magnifier differs from the traditional tesla-coil by having in fact
three coils. The primary excites the secondary and this is just an
ordinary non-resonant air-core transformer but the output from the
secondary is base-fed to the tertiary which is resonant at the primary

2.4     other configurations

Coils that are excited or driven to higher harmonics (over 1/4 wave)
break down prior to the top turns. For instance a coil driven to 3/4
wave resonance would still break down at the 1/4 wave point of the
winding. This problem is commonly seen on coils that are overdriven or
overcoupled, and I cannot imagine any way to insulate against this

However when you go to two or more coils, where you are working
with more than one output terminal, it is possible to achieve 3/4
wave signal processing. This requires a pair of 1/8 wave driver
coils, two primaries run in series off the same tank ciruit, and
two extra coils. Your outputs from the extra coils are still 1/4
wave with the peaks on the terminals, but taken as a whole the
system does efficiently process a 3/4 wave signal. This system
would be adding up lower harmonics (two 1/8th wave outputs, with
two 1/4 wave outputs) to get to efficient 3/4 wave signal
processing. This system would be unique in that it may not
require a ground, as the two tuned sets of coils could "beat" off
of each other; in other words the nul voltage current output from
the base of one 1/8 wave driver coil could be fed into the base
wire of the second driver. This system would require that the
series run primaries, and the drivers and extra coils be wound in
opposite directions to achieve phase differential. The base wire
current output of one coil becomes added input to the base of


3.      individuals components

A tesla-coil system is combined of many components that all have to be
of highest approprite quality in order to gain as good results as
possible. This is because the worst component often is the limiting
factor of over-all performance.

3.1     primary system

The tank circuit literally converts line current into a series of rapid
pulse dicharges with peak powers in the megawatt range.

Basically you take a high voltage pulse discharging capacitor and
connect it to a large heavy coil. The cap is charged with a high voltage
power supply (neon sign xfrmr, potential xfrmr, or power distribution
xfrmr run backwards). The circuit consisting of the cap and coil is
excited by discharging the cap through a spark gap. This way current of
hundreds of amps at thousands of volts oscillate through the coil.
Frequency of oscillation is dependant on the number of turns in the coil
and the size of the cap. This is the basis of the Tesla Tank circuit.

The primary circuit can be constructed in many different ways, with
either floating or grounded tank circuit. The configurations can be
specified with the following diagram and table:

        ------o--2------               1   2   3
     high     |        primary         Gap Cap -
  voltage     1        coil            Gap Cap Cap   "equidrive/balanced"
   source     |        connection      Cap Gap -

3.1.1   primary coil

The primary coil is a low resistance, heavy coil, through which
the currents produced by the pulse capacitance travel. Commonly
this consists either a pancake, "saucer" or helical coil wound
with soft copper tube with up to 15 turns.

3.1.2   capacitor

A pulse-discharge capacitor is a vital part of the primary circuit as it
enables the energy to discharge through the primary coil, spark-gap and
the wirings as quickly as possible thus creating the necessary
power-levels to excite the secondary coil.

Capasitors with adequate voltage rating can quite cheaply be constructed
of the materials commonly available even in the household supplies. Some
of the low-Q solutions include salt-water caps, glass-caps and
commercial caps not meant for AC or pulse-discharging found in surplus

A very good cap can be constructed of polyethylene or -propylene foil
and aluminium flashing in mineral-oil immersion. Commercial caps with
superiour performance are available as well.

More information: rqmsgs.txt   (theory and practical construction)
                  tesla.txt    (sources for commercial capacitors)

3.1.3   spark-gap

Spark gaps are the "brain" of the Tesla Coil. They are high the voltage
switches that allow the tank circuit capacitance to charge and
discharge. As performance of the spark gap switch is improved, peak
powers in the tank circuit grow without requiring additional input

More information: sparkg.zip (cylindr.*, airblst.*)

3.1.4   protective components

Tesla power supplies must be protected with extensive RF choking, and
safety gaps. This is especially important with neons, which are much
more delicate than pole pigs or potential xfrmrs, but in order to avoid
unwanted RFI and possible danger to other household equipment sufficient
protection should always be supplied. Bypass capacitors should be used
as well - for bypass capacitance across the power supply HV terminals
you WANT a dielectric with a high RF dissipation factor.

More information: chokes.zip

3.1.5   wiring

The wiring of the primary system should be as heavy as possible because
of the huge currents flowing through the conductors and connectors. All
the wiring should have as large and smooth a surface-area as possible to
avoid losses because the high-frequency currents flow on the surface of
the conductor and may cause corona-discharges from sharp points.

3.2     secondary system

Secondary system consists of a secondary coil that is connected to
the heavy RF-ground from the lower terminal and to a capasitive
hat or a discharge electrode from the higher terminal.

3.2.1   secondary coil

The secondary coil or "Tesla coil" is a hollow form with several hundred
turns of wire. This coil has a natural RF resonate frequency based
primarily on the length of wire used in the winding. The tank circuit
frequency is made to match the secondary natural frequency by tuning,
changing the number of turns in the heavy primary coil or changing the
value of the pulse discharging capacitor. When the tank circuit
frequency is matched to the secondary frequency, and the coils are
placed in close proximity, energy is exchanged and transformed.

More information: coilbld.zip  (step-by-step instructions with pictures)

3.2.2   capasitive hat

On top of the secondary on normal and tertiary on magnifiers there is a
discharge terminal that adds (isotropic) capasitance to the the system.
The effects include lowered resonant frequency and alteration of
electric field. The most common shapes are sphere and toroid terminal of
which the latter is better. These terminals can be quite large with
high-powered systems and especially magnifiers.

  3.2.2     ground connection

"There is no such thing as a RF system ground that is too heavy." RQ

The ground connection of the secondary coil serves as a transmission
line for high RF-currents from the coil system. These currents would
contaminate the normal household ground and thus need to be conducted to
a specific, heavy RF-ground.

3.3      tertiary coil

On magnifier systems one uses a third coil that is base-fed with
RF-energy from the closely coupled primary-secondary system. Huge
discharge terminal and thus large top capasitance is often used in these

3.4      control circuitry

A typical tesla-coil system operates at a multi-kW power-level and
thus needs adequate control circuitry to handle the power both at
normal situations and if something goes wrong.

3.4.1    variac

An elementary component is a variac, variable transformer, that enables
one to change voltage from 0V to maximum, say 240VAC, or even more than
that on good models to compensate losses, 270VAC typically. A variac is
not an isolation xformer but just uses a tap on a coil so it must be
handled with appropriate caution.

3.4.2    current limiting

With non-shunted xformers one has to use external current limiting. One
can either add resistive elements from ovens etc. or use inductive
limiting. Inductive limiting is most conveniently handled by contacting
a welder in series with the primary of the step-up xformer. Instead of a
commercial welder one can use a variable inductor that is rated
accordingly - old xformer-laminates are available for cores.

3.4.3    contactors

It is logical to use heavy contactors instead of hand-operated switches
at least as a final step before the xformer as if somethink goes wrong
one can switch the system off by just cutting off the control-voltage of
the contactor from a remote place where the operations panel is instead
of having to go near the live circuit leading to the tank-circuit.

3.4.4    fuses/circuit breakers

As there is a lot of RF present there is little point in using any
ground-fault indicators etc. that can't handle the RFI but appropriate
fuses and circuit breakers should be included. It should be noted that
surplus fuses etc. are not necessarily meant for this application and
should be tested before relying on - for example fuses meant for
protecting wiring can trip at a current multiple to the rating on the

3.5      test-equipment

There are several equipment one can use for determining parameters of a
tesla-coil system; here are some examples: Frequency counter,
oscilloscope and signal generator can be used for finding out the
resonant frequency of coils and primary circuit etc. A multimeter or
LCR-bridge measuring resistance, inductance and capasitance will help as
well: checking connections, measuring coil inductance and capasitance of
self-made capacitors. More advance equipment for measuring RFI emission,
radiation, electric field etc. can be used as well.

More information: rqpickup.txt     (pickup-coils)


 4. sources for more information

Check ftp.funet.fi directory /pub/sci/electrical/tesla for files;
rqmsgs.txt has basic information, tesla.txt is a list of resources.
There are pictures in jpg and gif -formats as well. Old postings to the
mailing-list are available in subdirectory ../maillist; there are
several megabytes of text with answers to most coiling questions.

< www-sites to be added - list someone? >


 5. acknowledgements

This article is Copyright 1996 by Kristian Ukkonen (kukkonen-at-alpha.hut.fi).
It may be freely redistributed as a text-file so long as the author's
name, and this notice, are retained and the text remains unmodified.

Thanks for texts etc. :
  Richard Quick.
  Marcus Young

< EOF : tesla.faq >