||Warning: This poor translation of the original Czech text may cause psychic
damage if you will continue reading! (especially for native English speaking people)
It's an air-core transformer working on its own resonant frequency used to
generate very high AC voltage. Primary coil consists of few turns of thick wire, secondary coil is wounded
by hundreds or thousands turns of thin wire on cylindrical skeleton in one layer. So it is two simple inductive
coupled resonant circuits.
Secondary coil with its own inductance and capacitance (between turns) plus capacitance of discharge
electrode determine own resonant frequency f0 = 1/[2*π*Sqrt(L*C)]. To maximize the power distribution from primary
circuit to secondary circuit is needed to tune primary circuit to the same frequency. Because between-turns
capacitance of primary coil is negligible is needed to connect an external capacitor.
More theory here. To do needed calculations easier I wrote a simple
electro & TC calculator in PHP.
Now we "only" need some power high-frequency generator.
Principle scheme is here:
The capacitor starts charging from high-voltage transformer connected to AC mains (it must be charged within one
half of AC mains period). When the voltage across the sparkgap exceed dielectric strength of air the sparkgap will
fire. Conductive sparks connect the capacitor to primary coil and the electrostatic energy will be transformed
to magnetical energy of primary coil. In secondary coil the damped oscillations is induced (as you can see on oscilogram).
If induced voltage is high enough there will be sparks jumping out to air. This high-frequency high-voltage
(and surrounding elmag. field) has interesting attributes. It can glow various gas-filled tubes, bulbs etc.
lying around Tesla Coil. If you touch it you don't get a classic shock (due to skineffect) but you will/may be
burned. Remember that more dangerous are voltages on primary side, HV supply
and capacitors. This may be lethal. For more informations about SGTC basics you should read this document:
James Kelly - Tesla Coil Theory.
Author doesn't accept any responsibility of any damages caused by electrical devices described here!
I have been fascinated by high voltage and discharges for a long time (no less by splendour of
show). I built a lot of various high voltage devices, (e.g. using burn coil from car, high voltage transformer from TV...).
But few months ago (in 2001) when I discovered a Tesla Coil on the Internet I found the right device for generating high
voltage. Tesla Coil is relatively unknown device in our country. I didn't find any Czech Tesla Coil website. I hope that
this site set it right little bit. But in Germany, U.K. or USA the Tesla Coil is a popular device which is built by lot
of amateurs. Since 1998 the TESLATHON gathering is done in the U.K.
In 2008 there was held the 1st Teslathon in Czech Republic.
When I acquired enough information from the Internet I started to build own TC.
I used PVC pipe (d = 40 mm, h = 290 mm) as skeleton of secondary. I wound about
900 tuns by hand on it. I used CuL d = 0,28 mm wire from an old relay. I varnished the winding
with resin in spirit to improve insulation. Then I wound primary coil using 3,5mm Cu wire 2 m long by hand on
a pot. First I wound about 8 turns on 70mm diameter and then I found that sparks jumps between primary and secondary
coil. Then I rewound primary coil on 120mm diameter. But only 4,5 turns. Whole system is mounted on plexiglass
base. Discharge electrode on top is made of tennis ball wrapped with alufoil. I measured the resonant frequency
about 1,26 MHz. For calculating primary coil inductance you may use my program
or online TC calculator that I programmed later.
my 1st TC
The building of TC isn't so difficult (only that 900 turns I was winding for 2 hours).
But problem with proper driving TC followed. First I tried classic static sparkgap. I used 200 W / 10 kV
transformer as high voltage power supply. I have problem with capacitor - I need 7 nF / 10 kVAC with
low tg(delta) and high dV / dt. I found only two impulse 1 nF / 15 kVAC capacitors which
I connected parallel together. Other solutions is large Leyden-bottles or serial/parallel matrix combination
of small usual capacitors called MMC - Multi Mini Capacitor. Sparkgap can be built in many ways. First I used
two steel balls from bearing but it got hot quickly and soldered down. Then I used divided sparkgap with 6
copper pipes. This sparkgap can be cooled better. I reach about 5cm sparks with it. I know it's not enough
but I have low capacity and only 200 W power transformer. The cool device is a rotary spark gap.
There are pairs of electrodes ones static and other rotating (motor driven). When pairs meets the spark
will jump. Because electrodes are moving the arc pulled after discharge is broken and air flows around and
ions are spreaded away. And you can simple change the spin-speed and change discharges per minute (BPM).
Sparkgap driver have an advantage that it is simple and can handle huge power (that any semiconductor can't do).
But it have some disadvantages:
But there are other choices. The function of sparkgap can be also
done by a silicon controlled switch or a vacuum tube. TC drivers can be divided into following
- it is very noisy and produce a lot of ozone
- it needs big and quality capacitor
- sparkgap electrodes are corroding
- it needs high voltage on primary side
- sparks cannot be touched (due to sharp RF envelope)
- power cannot be smoothly regulated in wide range
- it produces wide spectrum of EMI
I'm interested in SSTC and VTTC constructions. Here are links to my projects:
- SGTC - classic driver with a sparkgap described above.
- SSTC - uses semiconductor parts (BJT, MOSFET, IGBT) where the silicon switch changes its state
in every half of period of RF current. It can be powered directly from rectified mains (don't need
any HV transfromer) or battery or any DC power supply. There are various circuit topologies: flyback,
halfbridge, fullbridge, class-E oscillator, etc. in wide range of operating power ranging from a few
watts to several kWs. Sometimes it's used a TC with tuned primary coil (forming serial resonant circuit)
- DRSSTC (Double Resonant SSTC). To gain the effectivity the driving RF current can be pulse-modulated
with some kind of electronic interrupter circuit ISSTC (Interrupted SSTC). Then pulse power can be
increased many times (cause longer sparks) while average power remains the same (less heating and
stressing of semiconductors). Usually both ideas are joined into DRISSTC. Electronic drivers
can achieve not only a pulse modulation but even a continuous modulation. When modulated by audiosignal
it can reach interesting effect of audiable sparks. This effect is seriously used in plasma tweeters.
- OLTC - uses also a silicon switch (mostly a huge IGBT module) or thyratron but differently from
SSTC - the switch is turned on whole time during damped TC oscillations like a normal sparkgap.
It's powered usualy by rectified or doubled mains.
- VTTC - an oscillator with power vacuum tube (triode, tetrode, pentode) which has connected
tuned TC primary (parallel resonant circuit) in its plate circuit. Bigger tubes are usualy powered by
microvawe oven transformer. Interrupter circuit can be also used - IVTTC. Audiomodulation is possible
but requires much more powerfull modulator source.
Meetings of TC builders in Czech Republic:
1st Teslathon 19.-20.7.2008
2nd Teslathon 11.-12.10.2008
3rd Teslathon 3.-5.4.2009
153th Tesla's birthday on Google 10.7.2009
4th Teslathon 12.-13.9.2009
5th Teslathon 6.-8.8.2010
6th Teslathon 20.-21.8.2011
7th Teslathon 18.-19.8.2012, report by Ivo
8th Teslathon 27.-29.9.2013
9th Teslathon 5.-7.9.2014, report by LTX
10th Teslathon 11.-13.9.2015
11th Teslathon 16.-18.9.2016
Another nice photos you can find on other foreign websites associated as Tesla Coil webring.
Here are also a few new Czech sites: