The Basic Components of a Tesla Coil
The Tesla coil is in simplest terms an air core transformer which operates in a
resonant mode.
The coil contains only five main components. Also necessary is some electronic
filtering to protect the coil parts and reduce the effects of radio frequency
Interference (RFI). Tesla coils ideally should also be on their own circuit.
The power transformer
This component is often the hardest to obtain. It usually takes the form of a
neon sign transformer or oil burner ignition transformer. Neon sign type
transformers come in a variety of output voltages and current rating. The higher
the voltage and the more current the greater the secondary output will be. 9000
and 12000 Volt neon transformers are the most useful for Tesla coil use.
The Safety Gap
The safety gap is placed across the HV leads of our supply transformer. Its
purpose is to break down and conduct when voltages at or above its breakdown
voltage appear. This limits the voltage to a safe value protecting the secondary
of the neon transformer from damage due to overvoltage or RF kickbacks.
RF Chokes
RF chokes offer a high impedance to the high frequency RF energy generated by
the coil. They are placed in series with each HV line to prevent RF energy from
backing up into the transformer secondary windings. They provide another level
of protection for our neon transformers.
The Spark Gap
There are many gaps useful in Tesla coils. These vary from simple static gaps to
rotary gaps. No matter what type of gap is used the arcs need to be quenched
rapidly. Quenched means that the arc shouldn't continuously jump the gap, but
instead must be extinguished after a short time. Rotary gaps are often used
because they can be varied in speed and the break rate easily controlled.
However, a rotary gap is not well suited for use with neon sign transformers.
Neon sign transformers work better with a multiple static gap design with
several electrodes in series.
The Capacitor
This may be the most difficult of all the components. The electrical stress
placed on a Tesla coil capacitor is tremendous. The capacitor needs to be
voltage rated at least twice the RMS voltage of the high voltage transformer.
For example, if you are using a 12,000 volt neon transformer the capacitor may
see voltages of nearly 17,000 volts or more .Capacitors with this kind of rating
are expensive. Most coil builders will make their own capacitors from
polyethylene plastic and aluminum foil or flashing.
The Primary Coil
The primary of a Tesla Coil doesn’t operate like that of a normal transformer
where the degree of coupling is high to insure the greatest transfer of energy
to the secondary. If the coupling in a Tesla Coil were that high the primary and
secondary coils would saturate every time the spark gap fired and there would be
no free oscillation which could rise to a high voltage. Tesla coil performance
is limited by this inductive coupling between the primary and secondary. This is
why a Tesla coil is loosely coupled, allowing the secondary a measure of freedom
to ring up to a high voltage. To insure that the coupling can be loose enough
make the primary twice the diameter of the secondary. Or use a flat spiral or
inverted conical type primary that couples less. You should have 12 or more
primary turns.
The Secondary Coil
This is the part of the coil that people put the most emphasis in. Its
construction is not at all mysterious. It is a tightly wound coil, usually of
magnet wire. The turns should be tight next to each other. The form for the coil
is usually PVC plastic pipe. The plastic should be very dry and very smooth. PVC
is dried by baking at a moderate temperature (130° F.) for 30 minutes to drive
off moisture. It is then sealed with polyurethane varnish inside and out. The
wire is then wound on tightly and without any kinks. Several more coats of
varnish are applied over the winding to secure it and help improve the
insulation. The bottom of the secondary must have an excellent RF ground
connection to it.
The Discharge Electrode
This is usually a sphere or toroid on the top of the Tesla coil secondary. This
adds capacitance to the secondary and lowers the resonant frequency. The larger
the discharger the more the frequency of the secondary will drop from its
natural resonant frequency. The largest terminal that will still allow sparks to
break out from it will give the longest sparks. The toroid shape is preferred
because of the shielding effect that is given to the top turns of the secondary
coil. This shielding very often completely eliminates the corona discharge
around these turns.
The RF Ground
The ground establishes a reference for the secondary to work against. The RF
current is very high at the base of the secondary so this current must be
carried away via a low inductance low impedance connection to ground. Note that
the bottom of the secondary is NOT connected to the primary coil. Many old
designs show a connection between the secondary and primary but this is an
unsafe design as it places possibly lethal 60hz energy from the neon transformer
on the secondary coil.
These are the main components of most coils and construction is pretty straight
forward. Avoid any sharp points that may promote arcing or corona. Use the
shortest wire connections possible. Some space is necessary for the high
voltages involved. Of utmost importance is the tuning of the system for maximum
output. Reaching the point of best resonance will give the greatest streamer
length.