THEORY
OF OPERATION
Tesla Coils are simple devices. You don't need a Ph.D. in Electrical
Engineering to understand how they work.
The basic setup incorporates to major parts: the
primary coil and the secondary coil. The secondary coil has MANY more windings
than the primary coil and is placed inside of the of the windings of the
primary coil. This concentric configuration forms a transformer. A transformer
of this type will TRANSFORM a small voltage across the primary coil into
a voltage on the secondary coil. The voltage on the secondary coil is directly
proportional to the ratio of windings on the secondary to the windings
on the primary. Wow that was a mouthful. In short , the voltage on the
secondary coil will be much larger than that on the primary coil. This
is because the primary coil usually has only 2-10 windings while the secondary
has as many as 1000. It should also be noted that no electrical connection
exists between the primary and secondary coils, they are MAGNETICALLY COUPLED.
Magnetically coupled means that energy is transferred between the primary
and secondary coils via a common magnetic field.
On the primary coil side the circuit consists only of the primary coil
itself, a capacitor, and a high voltage source. By properly connecting
the capacitor and primary coil, a resonant circuit is obtained. The term
resonant circuit implies that the circuit will generate a voltage that
will oscillate with time. Furthermore the term resonant implies there is
some frequency at which the amplitude of the oscillating voltage will be
maximum, the resonant frequency. The idea of resonance is not constrained
to electricity. You can think of the primary circuit behavior to be very
much like pushing a child on a swing. When you put some energy into the
child (by pushing) the child and swing will oscillate. If you continue
to add energy to the child/swing at the right times you can make the child
can swing higher; you would then be adding energy to the child/swing system
at its resonant frequency. The electrical analogue of this child/swing
behavior is employed in the primary circuit. By charging capacitor (like
pushing the child) at the resonant frequency of the coil, a large output
voltage appears across the primary coil. As a result an even larger voltage
is present on the secondary coil owing to the aforementioned transformer
action between the two.
The secondary side consists of the same to circuit elements as the primary
side. The secondary coil and a capacitor. There is one distinct difference
in how they are connected however. The capacitor and secondary coil are
connected together at top of the secondary. The bottom of the secondary
is connected to 'ground'; a steak is literally driven into the earth. The
capacitor used is unique. All most all capacitors have two 'terminals',
the capacitor in the secondary circuit uses only one terminal. Now by selecting
the proper values for the capacitor and the secondary coil, the resonant
frequency of the secondary circuit can be made identical to that of the
primary circuit. This is done so the maximum amount of voltage can be developed
on the top of the secondary circuit.
Now when the primary circuit is operated, it causes a very large voltage
to appear on the capacitor of the secondary circuit. This voltage is high
enough to make the surrounding air conduct an electrical current and you
get the desired lightning bolts.
Pictured
at the right is a Tesla Coil in action. The long cylinder in the middle
is the secondary coil. Atop the secondary coil is a doughnut-shaped piece
of metal that acts as the single terminal capacitor. As you can see this
is where the voltage is developed and the arcs eminate from. If you follow
the large bolt in the middle of the picture you'll see that it is striking
the top winding of the primary coil. If you just to the left of the primary
coil you can see the primary capacitors (they look like two gray boxes.
You might be asking yourself how long the arc from a coil can be. The
answer is that it depends on a number of factors. First the ratio of the
primary and seconday coil windings has a strong influence on the voltage
(and therefore arcs) appearing at the top of the secondary coil. Second,
the more energy you have available to put into the primary coil, the longer
you can make the arcs. Finally it depends on how well the resonant frequencies
of the primary and seconday circuits are matched. The better the matching,
the longer the arc. The latter of the two factors have a direct analogue
with the child/swing system discussed earlier. Assuming you push at the
right time, the stronger you are the higher the child can swing. In short,
you need to push the child at the right time or it will not matter how
strong you are.
Finally I thought you might be interested in some typical arc lengths
from coils. The average modern 'garage' coil typically ranges from 1 foot
to 10 foot long arcs. Some of the exhibition coils are capable of arcs
upto 30'. The longest arc length that I have ever heard is 135 feet. The
135 foot bolts came from Nikola Tesla's Colorado Springs coil. To put the
power involved in generating such an arc in perspective, at one point Tesla
drew so much power from the power power plant that it melted the generator.
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