The Tesla coil is well-known for producing extremely high voltages. In this section, we’ll explain how the Tesla coil can reach voltages over a quarter million volts using coupled resonant circuits. We’ll build up from the fundamentals, to give you a thorough explanation of what’s going on.
Current, Magnetic Fields, and Induction
Let’s start with the basics of electromagnetism. One of Maxwell’s equations, Ampere’s law, tells us that current flowing through a wire creates a magnetic field around it.
If we want to use this magentic field to our adavantage, as we do in an electromagnet, we coil the wire.The magnetic fields from the individual turns add together in the center.
A constant current makes a static magnetic field. What happens with we put a changing current through the wire? Another of Maxwell’s equations, Faraday’s law of induction, tells us that a magnetic field changing in time induces a voltage across the wire proportional to the rate of change of the magnetic field:
If the current is abruptly shut off, Faraday’s law tellls us that there will be a sharp spike of voltage. If an oscillating current flows through the coil, it induces an oscillating magnetic field inside it. This, in turn, induces a voltage across the coil which tends to oppose the driving current. Intuitively, the magnetic field is “stubborn,” inducing a voltage that opposes any change to the field.
A transformer takes advantage of the law of induction to step AC voltages up or down. It consists of two coils of wire around a core. The core is soft iron or ferrite, materials which are easily magnetized and demagnetized.
An oscillating current in the primary winding establishes an oscillating magnetic field in the core. The core concentrates the field, ensuring that most of it passes through the secondary. As the magnetic field oscillates, it induces an oscillating current in the secondary coil. The voltage across each turn of wire is the same, so the total voltage across the coils is proportional to the number of turns:
Because energy is conserved, the current on the side of the transformer with the higher voltage is smaller by the same proportion.
The Tesla coil is a very souped-up transformer. Let’s briefly consider what would happen if it were a perfect transformer. The primary winding has six turns and the secondary has about 1800 turns. The primary is driven with 340 volts, so the secondary will have 340V x 300 = 102kV across it. That’s a lot! But not quite a quarter million. Additionally, becuase the Tesla coil is air-cored and the coils are positioned relatively far apart, only a small fraction of the magnetic field produced by the primary is actually interlinked with the secondary.
To understand how a Tesla coil works you need to understand a couple of points about its components.
The Tesla coil's primary and secondary coils are both inductors in electrical terms. When the current flowing through an inductor changes, it will create an opposing or reverse voltage.
2) Spark Gaps:
A sparking plug in a car is a simple spark gap, its break down voltage depending on the electrode gap. Once a spark gap conducts it has the ability to carry on so long as a reasonable current is flowing (hot ionized air in the gap).
A good analogy for a capacitor is to regard it as a sponge that is placed on some spilt water and left to slowly soak it up. If you leave it a minute then give it a very quick, hard squeeze, a large amount of water is released all at once, this is one minutes worth of slow soaking-up released in a mere fraction of a second.
In a Tesla coil the so called soaking-up stage lasts for a few milliseconds, but the squeezing-out can be a thousand times quicker, lasting for a few micro or millionths of a second.
The property of resonance is fundamental to the operation of all Tesla coils.
A good analogy is the garden swing. If it is left to swing on its own it does so at its resonant frequency, only slowing down due to friction and gravity.
If you stand behind the swing and push it just as it swings away from you each time, it will get higher with each subsequent push. This is because you are adding power at, and only at, the correct time-point in the swings cycle.
You are therefore adding momentum at the same time interval as the swings resonant frequency, this means the push you gave it, is in resonance with the swing.
Resonance does not increase the overall amount of energy, it only facilitates its transfer.
So if you're looking for tesla related, so-called 'free energy, or 'zero point' energy information, which some people seem to associate with tesla coils, this is not the site for you!
5) Resonant Circuit
If a capacitor is placed across an inductor you will have a resonant circuit. As the capacitor discharges, it sends current into the inductor that will then store this as energy in its magnetic field. But as the capacitor discharges the current feed into the inductor diminishes. This then causes its magnetic field to collapse and generate an opposing voltage that goes back into charging up the capacitor, and the cycle starts all over again. The number of times that this 'back and forwards' cycle happens per second, is its resonant frequency, expressed in Hertz (Hz). Because of resistive losses the current is reduced every cycle down to zero. There is no such thing as free energy!
An LC circuit can have an inductor and capacitor in series or parallel. Here, we are using series LC circuits like this:
Consider what happens when you don’t drive the circuit (assume that the AC source in the above figure is replaced by a wire), but start out with the capacitor charged. The capacitor wants to discharge, so charge flows around the circuit, through the inductor, to the other plate. In the process, a magnetic field builds up inside the inductor. When the charge on each plate of the capacitor is zero, current stops flowing. But at this point, the inductor has energy stored up in a magnetic field - which tends to oppose change. The magnetic field collapses, inducing a continuing current in the same direction, thereby recharging the capacitor and restarting the cycle in the opposite direction.
The resonant frequency of an LC circuit, or the frequency at which the energy cycles between the capacitor and inductor as described above, is:
Driving the circuit at its resonant frequency adds energy during each cycle. By providing a succession of well-timed pushes, we can build up to extremely high voltages! In the Tesla coil, a spark breaks out and discharges the circuit once the voltage is high enough.
Using different values of capacitance and inductance will give a different frequency.
SEQUENCE OF OPERATION
In the circuit of Fig 1 above, the capacitor ('C') is charged up by a high voltage source, like my example of the sponge soaking up water.
Once the capacitor attains a high enough voltage the spark gap fires and conducts (Fig 2 below). The spark gap is now a short-circuit that completes the resonant circuit (shown in red) of the primary inductor and capacitor.
The spark gap firing is virtually an instantaneous discharge of the capacitor energy into the inductor and is the same as the example of the sponge being instantaneously squeezed out.
The inductor stores this energy in its magnetic field with the lines of force cutting into the secondary coil and inducing a voltage into it. Once the capacitor is empty the current flow into the inductor stops, and its magnetic field collapses causing a reverse current to flow back into the capacitor again.
This back and forth diminishing cycle (called the 'Primary Ringdown') of capacitor to inductor and back, continues untill there is insufficient current flowing to keep the spark gap conducting. The point to remember is that every time the cycle occurs, more energy is transferred to the secondary, so the inductors magnetic field stores less energy on each cycle.
Unfortunately every time the spark gap conducts, losses occur in the form of heat and light, so you want the minimum number of cycles that are consistent with getting all of the available energy transferred to the secondary.
Usually after two, three, or possibly four cycles the majority of the energy has been transferred and the primary current has dropped enough to allow the spark gap to stop conducting (called quenching).
Once the spark gap has quenched it allows the capacitor to get a fresh charge and the whole affair can start again.
The amount of energy available to be sent to the primary (measured in Joules) is equal to the 0.5 x C x V^2
C = Farads
V = voltage that the gap fires at.
You can see here that doubling the value of C (provided your power source is robust enough) will give you twice the power. But doubling the voltage that the capacitor is charged up to will give 4 times the power, because the voltage value is squared, that's why if you want spark length its best to go for a higher voltage power source.
While the primary circuit is resonating and transferring its energy, the following is also occurring in the secondary circuit at the same time .........
The toroid on top of the coil acts like a capacitor with respect to the surrounding ground.
in reality the toroid discharges through the air to earth. If you now replace the toroid with the symbol for a capacitor (Fig 3) and re-arrange things, you end up with Fig 4.
This means that the secondary coil is also a resonant circuit and it behaves much like the primary circuit. The secondary's energy is therefore also resonating back and forth between the coil and the toroid. However it does not dampen down the same as the primary does, in fact it is steadily increasing.
This is because at just the right time-point in its cycle (like you pushing that swing in the example) another magnetic field from the primary circuit, which remember is also resonating at the same frequency, transfers a bit more of its stored energy into the secondary circuit.
Therefore as the primary ringdown is occurring causing the primary to loose its energy, the secondary is gaining power in what is called the Secondary Ring-up.
Remember the primary and the secondary need to have the same resonant frequencies for them to interact successfully. Typically this is in the hundreds of Kilo-Hertz.
Eventually the voltage on the surface of the toroid at the top, rises so high that the curved surface cannot retain the charge anymore, and breakout occurs. This will either be a misty purple corona discharge or, if all components are suitably balanced to one another, a whitish solid streamer down to earth or into the air.