The Amazing Nikola Tesla and His Coils

In the early twentieth century, Nikola Tesal invented AC currents while dreaming on the beach. He is also responsible for many other creations, the majority of them still 'unpublished,' including AC brushless motors, radio transmission (non-Marconi, that is), electric turbines, robotics, multiplexing transmitters, and underwater torpedoes. Tesla's intellect was on part with the of Sir Isaac Newton in this his experiments opened entirely new fields of scientific endeavor. Some people even go as far as to say that Tesla invented to Modern world.

But why don't you already know about Nikola Tesla? Is it because he believed in aliens from Venus? One conspiracy says that he has been intentionally erased from history because of his extensive knowledge of technology. Since his death, Tesla's notebooks have never been recovered. It is a very interesting to note that Tesla's ideas have made a recent comeback in popul thought in light of our 'newly discovered' national energy crisis.

In one opinion, the Serbian immigrant Nikola Tesla was the most brilliant and gifted electrical engineer to have ever lived. Ironically, the business needs of his inventions were constantly at odds with corporate and other selfish interests. Essentially, he created, and then lost, all stock interest in several of the largest electrical power corporations in the world, including both General Electric and Westinghouse. Tesla was never secure in the ways of the American businesses of his own time. One main fault of his was that he continuously allowed himself to learn about American business the hard way, losing time, money, and influence in the process. He e ven fell for the lies of his employee, the famous Thomas Edison, who was always a businessman foremost. Tesla invented entire classes of engineering; however, he failed to patent little things such as the concept of the AC current of the induction motor. He considered these ideas too simple and self-evident to warrent a patent. At one time, Tesla owned a contract that would earn him $2 per horsepower transmitted using AC technology. This could have made Tesla billions of dollars, even during his time. However, Tesla was forced to give over to corporate interest so that he could instead maintain funds for his laboratory.

In 1901, Nikola Tesla began construction on his now legendary Wardenclyffe Tower, a mammoth multi-stage Tesla Coil structure rising 187 feet into the air. Tesla advertised the colossal tower as a radio station, but he had secretly hoped to demonstrate that this facility could transmit power around the globe -- without wires. Unfortunately, Tesla's principal investor J.P. Morgan quickly grew cool the the idea of wireless power upon discovering that there was no way to meter the electrical power delivered. In 1904, J.P. Morgan cancelled funding for Wardenclyffe Tower, and the project collapsed. The project languished until WWI, when the tower was demolished to prevent its use as a landmark for German submarines.

During his lifetime, Nikola Tesla was friends with Mark Twain, a writer often believed to be a remarkable intelligent person. Mark would bring the ladies over to Tesla's laboratory for fun and amusement. Since Tesla rarely got out, Mark was a very good friend. Once, Tesla made a small vibrating wooden box that was big enough for a person to stand atop. He asked Twain to stand on the box. When Tesla turned on the box, Twain loved the physical sensation that the box produced as it resonated his insides. Twain felt that the experience was wonderfully beneficial to his health. Tesla warned his friend to turn the box off quickly, but Twain ignored all of the warnings and continued to enjoy the vibrating experience. That is, until he jumped off the box and fan as quickly as possible to the bathroom.

Here is an image of the famous one million volt cone. Notice the conical shape of the old style secondary coil as opposed to the cylinders most often used today. This is an interesting configuration and has come into particular interest regarding the generation of powerful electric arcs, as opposed to the generation of long electric arcs. Since Tesla's goal was to transmit power through the atmosphere, he was primarily interested in generating intense electromagnetic radiation. The larger windings at the bottom of the secondary are able to absorb the most current from the primary, while the tapered format linearly increases the voltage in a proportional amount according to the electromagnetic fall-off according to the destance from the primary. These conical coils often had no top-load capacitor and relied on the internal capacitance of the coil itself. This experiment clearly shows the distributed parameters in the components.

Tesla employed a glass blower to make interesting vacuum- and gas-filled tubes for experimentation called Geissler Tubes. Shown here is a small collection of some interesting Geissler Tubes.

The Tesla Coil

Components of the Coil

The tesla coil is simple. It's only five components are described below.

1. (Iron Core) Transformer: a conventional transformer boosts live voltage to around 12,000 volts. This energy then charges the next component.

2. High Voltage Capacitor: this capacitor is charged by the transformer. A capacitor consists of two conductive plates separated by layers of insulation. Electric charge builds in the capacitor with positive on one side and negative on the other. It eventually builds to a potential that can break down the air resistance of the next component.

3. Spark Gap: the capacitor's discharge of energy across the two electrodes of the spark gap is very sudden. The setting of the spark gap (i.e. the air distance between the electrodes) determines how rapidly these disharges will take place into the next component. The closer the electrodes are, the high frenquency with which the energy will discharge across the cap. This causes the energy to bounce back and forth between the capacitor and the primary coil (component #4): a rapid, elastic interplay of electric energy.

4. Primary Coil: the burst of energy coming through the spark gap from the capacitor is vented into the heavy wire of the primary coil. The pulsating energy of the primary circuit is induced into the next component. The combination of capacitor, spark gap, and primary coil are often together referred to as the 'tank' of the coil

5. Secondary Coil: normally has up to 1,000 turns of slender wire but may also have many more. Each secondary favors a particular rate of vibrations, like a guitar string, and the wire length helps to determine the frequency. Think of pushing a child on a playground swing: the swing in the secondary and you are the primary coil. If your timing is wrong when you push the swing, the momentum won't build and will instead go awry. However, if the timing of your pushing is just at the end of the cycle, these pushes will give great momentum to the swing itself: this is you achieving resonance with the swing. It is the same thing with the tuning (or timing, in our example) of a Tesla Coil. The trick is to coax the primary into a state where the timing will be such that each time the rush between the capacitor and coil takes place, the energy reinforces the oscillation of the secondary. In this resonant state, the oscillations can be made to swing to tremendous values.

Why the Tesla Coil Works

Ideally the primary is pulsing at that perfect moment in each and every cycle. In this ideal state of resonance the gain would be gigantic. But it is not necessary to pulse it at every cycle to get the gain any more than it would be necessary to push the swing at every cycle to get its amplitude to build. The primary can be oscillating at a fraction of the secondary's favored frequency, and, if the timing is right, you can still get the resonant gain. So you can have a relatively low frequency in the primary, and the secondary can be vibrating at its higher frequency. The Tesla Coil has been called a resonant transformer and it has also been called a high-frequency coil.

Resonance is the key to this phenomenal gain. Tesla said that there is "practically no limit to the power of an oscillator." This is the buried secret of Nikola Tesla. Through the magic of resonance, energy "almost without limit" is free for the asking.Forget all that circuit theory stuff. Instead, and realize that the Tesla coil works by magic. This circuit monster can only be understood by thinking lots about the speed of light and how magnetic flux transfers between coupled resonators. Tesla described his coil as a high-voltage, air-core, self-regenerative resonator transformer. This special transformer generates very high voltages at a high frequency. The high frequency aspect of a Tesla Coil is part of the magic. If you take the time to build a Tesla coil for yourself you can observe its amazing behavior. A Tesla coil is more than just the sum of its parts. The amazing beauty, when the coil runs, of brilliant fractal arcs crawling in space is absolutely breathtaking.

The tank creates high current waves that can be rudely tuned by adjusting the capacitor, the primary inductor, and the spark gap. When the frequency of these waves are at resonance they build up in the tank and create waves of magnetic flux. This flux cuts the turns of the secondary coil and induces a corresponding voltage. As it turns out, there are multiple resonant frequencies where the secondary coil will ring up, and the tank will induce a high voltage sinusoidal quarter wave onto the secondary coil. Strangely, these peaks do not occur at the harmonics of the fundamental frequency. The voltage rise from the bottom to the top of the secondary coil is shaped like the first quarter wave of a sine wave. The minimum voltage is at the bottom of the secondary coil. The maximum voltage is at the top of the secondary coil. The between the voltage is distributed like a sine wave. This observation shows that the secondary coil is acting in a distributed fashion and not behaving like a lumped inductor. The secondary coil then forms another LC resonant circuit that is composed of the the secondary (inductor) and the top load (capacitor). The two coupled circuits, primary and secondary, then pass energy in waves back and fourth, converting electricity to magnetism, and converting magnetism back into electricity. If the voltage in the top of the secondary exceeds the breakdown voltage in the air, very long streamers can be seen to ionize a channel and break away from the top load. The formation of a streamer changes the frequency of the Tesla Coil, which might extinguish it, but when a streamer strikes an object such as the ground, the energy is definitely removed from the top load capacitor. Then, the whole resonating process has to start over again.

My observations with a spark gap coil is that by placing two electrodes close enough together in any phase or handy frequency, such as 60HZ, they will create a push-pull current at about 120HZ. This happens when the electrodes reach their two maxima per revolution. Because the electrodes are close enough together at each maxima the air breaks down and ionizes a channel. This arc channel persists and conducts electricity. While the power is flowing through the spark gap the rest of the charging circuit is shorted out of the picture. While the power is flowing through the spark gap the capacitor is dumping its stored power into the primary inductor. While the spark gap is turned on, waves of energy exchange between the primary inductor and the primary capacitor. This energy exchange decays as it transfers into the secondary resonator or stops if the ionized path lets go. While the spark gap is on, energy from the primary coil wiggles into the coupled magnetic field in the secondary coil. When the primary tank and the resonator are tuned to the same frequency, energy transfer is most effective. Finally when the spark lets go, the energy exchange is stopped, and the capacitor builds up charge again. The tank capacitor should be chosen small enough so that it can charge up fully within the time for a single break, but large enough so as to knock on the primary inductor as hard as possible. Most capacitors are not strong enough for this task, and often times special capacitors must be used for this purpose.

Building Your Own Coil

Here is a simple reasonable wiring diagram for a classic Tesla Coil including a general parts specification. If you are close to building your own Tesla Coil for the first time, then this diagram should help with the overall idea, and how exactly to wire things up. Use the thickest wire you can find for the connections. An extra ohm makes a difference. Probably the most important aspect to success with a beginning Tesla Coil is the capacitor. You must use quality high voltage pulse rated capacitors.

How to Build a Tesla Coil Diagram
Types of Coils

Classic Tesla Coil: air core dual resonant transformer, this was the type of coil that Nikola Tesla himself created at the beginning of the 20th century, classic coil uses a tuned tank circuit componsed of an inductive primary and a matching capacitor,The capacitor in the tank circuit is charged with a high voltage AC transformer. Periodically, when the capacitor has reached a certain level of energy, a simple spark gap breaks down to ionize a short plasma channel. This plasma channel conducts the power stored in the capacitor through the primary inductor in concentrated pulses. This happens over and over again. One way to think about this behavior is to imagine the effects of repeatedly hitting a bell with a hammer. The bell will ring louder and louder. As long as this repeating charge and discharge process continues the electrical bell will continue to ring. An electrically independent secondary inductor is placed close enough to the primary inductor so that the two inductors are slightly coupled via magnetic field lines. The secondary inductor is usually connected to a small capacitive element on the top such as a sphere, metal doughnut, or top load. The top load capacitor is somewhat oddly defined. One plate of the capacitor is the outside surface of the element, while the other plate of the capacitor is the ambient environment, spreading out to the surface of the Earth. When the circulations of magnetic and electric fields are in tune, a resonant harmonic is established in the secondary coil system. The resonant harmonic condition in the secondary will pick up a natural frequency and will continue to build up energy until leaders and streamers of electric lightning ionize channels in the air. For all modern advancements in technology, the classic Tesla Coil is still the most compelling and powerful design.

Solid State Tesla Coil (SSTC): in the SSTC, semiconductor transistors and gates (such as MOSFETs or IGBTs) are used to replace the classic style spark gap,Expensive high power switching technologies exist that are capable of directly replacing the spark gap at classic voltages and currents. However these parts are very expensive. Therefore, the development of solid state Tesla Coils at affordable cost has been the primary development mode. A solid state Tesla Coil is essentially the same design as the classic Tesla Coil; however, the spark gap has been replaced with a lower voltage transistor switching system. Since semiconductors are only able to switch at one or possibly two kilovolts, the primary coil is usually reduced in the number of turns with an increased secondary coupling. This design parameter change requires a corresponding increase and sensitivity to tank capacitance. This design parameter change is also responsible for the characteristic pointy jet looking streamers. The first solid state coil designs were open loop designs. In an open loop design, a frequency is injected into the primary tank, in this case, in the form of a square wave. Sometimes there is no tank capacitor at all, because the self capacitance of the primary coil is enough. The frequency is adjusted in hopes of finding one of the various resonant harmonics. Methods for delivering more power into the primary inductor have evolved. One method is to apply an H-bridge configuration to the primary coil in order to reverse the coil stimulus polarity and add power to the waveform on the way up, as well as on the way down. Because semiconductors heat up quickly, solid state Tesla Coils are often run in burst modes. The time between bursts is used to cool off the transistors, and to recharge the energy capacitors for the next burst. This explains the characteristic "popping" sound these types of coils often make. Solid state Tesla Coils almost always have a break out point from which the electric arcs emanate. This is because the corresponding energies are usually lower than in a classic design.

Dual Resonance Solid State Tesla Coil (DRSSTC): dramatic advancement from the solid state TC, What makes this style of Tesla Coil unique is the idea that energy can be fed into the primary tank at precise intervals forcing a resonance to occur. In classic Tesla Coil operation, during the break of the spark gap, a great impulse of energy is transferred into the primary tank. This massive impulse of energy causes an damped, or ring down, type of oscillation to occur. In a dual resonant Tesla Coil, smaller pulses of energy are applied to the primary tank circuit at just the right intervals. This causes the primary tank to respond in an unstable, under damped, or ring up, type of oscillation. This design is a true Tesla Coil and therefore possesses the magic coupling of two resonant air core transformers. The term dual resonant stems from the fact that there are two resonant methodologies involved with this technology. The first resonant component is the solid state closed loop feedback circuit that is driving the primary tank. The second resonant part is the Tesla Coil, or the resonant air core transformer. This type of Tesla Coil is an amazing breakthrough for many reasons. One of them being overall weight, because there is no high voltage transformer or heavy voltage controller stacks or reactors or power controllers to carry around.

Vacuum Tube Tesla Coil: very similar in many aspects to the SSTC, but the design is more beautiful,the primary circuit is stimulated using an oscillating vacuum tube, or sets of vacuum tubes in various one sided, two sided, or H-bridge configurations. Radio vacuum tubes such as the 833C and other vacuum tubes have been used with success. An analog oscillator, such as the Armstrong oscillator, is often selected to excite the primary tank. A vacuum tube such as the 833C can switch at about 4 kilovolts. One must stay below about 8 kilovolts when working with vacuum tubes to avoid accidental x-ray emissions. The oscillator can be constructed using a simple current loop to control the vacuum tube grid. It is quite interesting to note that vacuum tubes function in a common sense way. An observer can see the electrons flowing inside the tube as a faint blue corona. In a vacuum tube the grid interposes itself between the anode and the cathode and when the grid is charged it interrupts the flow of electrons. It is thought that vacuum tubes actually operate by altering the real flow of electrons. This is in contrast to solid state electronic gates and transistors. Solid state electronics are theorized to function by altering the flow or motility of holes. Holes are theorized to be anti-electrons, or possibly electrons moving backwards in time. It is inspirational to watch the flow of real electrons inside an vacuum tube. It is not very inspirational to watch a black chunk of silicon sitting there. The primary oscillator is sometimes enhanced to produce different effects with the addition of a staccato circuit. The staccato circuit is typically a solid state modulator for driving the vacuum tube filament. Such confluences of solid state and vacuum tube components are also inspirational. The filament of the vacuum tube is often called the heater, and originates the sea of potential electrons. By modulating the heater with various pulse width modulation techniques a variety of visual and audio effects can be produced. The elegance of the vacuum tube Tesla Coil, it's low parts count, and ability to "see" the flying electrons make this type of Tesla Coil truly a form of electronic art.

Oudin Coil: developed in 1899, by French physician Paul Marie Oudin, this device was initially employed in electrotherapy. Potential relationships between biological energy and electric energy are often thought to exist. In general, this type of Tesla Coil is focused on creating and modulating high frequency discharges in order for a person to be exposed to the electric radiation. Particular frequencies were thought to stimulate or inhibit various biological functions. Experimental charts of frequencies and effects were compiled and used for everything from preventing disease to enhancing sex. This type of coil is not technically a Tesla Coil, but only a kind of amplified induction coil.

Spark Gap Tesla Coil (SISG): the SISG evolution was invented by Terry Fritz in 2006 and used in his interesting and amazing piranha coils. The SISG spark gap is an inventive semiconductor replacement for a spark gap, and functions with DC currents instead of AC currents. This ingenious little modular circuit consists of replicated modular sections in series. Typically, each section has a 900 volt breakdown. When the voltage across the SISG is below a certain threshold, no conduction occurs. However, when the voltage across the SISG gets above a critical value the SISG conducts for a short interval. The timing of this conduction period is controlled by a tiny 1/4 watt resistor running along the bottom of the board. Amazingly, chains of SISG boards have been put together with total conduction voltages up to 15kv. Chains of SISG boards have been successfully run on sizeable pole pig arrangements. The SISG modules have proven to be quite resilient, and long chains of SISG modules have proven to work and fire together in concert. The failure mode of a single SISG segment tends to be into conduction, and therefore chains of SISG segments continue to work even if a few of the segments have burned up. One drawback to this type of coil is they can be difficult to tune. In the authors opinion, this is due, at least in part, because the break rate difficult to modify. For an explanation consider the break rate is directly proportional to the drive current charging the tank capacitor. Since the DC tank voltage is typically established using a filtered high voltage AC supply, the stiffness of the power supply becomes a considerable issue, and tends to constrain the break rates to modes of 60hz. Another difficulty with this type of coil is controlling the conduction interval. Since each SISG module has its own controlling resistor, it is difficult to accurately adjust long string of modules.

Bipolar Tesla Coil: this physical Tesla Coil configuration demonstrates the symmetry of the electromagnetic field oscillations. Every type of driver can be applied to monopole as well as bipolar Tesla Coils. The pancake coil in the center creates a symmetric magnetic field. This field is then coupled to two resonators at the same time, as shown in this photo. An interesting design is that a bipolar coil usually does not have a ground connection. Arcs are formed between the left hand, and right hand side top loads, according to the difference in potentials that are developed there. This is in contrast with the standard upright monopole configuration where potential is developed in the top load relative to the ground connection tied to the bottom of the secondary inductor. For this reason bipolar coils are a good choice for coilers in apartments or other dwellings where a decent connection to ground is difficult to obtain.