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By Ryan Ries Several years back, during my senior year of highschool, I, with the help of one of my best friends, built what is known as a Tesla Coil. It was a large undertaking, and it was one of those projects that's never really finished. It can always be improved. We built the Tesla Coil for no other reason than that we thought it would be cool, but it had the unintended consequence of proving very handy for school-related science projects, even after we had moved on to college and had abandoned the physical project. I once had much more information on Tesla Coils and my coil in particular here on this website, but I somehow lost all the web documents and pictures. I did happen upon a CD that still had a document that I had written for a highschool computer animation / multimedia presentation class. So it doesn't go into too much detail, but I'm going to put it up here in lieu of all the old photographs I used to have. The Veretus Coil is the name of our Tesla Coil, and is an application and exploration of the experimental work of Nikola Tesla. It is intended to teach some of the principles and theories of high voltage, electromagnetic forces, and radio wave propagation, among many other things. A Tesla Coil is, in simplest terms, an air-core resonant transformer. The most outstanding quality of the Tesla Coil transformer is that it operates in a resonant mode, where voltage transformation does not rely completely on turns ratio, like standard AC transformers. The high voltages produced in this project are extremely dangerous, and great care must be taken when working on it. Nikola Tesla dreamed of worldwide free energy for the masses. He was no doubt brilliant, but nevertheless, the man is often erased from history books. Countless electrical inventions are accredited to Tesla. Tesla's system of alternating current proved to be superior to his former employer Thomas Edison's method of direct current. Perhaps it was the fact that this Serbian-American immigrant beat our own Thomas Edison that caused some nationalistic Americans to hide the work of Tesla. Perhaps it was also Tesla's outrageous claims that he could trigger apocalyptic earthquakes at any point and time, or that he had created a death-beam capable of destroying hundreds of airplanes in mid-air that drove Tesla's ideas out of the history books. -- Nikola Tesla While working on the Veretus Coil, we hope to gain a great deal of knowledge about the concepts surrounding high voltages, its relation to electromagnetism, and radio waves. We also hope that in becoming better at the science, we can increase the efficiency of the coil, creating better energy discharges from the same amount of input power. Calculation and experimentation is the best route to attaining that knowledge. A basic Tesla Coil diagram:  The spark gap is initially an open circuit. Current from the power supply charges the primary tank capacitor to a high voltage. The voltage across the capacitor increases steadily with time as more charge is being stored across its dielectric.  Eventually the capacitor voltage becomes so high that the air in the spark gap is unable to hold off the high electric field and breakdown occurs. The resistance of the air in the spark gap drops dramatically and the spark gap becomes a good conductor. The tank capacitor is now connected across the primary winding through the spark gap. This forms a parallel resonant circuit and the capacitor discharges its energy into the primary winding in the form of a damped high frequency oscillation. The natural resonant frequency of this circuit is determined by the values of the primary capacitor and primary winding, and is usually in the low hundreds of kilohertz.  During the damped primary oscillation, energy passes back and forth between the primary capacitor and the primary inductor. Energy is stored alternately as voltage across the capacitor or current through the inductor. Some of the energy from the capacitor also produces considerable heat, light and sound in the spark gap. Energy dissipated in the spark gap is energy that is lost from the primary tank circuit, and it is this energy loss which causes the primary oscillation to decay relatively quickly with time. Energy is gradually transferred from the primary resonant circuit to the secondary resonant circuit. Over several cycles the amplitude of the primary oscillation decreases and the amplitude of the secondary oscillation increases. The decay of the primary oscillation is called "Primary Ringdown" and the start of the secondary oscillation is called "Secondary Ringup". When the secondary voltage becomes high enough, the toroid is unable to prevent breakout, and sparks are formed as the surrounding air breaks down. Eventually all of the energy is transferred to the secondary system and none is left in the primary circuit. This point is known as the "First Primary Notch" because the amplitude of the primary oscillation has fallen to zero. It is the first notch because the energy transfer process usually does not stop here. In an ideal system, the spark gap would cease to conduct at this point, when all of the energy is trapped in the secondary circuit. Unfortunately, this rarely happens in practice. If the spark gap continues to conduct after the first primary notch, energy begins to be transferred from the secondary circuit back into the primary circuit. The secondary oscillation decays to zero and the primary amplitude increases again. When all of the energy has been transferred back to the primary circuit, the secondary amplitude drops to zero. This point is known as the "First Secondary Notch" because there is no energy left in the secondary at this time. This energy transfer process can continue for several hundred microseconds. Energy sloshes between the primary and secondary resonant circuits resulting in their amplitude increasing and decreasing with time. At the instants when all of the energy is in the secondary circuit, there is no energy in the primary system and a "Primary Notch" occurs. When all of the energy is in the primary circuit, there is none in the secondary and a "Secondary Notch" occurs. After several transfers of energy between primary and secondary, the energy in the primary will become so low that the spark gap will cool. It will now stop conducting at a primary notch when the current is minimal. At this point any remaining energy is trapped in the secondary system, because the primary resonant circuit is effectively broken when the spark gap opens. Keep in mind that his entire process may repeat itself several hundred times every second. Sparks may grow in length over time because every streamer tends to follow the hot ionized channel of air created by the previous one. The first essential part of the Tesla Coil design is the step-up transformer. Several kinds of transformers may be used to step up the voltage on the supply line. For our experiment, we started with neon sign transformers, NSTs. (We later moved to pole pig power.) NSTs usually range anywhere from 6KVAC (kilovolts AC) to 15KVAC, and from 20mA (milliamperes) to 90mA. Other transformers may come from automobile ignition coils, microwave oven transformers, X-ray machines, television flyback transformers, and utility distribution transformers. (Commonly referred to as pole pigs.) NSTs are most often used because of their good balance of availability and power. The most powerful practical transformer would be a distribution transformer. Along with their extreme power, however, pole pigs have several serious drawbacks. They often weigh several hundred pounds, are superbly dangerous because of the high currents (about an amp at 14KVAC), and are very expensive. The spark gap is the next essential part of the Tesla Coil, and performance of the coil depends greatly on its design. As you already learned, the spark gap acts as an SPST switch; the only practical type of switch capable of switching such high voltages and currents at such high speeds. We use a large amount of air pressure blowing through the electrodes of the gap to keep it cool. If the gap were not kept cool, it would never quench because the arc would ionize and heat the air between the electrodes, only making it easier for following arcs to make it across the spark gap. We call this an air-blasted static spark gap. (Static because it uses no moving parts.) Some spark gaps employ several gaps in series, which is another method to keep the spark quenching at a high rate. Still other spark gap designs use rotary motors to control the electrodes. Rotary spark gaps can be synchronous (operating using the frequency of the supply voltage to control their speed), or asynchronous (rotation speed is controlled by the operator.) Rotary spark gaps are often the most efficient because of their consistency. Static gaps are trusted to find their own break rate. The capacitors are the next essential part of the Tesla Coil circuit. The value of the capacitor cannot be too low, or too high. The capacitors need to resonate with the high voltage power supply so they can efficiently process all the energy from the power supply in every cycle, thus giving the spark gap a much better opportunity to quench at the instant when all the voltage is stored in the secondary and none is left in the primary. High voltage capacitors are very hard to come by, and even then, they are expensive. We have made several attempts to construct our own capacitors, but with minimal success. We made a large amount of "saltwater capacitors", but the capacitance of them was so low we would have needed 30 of them or more to get the rating that we desired. We also tried making a stacked plate capacitor, which would have worked wonderfully, but the capacitor always failed because of microscopic flaws in the material. Commercial capacitors are made with a clean-room process, and with the best materials they can find. So, we settled on what the rest of the Tesla Coiling community likes to call an MMC, or multi-mini-capacitor. We bought 100 small capacitors from a friend for $20. Each capacitor was rated .21uf at 1250VDC. Since we were using AC, the 1250VDC had to be degraded to 625VAC. We connected 20 of them in series to increase the voltage rating to 12500VAC. Capacitors in a Tesla Coil circuit always need to be over-engineered, because if the circuit is even slightly out of tune, excess voltage from "missed firings" will accumulate in the capacitor bank, showing them a much higher voltage than they were rated for. However, connecting capacitors in series also decreases the overall capacitance. To remedy this, we connected several identical strings in parallel. This component is referred to as the primary capacitor because there is also capacitance in the secondary circuit, and the two needn't be confused during calculations. The primary coil is the inductor that transfers energy to the secondary coil by means of magnetic coupling. The primary needs to resonate at the correct frequency in order to excite the secondary coil and allow the transfer of energy. To make this a less painful process, there is a tap point of the outside of the primary, which can easily be moved around to adjust the frequency and inductance. High frequency voltage has a property called the "skin effect". This effect states that the high frequency voltage travels along the surface of a conductor, not penetrating it. (In theory.) This would make the Tesla Coil much less dangerous, but the skin effect doesn't usually apply to humans. The human skin is not a very good conductor, so the energy will most likely seek a better conductor first, such as your blood vessels, and then travel along the surface of those. Also, because of the poor frequency response of the human nervous system, the initial shock may not even be felt by the person - only the burns left behind. It is because of this skin effect that the most efficient primary coil material is hollow copper tubing. Primary coils can be designed in a flat spiral, saucer, or vertical helix. The different designs differ only in their coupling characteristics, and on high-powered coils, the flat spiral design works best to prevent over-coupling. The secondary coil is usually the most interesting part of the Tesla Coil and most coilers often put the most emphasis on the design of it. Magnet wire is wound around an air-core coilform to produce an air-core resonant transformer. It needs to be designed with the correct height/diameter ratio and wire size in mind so that it can resonate efficiently with the primary circuit. The resulting coilform has its own self-capacitance and inductance that is very important to the output voltage. After winding, the entire coil is painted with polyurethane as an insulative coating and to hold the magnet wire in place. It is imperative that none of the wires are crossed at any point. This would cause a short circuit, which would produce a great amount of heat at the crossover, which would melt the wire. The secondary also needs to have a very strong, low impedence earth ground to provide potential difference. The better the ground, the better the output. The discharge terminal, or toroid, is the last stopping place for the energy before it breaks out into the air. We call it a toroid because it is a doughnut - or toroidal - shape. The toroid adds self-capacitance to the secondary circuit, and lowers the resonant frequency of the secondary. The largest toroid that still allows sparks to break out will be the best toroid. When the toroid is too large, sparks cannot break out, and the Tesla Coil becomes a giant RF emitter. To break out from a toroid that is too large, the best solution is to add more primary capacitance. This is the final result of all the work that goes into the Tesla Coil. When the voltage builds to such high amplitude the surrounding air can no longer hold it off, the air ionizes and a streamer breaks out to an object of potential difference. A simple but fairly accurate equation can be used to find the estimated maximum streamer length. Spark Length = 1.2 x sqrt(input power) Spark Length = 1.2 x sqrt(5000 watts) Spark Length = 7 Feet The coil transmits energy into the air as well. A fluorescent tube ban be lit about 15 feet away from the discharge terminal. The RF and EMF field generated by the coil is so strong that telephones and radios cease to function within the field of the coil. This is an obvious drawback, one of many that may have put an end to Nikola Tesla's dreams of worldwide free energy. Input Transformer Output Voltage: 14400 VAC Output Current: 0.811 Amps Input Frequency: 60 Hz Input Voltage: 240 VAC Matched Capacitor: 0.3uf Primary Coil Design Category: Flat Spiral Inside Diameter: 11 Inches Wire Diameter: 0.25 Inches Wire Spacing: 0.25 Inches Total Turns: 16 Turns Average Radius: 9.38 Inches Total Diameter: 26.5 Inches Wire Length: 78.54 Feet Total Inductance: 0.1404mH Required Primary Capacitance: 0.025uf Secondary Coil Outside Diameter: 9.0 Inches Outside Radius: 4.5 Inches Outside Circumference: 28.3 Inches Winding Length: 32.23 Inches Wire Length: 2456.92 Feet Wire Gauge: 21 AWG Turns per Inch: 33.3 Turns Total Turns: 1042.75 Turns H/D Aspect: 3.58 Total Inductance: 60.66mH Medhurst K: 0.66 Self Capacitance: 15.05pf Resonant Frequency: 166.59 KHz Resonant Frequency w/ Toroid: 95.93 KHz 1/4 Wavelength: 1477.07 Feet 1/4 Wavelength w/ Toroid: 2564.97 Feet Topload (Toroid) Tube Center to Center Diameter: 22.0 Inches Height: 6.0 Inches Topload Capacitance: 30.33pf Spark Gap Design Category: Single Static, Air Blasted Forced Air Pressure: 0 - 120 PSI Electrode Spacing: 0 - 2 Inches Theoretical Veretus Coil Output Voltage Ep - Energy (joules) stored in primary capacitor Cp - Capacitance (nanofarads) stored in the primary capacitor Vp - Voltage in the primary circuit Es - Energy stored in the secondary coil Cs - Capacitance (picofarads) stored in the primary inductor Vs - Voltage in the secondary coil Ep = 0.5 x Cp x Vp2 Ep = 0.5 x 30nf x (14400)2 = 3.11 Joules Es = 0.5 x 15.05pf x Vs2 = 3.11 Vs2 = 3.11/(0.5 x 15.05pf) Vs = 413,289 Volts Theoretical Veretus Coil Output Current Ep = Primary voltage Ip = Primary current Es = Secondary voltage Is = Secondary current EpIp = EsIs 14400 x .81 = 413289 x Is (14400 x .81)/413289 = Is Is = 0.0282 Amps Voltage Gain = sqrt(Cp/Cs) Voltage Gain = 28.7 times And finally, here is the short video that my friend Robert and I made in highschool for a class: |