Solid State Tesla Coils and Their Uses Sean Soleyman Electrical Engineering and Computer Sciences University of California at Berkeley Technical Report No. UCB/EECS-2012-265 http://www.eecs.berkeley.edu/Pubs/TechRpts/2012/EECS-2012-265.html December 14, 2012 Copyright © 2012, by the author(s). All rights reserved. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission. Solid State Tesla Coils and Their Uses by Sean Soleyman Research Project Submitted to the Department of Electrical Engineering and Computer Sciences, University of California at Berkeley, in partial satisfaction of the requirements for the degree of Master of Science, Plan II. Approval for the Report and Comprehensive Examination: Committee: Jaijeet Roychowdhury Research Advisor 2012/12/13 (Date) Michael Lustig ,tG/t; (Date) Abstract – The solid state Tesla coil is a recently- discovered high voltage power supply. It has similarities to both the traditional Tesla coil and to the modern switched-mode flyback converter. This report will document the design, operation, and construction of such a system. Possible industrial applications for the device will also be considered. I. INTRODUCTION – THE TESLA COIL For reasons that will be discussed later, the traditional Tesla coil now has very few practical Around 1891, Nikola Tesla designed a system uses other than the production of sparks and special consisting of two coupled resonant circuits. The effects. Nevertheless, numerous hobbyists and primary circuit is a spark gap oscillator that can be professional engineers have continued to study the charged from a high voltage power supply. When machine and produce improvements over the the voltage across the capacitor is sufficient to turn original design. As a result, modern technology has on the spark gap, the primary circuit begins to found its way into an age-old invention. Recently, it oscillate and transfer its energy to the secondary has become practical to replace the spark gap of a resonator. After the transfer is complete, the spark Tesla coil with an entirely different type of switch. gap turns off so that the primary circuit can re- charge for the next cycle. In this manner, each cycle II. THE SOLID STATE TESLA COIL (SSTC) adds energy to the secondary circuit. If a solid state inverter is used to feed energy into the system, the primary coil no longer needs to be driven by a resonant circuit. In fact, it can even be eliminated entirely. This is known as the base feed drive method. It has some major shortcomings, but provides a starting point for analyzing the behavior of the more practical two-coil SSTC. The base-fed SSTC is nothing more than an air-core inductor connected to a signal generator that is set The system was originally conceived as a wireless to the coil’s resonant frequency. A simple low- transmitter, and was indeed used for this purpose power experiment can demonstrate the behavior of until the 1920s. Since then, most interest in the such a system. In the following circuit, a 1V RMS, device has centered on its ability to generate high 637kHz sine wave is used to power a 50V neon voltages across the secondary circuit load. One indicator lamp. The resonator coil is wound with a physical arrangement of the circuit has proven to be single layer of 30AWG copper magnet wire on a especially convenient for generating electric arcs, plastic tube. streamers, and corona discharge. A secondary coil is wound as a single-layer solenoid and is oriented in a vertical configuration. A metal terminal with a large surface area is affixed to the top of this coil, and serves as one plate of a capacitor in the secondary resonant circuit. The base of the secondary coil is connected to the ground, which forms the other end of the capacitor. If sparks are emitted from the metal terminal, they can be modeled as a resistive load, completing the secondary RLC circuit. The neon lamp circuit provides a very convenient method for measuring the resonant frequency of a solenoid if an oscilloscope is not available. It also provides additional insights that will be useful when By this method, the inductance of the resonator is designing circuits for high power tests. found to be 11.0mH. This number agrees with the measured value of 12.04mH. The voltage rise that enables the neon lamp to turn on can be explained by modeling the system as a The resonant frequency of the system is more narrowband impedance matching circuit. difficult to estimate because several factors contribute to the effective capacitance of the coil and the load. ETesla6 is an open-source program that uses a finite element simulation to provide a reasonable estimate [2]. More information is available from the Tesla Secondary Simulation Project [3]. The simulated resonant frequency is 669kHz, with a capacitance of 5.03nF. The measured resonant frequency of this coil is The neon lamp has both capacitive and resistive approximately 660kHz when no load is connected. components. It is in series with the coil, which has a It drops to around 640kHz when large streamers are large inductive component. Since most high voltage emitted from the discharge terminal, and can go loads consist of electrodes separated by a gas or much lower if a load with a high capacitance is vacuum, this voltage rise effect applies to other used. For example, if the free end of the neon lamp Tesla coil circuits as well. It is, however, difficult to is instead connected directly to ground, the resonant predict the exact voltage rise unless detailed models frequency drops to 410kHz. of both the load and the coil are available. To summarize, the resonant circuit of a solid state As mentioned before, the resonant frequency of the tesla coil will be modeled as a driven RLC circuit. coil connected to a neon lamp has been found to be To characterize the RLC circuit, three parameters 637kHz. Is this the only resonant frequency? It must be identified. The inductance can easily be would be if the coil was an ideal inductor. However, found by calculation or measurement, and the it turns out that that 637kHz is actually the quarter capacitance can be determined from the resonant wave resonant frequency of the system, and that the frequency. The resistance is a characteristic of the neon tube will also light up at 1570kHz if the drive load, but may be subject to an impedance voltage is increased slightly. An oscilloscope was transformation if it is in parallel with a capacitor, as used to find an additional resonant mode at is often the case. In practice, the system can be built 2984kHz. It is important to note that these higher without knowledge of the exact value of this resonant modes do not occur at exact integer resistance, and can then be adjusted to provide an multiples of the quarter resonant frequency. appropriate output power. Therefore, if a square voltage waveform is used to drive the coil (as it will in a later section), the The first objective of this project is to design a harmonics do not excite the resonator. This means power supply capable of delivering hundreds of that although the drive voltage is a square wave, the watts of power at approximately 100000V. The current is a perfect sinusoid, as would be expected base-feed method is not practical for such a task. for a simple RLC circuit. Although it has been demonstrated that the resonator is able to produce a step-up effect by In practice, the Tesla coil resonator is only operated itself, the exact voltage produced by such a circuit at the quarter-wave resonant frequency, and is depends heavily on the characteristics of the load, is modeled as a series LC circuit. The inductance can difficult to predict, and is limited to a factor of be estimated using Wheeler’s approximation for a around 1:50. Much better results can be achieved by single-layer solenoid, where r and l are in inches using the transformer action between a short and n is the number of turns [1]: primary coil and a long secondary coil. A step-up ratio of around 1:100 can be achieved using this method, and this will be multiplied by any resonant voltage rise that is also present. III. THE AIR CORE TRANSFORMER The transformer described in this section is designed to generate electric sparks, and can also be used to power a gas tube. It consists of the resonator described in the previous section, along with a primary coil. The primary coil is wound with 3 As predicted by the MandK simulation program [2], turns of 14AWG wire, and can be moved up and the maximum coupling coefficient for this coil is down to provide more or less mutual inductance. around 0.35, and is achieved if the primary coil is a The top of the secondary coil is left disconnected, few inches above the base of the secondary. If the and is fitted with a small piece of wire that causes objective is to generate very high voltages, a high streamers to form from the sharp points. coupling coefficient is essential. The coupling coefficient represents the fraction of the secondary coil that is magnetically connected to the primary.