I present the following FAQ for your information. As per the authors request, I have not changed any of the text. I have only reformatted it in a table and added bookmarks. Spelling and other errors are not mine. In the future, I will attempt to make my own FAQ - Steve
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" And from the peace of darkness the neighbours
were cast into a firey |
| Contents 0. Warnings 1. Tesla himself 1.1 personal background 1.2 general inventions 1.3 what he means to us 2. Tesla-coil, types: 2.1 1/4 wave 2.2 bipolar 2.3 tesla magnifier 2.4 other configs 3. individuals components 3.1 primary system 3.1.1 primary coil 3.1.2 capacitor 3.1.3 spark-gap 3.1.4 protective components 3.1.5 wiring 3.2 secondary 3.2.1 secondary coil 3.2.2 capasitive hat 3.2.2 ground connection 3.3 tertiary coil 3.4 control circuitry 3.4.1 variac 3.4.2 current limiting 3.4.3 contactors 3.4.4 fuses/circuit-breakers 3.5 test-equipment 4. sources for more information 5. acknowledgements |
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The equipment (tesla-coils) described in this text-file are potentially lethal and should be handled with respect. The voltages present are very high, even megavolts are common, and especially at primary circuit have such high currents that death can be imminent in case of an accident. Without proper design large amounts of RFI or radio-frequency interference can result. |
1.1 Personal background Tesla was born in Smilja, Kroatia 1856. He arrived to New York in 1884 and founded a lab soon. His Colorado Springs experimental station was founded in 1889 and there he made his most interesting experiments with resonators of many kinds running up to gigawatts of power. His biggest project was the global transmitter of electricity, Wardenclyffe, at Long Island in Manhattan area of New York - he never finished this project because of lack of funds. 1.2 general inventions Tesla invented the AC-engine and multi-phase power distribution systems still use today, is the official inventor of radio, has his name as the unit of field flux density etc. 1.3 what he means to us < to be added - someone? > Back to top |
A Tesla Coil is an air core, RF resonate, transformer. It is a very efficient source of very high voltage RF energy. A simple coil setup is capable of producing more or less continous spark discharges several feet in length, or longer. 2.1 1/4 wave A normal coil system operates by having a VSWR, or voltage standing-wave resonance, -effect on the secondary coil and thus the peak of voltage appears on the top of the secondary. One can visualize the 1/4-wave nature by thinking about the normal sine wave starting from origo and continuing on the x-axis. The sine-wave has it's peak value at 90 degrees or pi/2 which is one fourth of full cycle or 2*pi and the 1/4-wave system has this very peak of voltage on the top of secondary. The formal name for the secondary is a slow-wave helical resonator. 2.2 bipolar On a bipolar system the 1/4-wave peak appears at both ends of the secondary so this could be called half-wave system. One can visualize this kind of a set-up by thinking that the bipolar secondary is in fact two 1/4-wave secondaries with grounds connected together at the middle of the coil. In fact, some set-ups do use separated coils to get more practical systems. 2.3 tesla magnifier A magnifier differs from the traditional tesla-coil by having in fact three coils. The primary excites the secondary and this is just an ordinary non-resonant air-core transformer but the output from the secondary is base-fed to the tertiary which is resonant at the primary frequency. 2.4 other configurations Coils that are excited or driven to higher harmonics (over 1/4 wave) break down prior to the top turns. For instance a coil driven to 3/4 wave resonance would still break down at the 1/4 wave point of the winding. This problem is commonly seen on coils that are overdriven or overcoupled, and I cannot imagine any way to insulate against this breakdown. However when you go to two or more coils, where you are working with more than one output terminal, it is possible to achieve 3/4 wave signal processing. This requires a pair of 1/8 wave driver coils, two primaries run in series off the same tank ciruit, and two extra coils. Your outputs from the extra coils are still 1/4 wave with the peaks on the terminals, but taken as a whole the system does efficiently process a 3/4 wave signal. This system would be adding up lower harmonics (two 1/8th wave outputs, with two 1/4 wave outputs) to get to efficient 3/4 wave signal processing. This system would be unique in that it may not require a ground, as the two tuned sets of coils could "beat" off of each other; in other words the nul voltage current output from the base of one 1/8 wave driver coil could be fed into the base wire of the second driver. This system would require that the series run primaries, and the drivers and extra coils be wound in opposite directions to achieve phase differential. The base wire current output of one coil becomes added input to the base of another. Back to top |
A tesla-coil system is combined of many components that all have to be of highest approprite quality in order to gain as good results as possible. This is because the worst component often is the limiting factor of over-all performance. 3.1 primary system The tank circuit literally converts line current into a series of rapid pulse dicharges with peak powers in the megawatt range. Basically you take a high voltage pulse discharging capacitor and connect it to a large heavy coil. The cap is charged with a high voltage power supply (neon sign xfrmr, potential xfrmr, or power distribution xfrmr run backwards). The circuit consisting of the cap and coil is excited by discharging the cap through a spark gap. This way current of hundreds of amps at thousands of volts oscillate through the coil. Frequency of oscillation is dependant on the number of turns in the coil and the size of the cap. This is the basis of the Tesla Tank circuit. The primary circuit can be constructed in many different ways, with either floating or grounded tank circuit. The configurations can be specified with the following diagram and table: ------o--2------ 1 2 3 high | primary Gap Cap - voltage 1 coil Gap Cap Cap "equidrive/balanced" source | connection Cap Gap - ------o--3------ 3.1.1 primary coil The primary coil is a low resistance, heavy coil, through which the currents produced by the pulse capacitance travel. Commonly this consists either a pancake, "saucer" or helical coil wound with soft copper tube with up to 15 turns. 3.1.2 capacitor A pulse-discharge capacitor is a vital part of the primary circuit as it enables the energy to discharge through the primary coil, spark-gap and the wirings as quickly as possible thus creating the necessary power-levels to excite the secondary coil. Capasitors with adequate voltage rating can quite cheaply be constructed of the materials commonly available even in the household supplies. Some of the low-Q solutions include salt-water caps, glass-caps and commercial caps not meant for AC or pulse-discharging found in surplus stores A very good cap can be constructed of polyethylene or -propylene foil and aluminium flashing in mineral-oil immersion. Commercial caps with superiour performance are available as well. More information: rqmsgs.txt (theory and practical construction) tesla.txt (sources for commercial capacitors) 3.1.3 spark-gap Spark gaps are the "brain" of the Tesla Coil. They are high the voltage switches that allow the tank circuit capacitance to charge and discharge. As performance of the spark gap switch is improved, peak powers in the tank circuit grow without requiring additional input power. More information: sparkg.zip (cylindr.*, airblst.*) 3.1.4 protective components Tesla power supplies must be protected with extensive RF choking, and safety gaps. This is especially important with neons, which are much more delicate than pole pigs or potential xfrmrs, but in order to avoid unwanted RFI and possible danger to other household equipment sufficient protection should always be supplied. Bypass capacitors should be used as well - for bypass capacitance across the power supply HV terminals you WANT a dielectric with a high RF dissipation factor. More information: chokes.zip 3.1.5 wiring The wiring of the primary system should be as heavy as possible because of the huge currents flowing through the conductors and connectors. All the wiring should have as large and smooth a surface-area as possible to avoid losses because the high-frequency currents flow on the surface of the conductor and may cause corona-discharges from sharp points. 3.2 secondary system Secondary system consists of a secondary coil that is connected to the heavy RF-ground from the lower terminal and to a capasitive hat or a discharge electrode from the higher terminal. 3.2.1 secondary coil The secondary coil or "Tesla coil" is a hollow form with several hundred turns of wire. This coil has a natural RF resonate frequency based primarily on the length of wire used in the winding. The tank circuit frequency is made to match the secondary natural frequency by tuning, changing the number of turns in the heavy primary coil or changing the value of the pulse discharging capacitor. When the tank circuit frequency is matched to the secondary frequency, and the coils are placed in close proximity, energy is exchanged and transformed. More information: coilbld.zip (step-by-step instructions with pictures) 3.2.2 capasitive hat On top of the secondary on normal and tertiary on magnifiers there is a discharge terminal that adds (isotropic) capasitance to the the system. The effects include lowered resonant frequency and alteration of electric field. The most common shapes are sphere and toroid terminal of which the latter is better. These terminals can be quite large with high-powered systems and especially magnifiers. 3.2.2 ground connection "There is no such thing as a RF system ground that is too heavy." RQ The ground connection of the secondary coil serves as a transmission line for high RF-currents from the coil system. These currents would contaminate the normal household ground and thus need to be conducted to a specific, heavy RF-ground. 3.3 tertiary coil On magnifier systems one uses a third coil that is base-fed with RF-energy from the closely coupled primary-secondary system. Huge discharge terminal and thus large top capasitance is often used in these systems. 3.4 control circuitry A typical tesla-coil system operates at a multi-kW power-level and thus needs adequate control circuitry to handle the power both at normal situations and if something goes wrong. 3.4.1 variac An elementary component is a variac, variable transformer, that enables one to change voltage from 0V to maximum, say 240VAC, or even more than that on good models to compensate losses, 270VAC typically. A variac is not an isolation xformer but just uses a tap on a coil so it must be handled with appropriate caution. 3.4.2 current limiting With non-shunted xformers one has to use external current limiting. One can either add resistive elements from ovens etc. or use inductive limiting. Inductive limiting is most conveniently handled by contacting a welder in series with the primary of the step-up xformer. Instead of a commercial welder one can use a variable inductor that is rated accordingly - old xformer-laminates are available for cores. 3.4.3 contactors It is logical to use heavy contactors instead of hand-operated switches at least as a final step before the xformer as if somethink goes wrong one can switch the system off by just cutting off the control-voltage of the contactor from a remote place where the operations panel is instead of having to go near the live circuit leading to the tank-circuit. 3.4.4 fuses/circuit breakers As there is a lot of RF present there is little point in using any ground-fault indicators etc. that can't handle the RFI but appropriate fuses and circuit breakers should be included. It should be noted that surplus fuses etc. are not necessarily meant for this application and should be tested before relying on - for example fuses meant for protecting wiring can trip at a current multiple to the rating on the unit. 3.5 test-equipment There are several equipment one can use for determining parameters of a tesla-coil system; here are some examples: Frequency counter, oscilloscope and signal generator can be used for finding out the resonant frequency of coils and primary circuit etc. A multimeter or LCR-bridge measuring resistance, inductance and capasitance will help as well: checking connections, measuring coil inductance and capasitance of self-made capacitors. More advance equipment for measuring RFI emission, radiation, electric field etc. can be used as well. More information: rqpickup.txt (pickup-coils) Back to top |
| Check ftp.funet.fi directory /pub/sci/electrical/tesla for files; rqmsgs.txt has basic information, tesla.txt is a list of resources. There are pictures in jpg and gif -formats as well. Old postings to the mailing-list are available in subdirectory ../maillist; there are several megabytes of text with answers to most coiling questions. < www-sites to be added - list someone? > Back to top |
| This article is Copyright 1996 by Kristian Ukkonen (kukkonen@alpha.hut.fi). It may be freely redistributed as a text-file so long as the author's name, and this notice, are retained and the text remains unmodified. Thanks for texts etc. : Richard Quick. Marcus Young < EOF : tesla.faq > Back to top |