Editing Tesla coil (section)

==Modern-day Tesla coils==
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[[File:tesla-coil-discharge.jpg|thumb|left|[[Electric discharge]] showing the [[lightning]]-like [[plasma (physics)|plasma]] filaments from a 'Tesla coil']]
[[File:Tesla coil (discharge)..JPG|thumb|Tesla coil (discharge)]]
[[File:Tesla coil in terrarium (I).JPG|thumb|Tesla coil in terrarium (I)]]

Modern high-voltage enthusiasts usually build Tesla coils similar to some of Tesla's "later" 2-coil air-core designs. These typically consist of a primary [[tank circuit]], a series LC ([[inductance]]-[[capacitance]]) circuit composed of a high-voltage [[capacitor]], [[spark gap]], and [[primary coil]]; and the secondary LC circuit, a series-resonant circuit consisting of the [[secondary coil]] plus a terminal capacitance or "top load". In Tesla's more advanced (magnifier) design, a third coil is added. The secondary LC circuit is composed of a tightly coupled air-core transformer secondary coil driving the bottom of a separate third coil helical resonator. Modern 2-coil systems use a single secondary coil. The top of the secondary is then connected to a topload terminal, which forms one 'plate' of a [[capacitor]], the other 'plate' being the earth (or "[[ground (electricity)|ground]]"). The primary LC circuit is tuned so that it [[resonate]]s at the same frequency as the secondary LC circuit. The primary and secondary coils are magnetically coupled, creating a dual-tuned resonant air-core transformer. Earlier oil-insulated Tesla coils needed large and long insulators at their high-voltage terminals to prevent discharge in air. Later Tesla coils spread their electric fields over larger distances to prevent high electrical stresses in the first place, thereby allowing operation in free air. Most modern Tesla coils also use toroid-shaped output terminals. These are often fabricated from [[metal spinning|spun metal]] or flexible aluminum ducting. The toroidal shape helps to control the high electric field near the top of the secondary by directing sparks outward and away from the primary and secondary windings.

A more complex version of a Tesla coil, termed a "magnifier" by Tesla, uses a more tightly coupled air-core resonance "driver" transformer (or "master oscillator") and a smaller, remotely located output coil (called the "extra coil" or simply the [[resonator]]) that has a large number of turns on a relatively small coil form. The bottom of the driver's secondary winding is connected to ground. The opposite end is connected to the bottom of the extra coil through an insulated conductor that is sometimes called the transmission line. Since the transmission line operates at relatively high RF voltages, it is typically made of 1" diameter metal tubing to reduce corona losses. Since the third coil is located some distance away from the driver, it is not magnetically coupled to it. RF energy is instead directly coupled from the output of the driver into the bottom of the third coil, causing it to "ring up" to very high voltages. The combination of the two-coil driver and third coil resonator adds another degree of freedom to the system, making tuning considerably more complex than that of a 2-coil system. The transient response for multiple resonance networks (of which the Tesla magnifier is a sub-set) has only recently been solved.<ref name="Antonio1"/> It is now known that a variety of useful tuning "modes" are available, and in most operating modes the extra coil will ring at a different frequency than the master oscillator.<ref name="Antonio2"/>

===Primary switching===
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[[File:Maker Faire 2008 Tesla Coil.ogg|thumb|left|Demonstration of the Nevada Lightning Laboratory 1:12 scale prototype twin Tesla Coil at [[Maker Faire]] 2008]]

Modern [[transistor]] or [[vacuum tube]] Tesla coils do not use a primary spark gap. Instead, the transistor(s) or vacuum tube(s) provide the switching or amplifying function necessary to generate RF power for the primary circuit. Solid-state Tesla coils use the lowest primary operating voltage, typically between 155 and 800 volts, and drive the primary winding using either a single, [[rectifier|half-bridge]], or [[power electronics#Single-phase full-bridge inverter|full-bridge]] arrangement of [[bipolar junction transistor|transistors]], [[MOSFET]]s, or [[IGBT]]s to switch the primary current. Vacuum tube coils typically operate with plate voltages between 1500 and 6000 volts, while most spark gap coils operate with primary voltages of 6,000 to 25,000 volts. The primary winding of a traditional transistor Tesla coil is wound around only the bottom portion of the secondary coil. This configuration illustrates operation of the secondary as a pumped resonator. The primary 'induces' alternating voltage into the bottom-most portion of the secondary, providing regular 'pushes' (similar to providing properly timed pushes to a playground swing). Additional energy is transferred from the primary to the secondary inductance and top-load capacitance during each "push", and secondary output voltage builds (called 'ring-up'). An electronic [[feedback]] circuit is usually used to adaptively synchronize the primary [[oscillator]] to the growing resonance in the secondary, and this is the only tuning consideration beyond the initial choice of a reasonable top-load.

In a dual resonant solid-state Tesla coil (DRSSTC), the electronic switching of the solid-state Tesla coil is combined with the resonant primary circuit of a spark-gap Tesla coil. The resonant primary circuit is formed by connecting a capacitor in series with the primary winding of the coil, so that the combination forms a series [[LC circuit|tank circuit]] with a resonant frequency near that of the secondary circuit. Because of the additional resonant circuit, one manual and one adaptive tuning adjustment are necessary. Also, an [[interrupter]] is usually used to reduce the [[duty cycle]] of the switching bridge, to improve peak power capabilities; similarly, IGBTs are more popular in this application than [[bipolar junction transistor]]s or MOSFETs, due to their superior power handling characteristics. A current-limiting circuit is usually used to limit maximum primary tank current (which must be switched by the IGBTs) to a safe level. Performance of a DRSSTC can be comparable to a medium-power spark-gap Tesla coil, and efficiency (as measured by spark length versus input power) can be significantly greater than a spark-gap Tesla coil operating at the same input power.