TCBA Volume 17 - Issue 1
Page 9 of 18
In all Tesla coil systems of the capacitive discharge type, one of the key components is the spark gap or “switch” used to commutate the electrical energy within the energy storage capacitor to the primary coil of the oscillator. The resonator or secondary is truly secondary to this action and is pretty much a cut and dry process assembly. The “tank” circuit consisting of the three key components in bold type above, is where most loss and design limitations are set in a Tesla coil system. The primary coil is usually not a big issue in coil construction. The capacitor is a very big issue, but can easily be placed in the “easy to fix” category by more experienced amateurs using modern materials for their homemade capacitors or by purchasing special pulse discharge capacitors available from suppliers.
The “switch” in most all Tesla coils is some form of spark gap operating in air. This can take the form of two nails held in a wooden fixture, often found in a “first table top coil”, to elaborate series quench rotary systems demanded by very large or high power magnifier type Tesla systems. The search for the perfect switch is a never ending quest.
The demands made upon the switch changes across a broad spectrum of alterations made to systems. Among the key factors determining switch design are: coupling of the primary to the secondary (related to arc quenching), the power input to the system, (related to dimensioning, surface areas, etc.), the desired result from the system, (sparks, efficiency, RF output, spectral purity, etc.).
A perfect switch would:
- Switch on very fast (<1 microsecond)
- Be able to withstand very high voltages (>10,000 volts)
- Be able to carry incredibly high peak currents (>500 amps)
- Be able to withstand very high average temperatures (>500 degrees C)
- Have very low internal losses while performing the switching task. This demands a low “on” state resistance within the device. (Ron<1 ohm)
- Be able to be turned off rapidly while all the voltage and currents in the switch are at or near maximum levels. (<1 microsecond).
The common air spark gap can actually be optimized for almost all of these parameters! Only parameters #5 and #6 give problems in design of high quality Tesla coil air gap switches. In addition, the construction for even the highest quality spark gap systems is mainly mechanical with relatively simple, easy to obtain, and inexpensive materials. On the whole, anyone who has experience and has studied the issue for Tesla systems, will have to admit that the common spark gap, in air, is the finest Tesla coil switch currently available in the world! Still, we experiment with many different switch devices in our quest for that elusive perfect switch.
In this modern electronic age, one would think that the old spark gap would fall prey to modern advances within the art of electrical/electronic switch design. This is most definitely not the case! Modern switches for electronic usage fall into two broad categories; 1. Solid State devices, 2. Vacuum tubes.
Solid State devices
Solid state devices have come a long way, very fast. FET switches can exhibit “on” resistances in the milliohm range! FETs, Bipolar transistors and IGBT devices can switch moderately large currents (100 amps) in nanoseconds. None of the devices, in a single unit, can switch more than 1600 volts! SCR devices are not bi-directional and require special circuitry to commutate properly. Modern advances in very expensive solid state devices have taken voltages up to 6900 volts and can switch 3000 amps, (IGCTs), but are thousands of dollars per unit! Series strings of lower voltage devices (to achieve higher voltages) are expensive and blow out in unison if a problem arises. All solid state devices are at the mercy of electrostatic damage from high electrical fields. Tesla coils are high E-field generating devices! Finally, all solid state devices are strictly limited to very low operating temperatures (<<200 degrees C.) and performance of these devices degrades rapidly as temperatures of the device rise. Thus, most solid state devices fall on their face when considering items #2, 3, and 4 of the “perfect switch” description. These particular three criteria are key to the Tesla systems maximal performance curve, especially at power levels much above 1,000 watts.