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Frequency
Posted by Liviu Vasiliu on 10/2/2003, 12:04 pm
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Re: Frequency
Posted by Bert Hickman on 10/6/2003, 8:28 am, in reply to "Frequency" Vasil, The short answer: No. The longer answer: A Tesla Coil secondary does not behave as much like a long antenna wire, but more as a base-excited LC circuit made up of a large inductor in series with a small capacitor (the topload and self-C of the secondary), driven by a low impedance voltage source (transformer action from the primary) at the base. Connecting a small value of capacitance between the base of the coil and ground results in a marked change in the voltage distribution of the secondary, since the base will no longer be at a "hard" ground. Since the new ground path through the small capacitor will no longer be able to support a high level of base current, energy no longer transfers efficiently from the primary to the secondary. You'll develop significant voltage across the base capacitor but you'll get comparatively little actual coil output since the coil will no longer be able to build up energy efficiently. However, it is possible to use a much larger value of base capacitor. A large capacitor will introduce a comparatively low value of capacitive reactance, permitting high RF current from the base of the coil to easily flow to/from ground. However, this mode really can't be used for tuning. Instead, the technique is sometimes used for elevating the base of the secondary to a high DC potential in order to intensify the power of discharges to ground. But, in the process, it also creates a lethally dangerous Tesla Coil. It is also possible to connect a series chain of high voltage ceramic doorknob capacitors, positioning the chain in the center of a coil form, and designing the chain so that the approximate voltage distribution along the chain approximates the voltage discribution along the secondary. Matching the voltage distribution is important so that you dont develop excessive voltage differentials between the portions of the sceondary and the capacitor chain that might cause the insulation of the coil form to be overstressed and punctured. This configuration will connect the series chain of capacitors in parallel with the toroid's topload capacitance. In this manner, a relatively small topload can be made to "look" bigger electrically, lowering the resonant frequency and (up to a point) promoting streamer propagation with a physically smaller toroid. For example, a chain of twenty to thirty 400 pF 30 kV doorknobs will add 20 to 13.3 pF of capacitance to an existing topload. By adding or removing capacitors from the chain, you can gain a degree of secondary tuning. Best regards, -- Bert --
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