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Re: Evaluation needed (fwd)





---------- Forwarded message ----------
Date: Thu, 12 Feb 98 09:16:42 EST
From: Gary Lau  12-Feb-1998 0906 <lau-at-hdecad.ENET.dec-dot-com>
To: tesla-at-pupman-dot-com
Subject: Re: Evaluation needed

>From:  Dan Ressler [SMTP:danz_da_man-at-hotmail-dot-com]
>
>Hey all,
>Im having trouble understanding the theory of the tesla coil and the 
>tank circuit.  But after much pondering I have come up with an 
>explination that I think makes sense......so give me your input.  
>
>A tesla coil is simply a special transformer that uses the series
>resonant circuit to step up the voltage of the input power.  A series
>resonant circuit consists of a coil, a capacitor, and the resistance of
>the wire.  Energy is stored on one terminal of the capacitor 

        Energy is stored in the dielectric layer of a capacitor, not on
        either plate.

>and in the coil (the coil stores energy in a magnetic field, this is
>called inductance), 

        Energy in an inductor in is the form of a magnetic field but is
        not called inductance, inductance is a property of the coil and
        is the same regardless of whether any energy is stored in it or not.

>but as the capacitor fills up the magnetic field of the coil breaks down
>causing an inductive kick (theoretically this would result in a voltage
>in the coil infinitely high and infinitely short, but since the capacitor
>fills gradually the magnetic field is converted into electricity
>gradually).  This energy then travels back through the circuit and builds
>up on the other terminal of the capacitor, and the process repeats its
>self until the energy is damped out by the resistance of the wire.  This
>process is synonymous to a pendulum swinging back and forth.  If energy
>is applied to the circuit at the right frequency (the circuit's resonant
>frequency) the voltage in the system will build up to huge voltages (in a
>perfect world the voltage would reach infinity).  This is synonymous to
>pushing the pendulum at the right times so that its amplitude increases.
>In the case of the tesla coil the transformer adds energy into the tank
>circuit, not at the resonant frequency, but the voltage still escalates.
>When the voltage is high enough it jumps the spark gap.  The gap produces
>"white noise", or all possible radio frequencies, one of these
>frequencies is the resonant frequency of the tank circuit.  

        You've got this all backwards.  The first thing that happens is
        the primary capacitor charges from the high voltage neon
        transformer.  There may be some resonant action here, but for
        simplicity, let's say not, it's not pertinant to Tesla coil
        operation.

        As the voltage across the capacitor reaches the spark gap
        breakdown voltage, the gap fires and conducts.  It does NOT
        produce white noise (at least not much).  The conducing gap
        places the charged capacitor in parallel with the primary coil,
        and here is where the pendulum action takes place.  The energy
        alternately exists in the capacitor, then the primary coil.  The
        energy does not move from one side of the capacitor to the other,
        although the polarity of the charge does alternate on alternate
        "pendulum swings".

>The energy see-saws back and forth at the resonant frequency and the
>voltage jumps to extreme voltages.  

        Right, but the voltage rise occurs in the secondary, not the primary.

>Next, the secondary coil absorbs the
>energy in the tank circuit through coupling.  When the energy in the
>primary circuit reaches zero the spark gap extinguishes, and the above
>process begins again.  Meanwhile, the secondary coil is acting as a tank
>circuit by its self.  The winding of the coil have a small capacitance.
>The coil also has inductance, energy is stored in a magnetic field, and
>there is resistance.  These are all the components of a tank circuit.
>The tesla coil is designed so that the resonant frequencies of the
>primary coil and the secondary coil are the same;  and the secondary
>absorbs the energy of the primary coil at this frequency.  The result is
>the voltage climbs and climbs as it gets higher in the secondary coil.
>The voltage gets so high the electricity can flow through even air.

        Pretty close.  In addition to resonant rise, there is more to why
        the secondary voltage gets so high.  If the energy in the primary
        capacitor (when the pendulum is on that side and the cap is fully
        charged) is defined by E=.5*C*V*V, so must the energy in the
        secondary capacitance when it is fully charged.  Since the
        secondary capacitance is a tiny fraction of the primary
        capacitance, equating the primary and secondary energies shows
        Vs = Vp sqrt(Cp/Cs).  This is a gross oversimplification and
        ignores all losses, but in principal is true.

Gary Lau
Waltham, MA USA