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Re: Arc length vs pwr
Hi Mark,
I'd like to address some of your points...
> The discussion had began in reference to a ringing primary with a
> very light or no load (under coupled or non coupled) like a
> secondary coil. Under conditions of load (coupling) the secondary
> coil looks like a series resistance to the primary oscillator loop.
Agreed. It may be modelled as such. It can also be modelled as an
equivalent parallel resistance. Actually, it only looks resistive
under one set of secondary loading conditions anyway.
> If resonant rise didn't occur a tesla coil couldn't operate, the
> principles of resonance apply if its a tesla coil or just a L-C
> circuit driven by a generator.
I would class "resonant rise" in the coupled system as the action
whereby the primary imparts a nudge to the secondary with each half
cycle thereby transferring its energy to the secondary over a number
of cycles - i.e. secondary amplitude builds at the expense of the
primary (see diagrams below for single tuned circuits).
> The source of voltage in the primary circuit under condition of
> oscillation is the capacitor AND the inductor, the inductors back
> EMF is what charges the capacitor, the only reason we need the
> drive transformer is to get things started and to keep them going
> due to the limited Q of the circuit which is why the output rings
> down in level.
As you originally said, the transformer is no longer part of the
circuit once the gap has fired. In fact, the initial conditions are:
cap charged, no primary current. A quarter cycle after gap fire you
have cap empty, all energy in the mag field surrounding the primary
(some of which will be coupled to the secondary if present) and
primary current at maximum. A half cycle later, the cap is charged to
opposite polarity but with a bit less voltage due to losses and the
inductor current is zero, and so it goes until the energy is mostly
gone and gap ionization has reduced to the point where the gap ceases
firing. It is important to realize the phase relationships in the
circuit. The energy the cap started with is apportioned between
inductor and cap in proportions depending on which part of the 1/2
cycle you take a snapshot of.
The point is, the cap alone is the energy source initially.
Whatever it transfers to the magnetic field is no longer part of cap
energy.
I have measured this, and I have _never_ observed the cap rising
beyond its initial value after the gap has fired. This was one of the
things I checked thoroughly in my MOSFET gap modelling exercise. I
have posted a number of photographs taken off the scope of both
primary and secondary waveforms simultaneously to a number of people.
I also measured the primary alone with attendant photos.
______/ __________
| | (A) prior to gap fire
| o
_____ o
_____ Vc o
| o
| |
------------------
_______ generator in series_________
| | (B) with AC voltage source
| | in series.
----- o
----- o
| o
| |
-------------------------------------
Circuit (A) is definitely not equivalent to circuit (B). The
phenomenon of which you speak only occurs with circuit (B) because
there is an external energy source in the circuit.
> Actually, I make take a big flame for this, but
> there is really no such thing as "parallel resonance" all resonant
> circuits are series in nature and operation series and parallel
> only really pertain on how the drive signal is applied to it. In
> order to be able to measure this resonant rise with any degree of
> accuracy you'd have to be very sure the measuring device offered
> VERY LITTLE capacitive or resistive loading.
True. For example, let Lp = 100uH, Cp = 10nF and the closed-circuit
Q = say 50. Then the ESR of the circuit is: SQRT(L/C)/Q = 2 ohms.
The equivalent parallel resistance is close to ESR*Q^2 = 5 kohms.
Measuring with a 1 Mohm probe in parallel with 20pF across the
capacitor gives negligible loading.
Malcolm
PS - If you would like, I'd be happy to send you some photographs of
all this.