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30 BPS, 60 BPS tests




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From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Saturday, March 21, 1998 7:34 AM
To:  Tesla List
Subject:  Re: 30 BPS, 60 BPS tests

Tesla List wrote:
> 
> ----------
> From:  Hollmike [SMTP:Hollmike-at-aol-dot-com]
> Sent:  Friday, March 20, 1998 7:08 AM
> To:  tesla-at-pupman-dot-com
> Subject:  Re: 30 BPS, 60 BPS tests
> 
> John, all,
>    I have been reading all the arguments about this final charge left on the
> secondary.   Many have said that this cannot happen due to the low DC
> resistance to ground, but one notion came to mind:  As the amplitude of the
> output decays, there is some point where it dies out before the next bang.
> Being that there is a large inductor that resists the change in current, could
> it be possible that a small amount of charge be left in the toroid due to it
> not having enough potential energy to overcome the inductive reactance on that
> last current reversal before it dies out?  This, if possible, could store a
> minute amount of energy until the next bang.  I can't think of any way to test
> this notion, but thought I'd put it forward anyway.
> Mike Hollingsworth


Mike and all,

Excellent question! In my earlier days of coiling I wondered about this
too, and performed a series of measurements to see just what WAS going
on. Although these experiments were more focused on the feasibility of
"building up" secondary energy from one bang to the next in a disruptive
system, they are also applicable to the DC question at hand. 

First, a thought experiment for you... 
Suppose you were able to take a base-grounded, isolated secondary coil
and could instantaneously apply a high voltage to the top toroid from a
voltage source. For the moment, let's also assume that we are able to
suppress any corona or streamer losses. As you indicate, the secondary's
inductance prevents any initial current flow. This scenario corresponds
to the proposed case where somehow have a residual DC charge on the
toroid and no current flowing through the secondary. 

We now remove the voltage source and observe what happens. Current
begins flowing through the secondary to ground. The electrostatic energy
stored in the toroid and secondary self-capacitances begins to transfer
to the secondary inductance. As current climbs in the secondary winding,
the toroid voltage falls, and the RLC circuit begins to oscillate at the
frequency determined by the L, C and series R of the secondary and
groundpath. The total energy in the system at any time (1/2 CV^2 + 1/2
LI^2) declines exponentially through resistive losses, resulting in the
classical "ringdown" voltage envelope seen in an ideally-quenched Tesla
Coil. 

In the very best case [no external energy losses from streamers or EM
radiation], the overall envelope during "ringdown" will decline
exponentially. For all but the very largest of coils, the secondary will
ring down to zero with NO residual energy remaining in either the
inductive or capacitive elements prior to the next bang. This is the
case even with small to medium systems having secondary Q's of over 200
and breakrates of over 500 Bangs/Second. This has been confirmed by
numerous measurements made by myself and a number of other coilers on
the list. More typically, streamers become the dominant loss (a
desireable goal...), quite rapidly reducing the energy stored in the
secondary. Under heavy streamer loading, the effective secondary Q
typically drops an order of magnitude or more, and the decay envelope
tends to decay in a more linear fashion due to the nonlinear V-I
characteristic of the streamers. 

The bottom line: 
Because the secondary behaves very much as an RLC circuit, any DC
potential appearing on the toroid will cause the secondary to oscillate
and ring down at its operating frequency. It is physically impossible
for this system to sustain a steady-state DC potential in the absence of
an external energy source. Energy-wise, successive bangs are truly
independent events. It's the nature of the beast...

>From a frequent transient in Illinois,

-- Bert --