Measurements using field probe
From: Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent: Thursday, June 25, 1998 11:37 PM
To: Tesla List
Subject: Re: Measurements using field probe
Tesla List wrote:
> From: terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
> Sent: Wednesday, June 24, 1998 6:44 PM
> To: Tesla List
> Subject: Re: Measurements using field probe
> Hi Bert,
> Thanks for the note about the bad pointer. It should be fixed now.
> The pointer changed to:
> This is a newer version that has a bunch of typos fixed.
> I have been busy this week and haven't been able to play much with higher
> power strikes. I don't have much I would like to comment on now but there
> are many things I need to look at. The very high voltage stresses and where
> the missing energy is going are two. More latter.
> I would like to know about your light bulb experiment. How is was set up
> and what happened?
> I would also like to know where you disagree with the conclusions and why?
> Perhaps something was not explained well or there is something I need to
> look at closer.
> Thanks for your interest.
> Terry Fritz
Terry and all,
Thanks for fixing the pointer - the WORD97 file looks great!
The areas where I disagree tend not to be in the measurement techniques,
but in a couple of the conclusions. I'm not disagreeing with your
experimental data or measurement techniques, but only with some of the
interpretations. Further measurements may be needed to nail down these
areas. This message will focus on a two conclusions from your 4/25/98
paper: "Tesla Coil Primary Circuit Behavior Analyzed at High Bandwidth".
I agree with all of the conclusions except for #4 and #5.
Conclusion 4 states that "The loss of the arc at the zero crossings
results in heavy current ringing which seems to be related to the
self-capacitance of the primary coil and the inductance of the primary
Conclusion #5 states that "This phenomenon of the gap turning off at the
zero current crossings is probably responsible for the linear decrement
waves common to RLC spark gap circuits".
I'd contend that you're actually seeing high-current spikes not from
when the gap _extinguishes_ (at near zero primary inductor current), but
instead upon the gap's reignition. Upon reignition, the charged
parasitic capacitance in the primary circuit is now suddenly discharged
by the gap. This is more plausible, since this corresponds to maximum
capacitor (lumped and parasitic) voltage in the primary circuit. These
sudden current spikes are more consistent with sudden capacitor
discharge than inductive "kick". Sudden turn-off of current in an
inductive circuit would show a dramatic current decrease (and voltage
spike), but shouldn't show a sudden (>100X) current increase.
I'd also contend that little, if any, of this high current impulse
actually flows through the primary tank inductance. Thus little of this
energy will be coupled to a secondary (if it were present). A test could
be made by breaking the primary inductor in half, and inserting your
current probe into the midpoint. Because of the relatively small
parasitic capacitances in the primary circuit, the discharge time are
very short, but the peak currents are still quite high.
Since the energy stored in the primary circuit's parasitic capacitance
won't be coupled to the secondary when the gap fires, it does represent
a source of energy loss in a two-coil system. However, assuming that the
ratio of parasitic to tank capacitances is over 100:1 (150pf versus 17
nF in your model), the ratio of stored energies should be of comparable
magnitude. This would imply that relatively little of the total "bang"
energy energy is actually lost when the parasitic capacitance is
discharged, especially since this loss only occurs during a brief
instant following each current zero crossing at Fo.
The comparatively low energy loss from this mechanism causes me to doubt
that this is a significant factor in the linear decrement phenomenon.
Instead, it is more likely due to the combination of the non-linear gap
behavior over the conduction cycle at Fo. Gap losses tend to be more
closely approximated by the product of "clamped" gap arc voltage (Vg)
multiplied by the primary circuit current (Ip). During most of the
waveform of Fo, this loss will be directly proportional to the current,
leading to a linear decrement (i.e., a relatively constant proportional
loss per each cycle of Fo).
I agree with all of the other conclusions in this paper. BTW, I again
want to congratulate you on your excellent experimental AND theoretical
work! It's the "surprises" lurking in some of these measurements that
will help us break new ground and revise modern Tesla Coil theory. Way
to go, Terry!
-- Bert --