Measurements using field probe

From:  terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
Sent:  Friday, June 26, 1998 5:55 PM
To:  tesla-at-pupman-dot-com
Subject:  Re: Measurements using field probe

Hi Bert,
	My comments follow.

>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
>circuit wiring". 
>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. 

Excellent Point!!  The gap appears to extinguish due to the current dropping
to zero.  As the voltage again rises, The gap will have to reignite.  Even
though the gap is still very hot, there appears to be much commotion
involved in getting the gap going again.  Since this paper was written, I
found that my estimate for the primary self capacitance was way too high.
It appears that the parasitics of the wiring and such may affect the
re-ignition dynamics much more than the primary turn-to-turn capacitance.
Also,  The high current spikes shown may be due to the fiber-optic LEDs
saturating.  When the LEDs see high-frequency high-current pulses, instead
of faithfully reproducing those signals, the LEDs saturate and just emit
continuous light.  This appears as very high current on the scope but is
really just the LED sticking on.  I have experimented some with ways to fix
this but I have not figured it out yet.  Should be easy but the antenna
probe took time away.  If you notice The spikes are always positive
indicating that there was something wrong.  Also, when I calculated the
energy of the spikes The calculations produce numbers that are off by orders
of magnitude.  Still more work to be done...

>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. 

The top terminal capacitance filters out the spikes but the currents
measured with the field probe show the spikes are entering the secondary.
There are 90 degree out-of-phase spikes in the secondary current waveforms.
This would be expected if the current spikes are being circulated throughout
the coil.  There may be some evidence that these spikes in the secondary may
contribute to arcing but that is a very early observation.  There is a
chance that the antenna is picking up the spikes from the primary system so
there is more testing to be done to confirm this.  Fiber optic-probes in the
top terminal do not seem to show the spikes.  The secondary system should
strongly reject these far out of band events.  Still questions to be answered.

>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 original model had the tank inductor self capacitance way over
estimated.  There seems to be strong evidence that the spikes and what ever
goes into their production is draining energy from the system and producing
the linear decrement waveform.  Still more details to be investigated.  What
is needed is a better way to measure the waveforms with high bandwidth that
also produces realistic energy budget calculations.  Either the fiber-optics
need to be improved or perhaps a modified antenna system could be used.  So
far I have been totally impressed with the antenna's performance.  I know of
no problems with it.  It would be advantageous to make it much more
directional to isolate certain events.  Perhaps a tiny version could be used
to probe the primary system.

>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). 

Better testing is needed that can measure the voltages and currents
accurately enough to determine where, how, and how much energy is being
dissipated.  Still more work to be done here.

>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 --  

Many thanks for your comments!  The points you raise are valid and correct.
As I do these experiments, I see many areas that need further investigation
that I am unable to pursue do to lack of time.  There are many questions
that are unanswered any many more questions still to come.  Fortunately, the
antenna system is much simpler and cheaper than the fiber-optics and is
giving excellent results.  This allows others to pursue these question
independently as they wish.  I strongly encourage others to do their own
testing into the areas that interest them.  I will never have time to
examine all the questions these new probes have raised by myself.  I am
working on the higher powered secondary measurements and such now which
appears quite complex but the measurements appear very accurate.

All the best,

	Terry Fritz