Re: [TCML] VTTC-tuning. John got me started ;-)

[DC Cox <resonance@xxxxxxxxxxxx>]

Actually, we can measure the top terminal potential of any Tesla transformer
with reasonable accuracy.

In a recent textbook on HV engineering, authors state (after many tests and
measurements):

"HV max potential across a sparkgap is independent of waveform."

I should qualify this statement.  If RF, the spark has to be pulsed, not
continuous so it can "grow" to long lengths.

Years ago I set up some tests using an ignitron to produce single shot
pulses in an air core transformer.  Another, easier way for the average
experimenter to do this would be
to use a 10-20 MEG HV resistor to charge the HV cap off a rectified DC HV
power supply.  Adding diodes to the output of a pole xmfr and use 20 meg
resistors.  Be sure to discharge the cap
and short it out before touching any circuit that has had a cap charged on
DC, not AC, potentials.

This produces a single shot pulse of HV off the sec coil every 5-10
seconds.  I used a standard 1/2 inch rod gap off the toroid to fire to a
ground electrode which was also a 1/2" rod gap.

By comparing my distance measurements to 1/2 inch dia rod gap charts for
impulse generators, the HV was the same.

I double confirmed this by using single shots across a large 1 MEV antique
X-ray tube, and be careful measurement of X-rays, was able to confirm the
voltage was
equal to my simple spark gap measurements.

A coil producing 15 ft. sparks at 15 kVA, showed a max peak potential of 985
kV.  The spark gap distance in single shot pulsed DC mode was only 6 ft.
Much less than the full power
operation.

The math, however, is the same.  A coil produces the same potential whether
it is running in single shot mode or operating at 500 pulses/sec.  The spark
at high rep rate is much longer
due to the continuous pumping of the ion field above the coil, and also due
to certain metastable decays of ions in or near the spark channel.

Be comparing sparkgap measurements with X-ray measurements, and also Terry
Fritz's work, a simple equation emerges:

Vsec = Vpri x 70% x SQR (Ls/Lp).

My measurements were also in close agreement with pulsed field mode
measurements made on an "antenna" system developed by Terry Fritz several
years ago  Terry was kind enough to lend me his antenna system, and I did a
calibration with a 200 kV DC X-ray power supply on a toroid atop an unground
Tesla transformer.  The antenna was calibrated accurately at 200 kV with it
being placed at a distance of 12 feet from the toroid.  Then by running the
coil in single pulsed DC mode we could take calibrated measurements of the
max peak output.

All the math analysis techniques were reported in several references in Mark
Denocoli's paper on Resonance Transformers.  See the appendix of his
excellent paper.  Many of the equations worked out 50 years ago by Drude,
et. al., are still accurate today.

So, you can take all kinds of measurements, some simple, some complex, but
the equation listed above will be a reliable indicator of your coil's
potential.  All you have to do is hook up a nice Wavetek meter, take the
inductance measurements, then do some math based on the spark gap firing
voltage, and you have the answer.

One word of caution in your calculations:   Don't take a 12 kV NST and
assume the pri spark gap potential is 1.4 x Erms, ie, 16.8 kV, if your total
spark gap is set up at 200 mils (0.200").  It will only be a fraction of
this value because the correct Vpri to use is the "measured sparkgap
breakdown potential", not the max Epeak value of the pri power transformer.
In summary, Vpri is dependent on the total sparkgap setting in mils.

A few years ago I made careful measurements of sparkgap breakdown potentials
from 5 kV up to 26 kV (above 26 kV erratic corona and Trichel pulses made
measurements difficult at close gap settings), and posted this vs. the mil
spacing of the 1/2 dia. rod sparkgap.  This data is posted somewhere in the
archives for retrival if you're interested in the full chart.  This chart
and your inductance meter is all you need to get an accurate estimate of
your coil's output potential.  And the beauty is these measurements are
taken with the coil not operating.  Even outputs for new designs run in
JAVATC can be accurately calculated.

Regards,

Dr. Resonance







On Thu, Apr 9, 2009 at 6:43 PM, bartb <bartb@xxxxxxxxxxxxxxxx> wrote:

> Dex Dexter wrote:
>
>> I  measured nothing.
>>
> Ok. Then di/dt is based on some predictions? You stated that it works out
> near to DC's equation, so I assumed you had made a measurement.
>
>> And even if I measure current ,how to measure top volts to determine what
>> efficiency factors enter the formula?
>>
> Difficult for a direct measurement. DC Cox does have a method which he uses
> for approximation, but I have not tried it myself. Unfortunately, I don't
> remember the details and can't find any info about it on my pc.
>
> How did you and others found factor of 0.7 for a typical spark gap tesla
>> coil?
>>
> DC I expect is looking at the equation V = Vp*sqrt(Ls/Lp) and comparing
> those results to his measured approximations and inserting the efficiency
> factor of 0.7 as an average over whatever series of measurements were made
> (and this is only an assumption on my part). As you know, losses are not
> part of the equation but they exist.
>
> For Lp, the inductance at low frequency is fine. For the secondary, it
> would be more accurate to insert the high frequency inductance. In many
> coils, it's not a large change, but there are geometries where the distance
> between the two do become significant. As long as you always use the high
> frequency inductance, then your good for all geometries. As you know, the
> top terminal affects the peak potential due to breakout. Note that this
> equation or anything similar will never account for the top terminal
> geometry and the surface field developed due to the charge and where corona
> inception occurs followed by breakout. This is a physical limitation that
> should also be accounted for, but until we can directly measure the top
> voltage at the terminal of a Tesla coil with accuracy, then we can only make
> educated guesses.
>
> Take care,
> Bart
>
>
> Dex
>> --- bartb@xxxxxxxxxxxxxxxx wrote:
>>
>> From: bartb <bartb@xxxxxxxxxxxxxxxx>
>> To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
>> Cc:
>> Subject: Re: [TCML] VTTC-tuning. John got me started ;-)
>> Date: Wed, 08 Apr 2009 19:00:50 -0700
>>
>> Hi Dex,
>>
>> Moderators are not going to change an equation. They may later reply and
>> correct it with a reply, but they don't need to be messing with a persons
>> equation (what is obvious is subjective to how much attention is being paid
>> to the equation). Their looking for more of content as it relates to the
>> TCML and not necessarily every calculation detail.
>>
>> DC's equation as you state is a standard energy equation that has been
>> around forever. What DC is doing is showing are losses with the efficiency
>> factor of 0.7. The same equation is derived with the capacitances as well.
>> Your equation is dependent on di/dt. Have you measured this yet? If yes,
>> what was the change in current and over what time frame (0 to peak). May as
>> well state L also. This equation requires measurement to verify, so I can't
>> go any further without some data if you have it.
>>
>> Take care,
>> Bart
>>
>>
>>
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