Re: [TCML] Sudden and unexplained destructive racing spark flashovers

[DC Cox <resonance@xxxxxxxxxxxx>]

Solid state coils prefer tighter coupling, usually because the input
potential is much lower and the gain has to be a bit higher on the system.

I never go above 0.18 in my solid state designs.  some guys go 0.16 to 0.22
but this usually ends up reducing the output a bit.  0.14 to 0.18 values
seem to be a nice sweet spot
for DRSSTCs and MOSFET powered coils.  Pushing it above that value is risky
and usually diminishes the sec spark length.

You just can't beat a small gear motor driving the sec up and down via some
thick walled tygon tubing.  You can see the sweet spot for your particular
coil design.  Then, later,
use the 10 Amp hot water heater element process (in the archives) to take an
actual coeff. of coupling measurement.

Steve Ward's method of using the Envirotex epoxy, 1/2 to 3/4 inch thick on
the sec, actually allows an occassional sec strike.  The energy is coupling
capacitively thru the
epoxy and usually does not produce a hot pinpoint strike thus enabling the
coil to survive.  The energy is spread out across a few turns and doesn't
burn th sec coil at the point of strike.

Dr. Resonance

On Sun, Jul 5, 2009 at 2:09 PM, Dave Leddon <dave@xxxxxxxxxx> wrote:

> I've found your elevation estimates to be right on the money for every
> disruptive
> coil that I've built but I've noticed that solid state coil builders
> typically use
> couplings greater than your upper limits.  Have you seen the same thing in
> your solid
> state coil designs?
>
> Dave
>
> At 09:04 AM 7/5/2009, you wrote:
>
>> I'm just reporting from practical experience in coil building.
>>
>> If you exceed 0.14 in classic type coils, especially around 0.18 and
>> above,
>> racing sparks will occur.
>>
>> There is no reason to exceed this coupling value.  If you do your output
>> sparks will diminish in length, not grow.
>>
>> I've attached a flexible shaft coupling to the base of various sec coils
>> and
>> dynamically elevated and lowered them while the coil is running.  Various
>> size coils, 4" dia to 24 inch dia.  In all case max spark length is
>> achieved
>> around 0.12 to 0.14 range.  I've carefully measured the k value at the
>> setting that we achieved the long spark length for a given coil.
>>
>> Oddly, it appears to be nearly a linear relationship.  A 24" dia sec
>> performs best with 9" of elevation.  A 18" dia. sec performs best with 6.5
>> inches of elevation.  9" dia sec works best with 3.25" of elevation.
>>
>> At around 0.18 to 0.2 range most coils started to exhibit signs of
>> excessive
>> corona and occassional flashovers.  I've repeated these experiments with
>> coils sizes ranging from 4" to 24 inch dia and the results seem to be the
>> same.  Excessive coupling produces diminished output spark lengths.
>> Carefull measurements with oscopes indicated the frequencies were starting
>> to split indicating power was being wasted by not going into the
>> fundamental
>> resonant freq of the sec inductor.
>>
>> Dr. Resonance
>>
>>
>>
>>
>> On Sun, Jul 5, 2009 at 3:15 AM, Dex Dexter <dexterlabs@xxxxxxxxxxx>
>> wrote:
>>
>> > It's an experimental fact that too high coupling can be the cause of
>> > flashovers and racing sparks but...The point of max potential is at the
>> > terminal (top load) of the secondary in 1/4 wave resonance coils.Typical
>> > couplings used (0.1-0.2 range) can be hardly  high enough for the point
>> of
>> > max potential to be shifted somewhere else.Perhaps you are referring to
>> > increased secondary local voltage gradients vs higher coupling
>> > instead?That's a different thing and more possible to occur I think.
>> >
>> >
>> > Dex
>> >
>> >
>> > --- resonance@xxxxxxxxxxxx wrote:
>> >
>> > From: DC Cox <resonance@xxxxxxxxxxxx>
>> > To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
>> > Cc:
>> > Subject: Re: [TCML] Sudden and unexplained destructive racing spark
>> > flashovers
>> > Date: Mon, 29 Jun 2009 20:19:02 -0700
>> >
>> > Over coupling is one cause of racing sparks.  It produces more than one
>> HV
>> > point near the top 1/4 of the coil which can cause local ionization and
>> > eventual flashover in this area.
>> >
>> >
>> >
>> > Dr. Resonance
>> >
>> >
>> >
>> >
>> >
>> >
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