Re: Ballast damping resistor

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Weazle, All!

>When I put a 5000 ohm, 10 watt, damping resistor across the ballast
>choke, my spark intensity increased dramatically.  Could it be that
>the resistor is snubbing the inductive kick as the field collapses
>in the ballast, whereas before the energy was going elsewhere, to
>the detriment of the spark?  The resistor is just slightly warm
>after a several minute run.


I'd say you've hit the nail on the head.  This is a very close analogy
to the protection filter circuit due to Carter, details of which I
posted ages ago and which is lurking somewhere in the bowels of
hot-streamer-dot-com.  The optimum value of R varies according to the
power transformer primary stray+leakage inductance and reflected Tesla
primary capacitance in the circuit.

Ignore the ballast as regards inductance, what you're looking for is
the inductance seen by the ballast terminal which is connected to the
power transformer primary and looking towards the transformer (if that
makes sense); this will amount to a few millihenries perhaps (comments
welcome on the likely level of stray+leakage inductance in
transformers of various kinds) and all of this will be due to strays
and leakage in the transformer itself, the Tesla primary won't
contribute to this (because it's only microhenries and the resultant
seen in the power transformer primary is equal to these few
microhenries divided by the square of the transformation ratio, i.e. a
few thousandths of not a lot).

Likewise the capacitance is that seen looking into the transformer
primary, this will be significant and will equal the Tesla primary cap
value multiplied (not divided) by the square of the transformation
ratio of the transformer.  If you have a 240V/8kV radar transformer it
will be (8000/240)^2 x the bucket cap value.  The optimum resistance
is then given by R = 1,54 SQRT(L/C), which gives critical damping.
Higher values of resistance give under-damping (i.e. the ballast
"rings" for one or more cycles) while lower values require an
excessive power loss in the resistor and obtain less benefit from the
ballast inductor.

As long as the ballast is at least eight times the transformer
stray+leakage inductance, the resistor will provide damping of the
kickback, and it will have some effect even if it doesn't equal the
optimum value.  My guess is that previous to installing this resistor,
you were getting quite lengthy oscillations in the ballast, and now,
even if the resistance is not that for critical damping, the damping
you are getting is cutting any kickback after just a couple of cycles.

Dunckx
Geek #1113 G3