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Re: Another Terry's filter question

Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>

At 09:54 PM 3/28/2006, you wrote:
I recently posted a question to the list about Terry?s filter. I wanted to know 
if my string of twenty, 56 ohm, ½ Watt resistors (in place of the 100 Watt 1k
resistor) would fry when the safety spark gap fired. I mistakenly assumed that 
the safety gap was across the NST rather than the primary gap.  If the safety
gap were across the NST, and fired, the primary cap would discharge back
through each of the 1k resistors, resulting in a large momentary current
through the resistors.  It was this large momentary current that concerned me.
But, since the safety gap is not across the NST, the current that will flow
through the 1k resistor is only the 30 mA of the NST.   30 mA times 56 ohms is
only 0.168 Watts per each ½ Watt resistor.  The resistors are at 1/3 of their
rated power and will survive. Several people replied that the resistors would 
run hot.  Am I missing something?  Also, I do not think that the 1k resistor
will ever see the full 7500 volts of the NST.  7500 volts across a 1k resistor
calculates to 7 amps - from a 30 mA NST??  But again I may be missing
something.  Perhaps the small filter capacitor (charged to 7500 volts) on one
end of the 1k resistor, and the firing (shorted) primary gap on the other side
of the resistor, might momentarily place 7500 volts across the resistor?
For a 15/30 NST the current might really be about 45mA depending on the system and all. That gives 0.045^2 x 1000 = 2.025 watts.
The filter resistors are discharged at every firing through the resistors. The power there is: 

1/2 x C x V^2 x BPS

1/2 x 3300pF / 6 x (7500xSQRT(2))^2 x 120 = 3.7 watts.
The MOVs also act like capacitors normally. I forget the value, but I think it is close to the caps. So they add another say 4 watts.
So if we add it all up, it is about 10 watts. So I think you will be ok with 10 watts of resistors.
For the 100Watt resistors, that is 10% of there rating. With an astounding 300C them rise for 100 watts they will heat to 30C above the ambient temperature.
So, now that I realize that the safety gap is not across the NST, my question is 
why was it designed that way?  Since I want to protect the NST and not my
resistors, why not put the safety gap across the NST?


The more reliable MOVs directly protect the NST.

Cheers,

        Terry




Norman Scheinberg