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Re: Safety Gap Firing



Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Dennis,

All this means is that your main gap fires at a higher voltage than the safety gap. Sounds like your safeties are set correctly so you need to reduce your main gap setting. With non linear (SG) and capacitive (Cp) loads it is very easy to get voltages higher than the NST no load open circuit output. The NST being a current limited transformer looks inductive. When driving a capacitive load, the inductance will series resonate with the capacitance to some extent. With capacitance value lower than a certain LTR value (not sure on the exact value but this can be easily computed), the steady state voltage will be higher than the NST rating if allowed to rise. When the SG fires, an additional transient is introduced with a frequency associated with the natural frequency of the charging circuit (lower than 60Hz for LTR caps). This transient voltage will add to the steadystate 60 Hz voltage and can double the steadystate voltage. So, yes, even with the recommended 1.6*Cres LTR capacitor, the voltage can get much higher than the NST rating if allowed to. That is the main impetus behind the properly set safety gap.

Gerry R.

Original poster: otmaskin5@xxxxxxx
I'd sure appreciate some advice on this problem. My coil seems to run fine as I dial up the variac, but as soon as it hits the 110 volt setting, the safety gaps start firing. I've reverified the optimal primary tap point, so hopefully I've got the tuning right.

Here's some info on the coil that might be helpful:
   * 15/60 NST with a 35kv/0.03 uF Maxwell pulse cap
* spark gap - air-cooled segmented pipe - 6 gaps @ 0.03" each - 0.18" total * safety gap - 3 brass balls (1/4" dia.) - 2 outer balls connected to the NST terminals & center ball to ground. The hot-to-ground gaps are 0.130" or 0.26" total. This was the setting at which the safety gaps just wouldn't fire with only the variac & NST hooked up It doesn't make sense to me that the safety gaps can fire when the spark gap separation is greater. Any help for a struggling semi-newbie?

Dennis Hopkinton MA