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High voltage capacitor experiment?

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kellex

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I paralleled a 40 kv 200 pf doorknob capacitor to the output of a 15000 volt neon sign transformer which was then connected to a 1-1/2" spark gap. I switched on the power and a loud intense arc snapped across the spark gap. I immediately cut the power. My question is; what effect is this discharge having on the transformer, the capacitor and the 120 volt main supply?
 

chemelec

Well-Known Member
Why a 200 pf cap on the Output?
Considering the High Voltage is 60 Hz, it will have almost no effect.

If this is one of the Older, 30 or 60 mA, 15KV Neon transformers, Running that Spark Gap is Not a Problem.
Those Transformers are Current Limited.
 

DerStrom8

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At 60Hz the reactance of the 200pF capacitor is 1/(2*pi*60Hz*200pF) = 13.26 megohms. As chemelec mentioned, it has little effect in the sense that it is effectively a high value resistor across the output of the transformer. However, the capacitor will charge up until the voltage is high enough to jump the spark gap (at 200pF I would expect it to charge up rather quickly, depending on the NST current), at which point all of the stored energy will be released at once across the gap. This is what makes the spark so intense. It will do this at 120Hz with a 60Hz mains input, or 100Hz with a 50Hz mains input.
 

dr pepper

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For the very brief moment the transformer would experience a short circuit.
The transformer will have a high thermal mass so if there usnt too many sparks per second it probably wont come to any harm.
 

DerStrom8

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Most Helpful Member
For the very brief moment the transformer would experience a short circuit.
Good point, and the amount of time it sees that short circuit for is determined by the current output of the transformer. But again, NSTs are generally current-limited so this most likely wouldn't be a concern, regardless of the capacitor size.
 

tcmtech

Banned
Most Helpful Member
Given how a current limiting transformer is made it's entirely possible to get a LC resonance tank circuit going on the secondary that can ramp the secondary voltages up extremely high to the point it will burn out the secondary insulation thus ruining the transformer.

I've killed a few of them playing around like that not knowing what was actually happening.
 

dr pepper

Well-Known Member
Most Helpful Member
Yes, I'm not an 'expert' on the subject however if the tranny has magnetic shunts then this could be a problem using it as part of a marconi oscillator.
So long as the op doesnt have too large a gap things should be Ok.
 
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