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# Capacitor Voltage Rating

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#### Tyrone Faulkner

##### New Member
I am wondering what voltage rating I need for the capacitor in the attached circuit. Its basically the front end of a Tesla coil. The voltage source is a neon sign transformer 3kV open circuit at up to 15ma short circuit (2kV 10ma under load), 30K Hz. This is what I understand about the circuit. The voltage is rectified to pulsed DC via the full wave rectifier bridge. The capacitor acts like a short circuit so the nst effectively pumps the capacitor up to the breakdown voltage of the 2nd neon lamp (90V). When the breakdown voltage is breached the capacitor unleashes a 90V spike into the inductor. I'm not sure what roll the first neon lamp (spark gap) plays other than delivering a current rush to the capacitor.

If the AC from the nst is a sinewave the capacitor would receive about 0.182V in one pulsed DC period (t=0 to t=16.6 uS) (disregarding the effect of the first neon lamp). I'm thinking a capacitor with a voltage rating of at least 200V?
Two Questions:
1. What is the purpose of the first neon (spark gap)?
2. Would a capacitor with a voltage rating of around 200V suffice?

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There are many factors affecting the voltage across the cap, probably the biggest being reflected voltage from the primary, and that depends on coupling factor, turns ratio, spark gap length, temperature, probably the best way to get it working it put a high rating cap in like 1kv, and protect it with a spark gap limiting the voltage across it and the output of the system.
Tesla coil questions dont seem to appear on this forum that much.

I've just ordered a microwave oven cap. They're rated at 2100V.

Beware microwave caps can be dangerous.

Beware microwave caps can be dangerous.
How are they more dangerous than other similar rated caps?

Microwave caps are not particularly worse than any other, maybe not a good phrase to have said, its the voltage and energy stored within, 2kv would give you a really bad jolt, not something to mess with really, you need the right conditions & gear.

I will handle with care. Made myself a cap discharger and have a pair of rubber safety gloves. However I would be surprised if this circuit charged it to anything like 2KV.

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