On 12/29/18 10:13 PM, Steve White wrote:
I want to confirm my understanding of the maximum VR (voltage reversal) specification. If the spec states a maximum VR of 20%, I assume that this means that the capacitor can safely handle a 20% reversal of the maximum DC voltage spec. For example, if I had a capacitor with a maximum DC voltage rating of 50 KV and a maximum VR of 20%, then the capacitor could safely handle up 10 KV of reverse voltage or -10 KV volts. Is my interpretation correct?
the VR spec applies to a circuit where there's an exponential ring down over multiple cycles (this is true of most tesla coils). That is, the pulse looks like cos(2*pi*f*t) * exp(-b*t). The VR is the height of the first "negative" peak assuming it was charged to the full positive voltage.
Primary Q on a TC is often around 10, so the VR is around 85% (0.85) - this is pretty high.
A 20% VR cap, used in an 85% VR circuit will see a life approximately (20/85)^1.6 = approx 1/10th...
The shorter life is because the dielectric/plates is being mechanically stressed a bunch of times- first it's charged and pulled one way, then it's charged and pulled the other way, etc.
Or, another way to look like it is that the damped sinusoid is just like a series of pulses. The first at 100%, the second at 85%, the third at 72%, the next at 61% and so on.
BTW, there are different exponents for the voltage for other dielectric systems. I can't remember what they all are, but they sort of range from 6-9. It's somewhere in the list archives from Bert, I believe.
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