# Re: [TCML] Pulse capacitor spec question

```On 12/29/18 10:13 PM, Steve White wrote:
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```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?

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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.
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Primary Q on a TC is often around 10, so the VR is around 85% (0.85) - this is pretty high.
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A 20% VR cap, used in an 85% VR circuit will see a life approximately (20/85)^1.6 = approx 1/10th...
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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.
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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.
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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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