# Re: [TCML] Pulse capacitor spec question

```Hello Jim,

The real question I have is still unanswered. Is the VR spec based on the maximum DC voltage rating or is it based on whatever peak DC voltage the capacitor was originally charged to before the ring down? Using a simplistic example, if the original peak DC charged voltage on the 50 KV DC rated capacitor was 10 volts and the capacitor has a 20% VR rating, does this mean that the maximum rated voltage reversal is only -2 volts (10 x 0.2)? Or does it mean that the peak voltage reversal allowed is -10 KV (50 KV x 0.2)? In other words does the peak allowable voltage reversal scale with the peak charged DC voltage level or is it fixed according to the 50 KV peak voltage rating?

Steve White
Cedar Rapids, Iowa

----- Original Message -----
From: "jimlux" <jimlux@xxxxxxxxxxxxx>
To: tesla@xxxxxxxxxx
Sent: Sunday, December 30, 2018 9:00:23 AM
Subject: Re: [TCML] Pulse capacitor spec question

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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