I have some 25kv caps that are specially made for 80% reverse current but unfortunately they are 2.57uf and 100 lbs each so don't suppose anyone is building a coil large enough for those!!!Jim Heagy -----Original Message-----
From: Steve White <steve.white1@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Mon, Dec 31, 2018 12:11 pm
Subject: Re: [TCML] Pulse capacitor spec question
Thanks to all for responding to this question. I think that I finally have a handle on this. My original question about the meaning of the VR spec was prompted by wondering how long my Maxwell capacitors would last on my coil. The VR spec is not very specific. For example, for my 37667 capacitors, the VR spec is stated as 15% not to exceed 50%. That is rather ambiguous to me and was not stated what the reference point was.
So, using Jim Lux's link, I did the lifetime calculations using the supplied equations. Using the 15% VR spec, my peak DC charged voltage of 19 KV, and the peak voltage spec of 70 KV (6 capacitors in series-parallel), the expected life time came out to be 40,000 hours! This is assuming a new capacitor. Since mine are used, if I assume that the life time was half gone when I got them, it still computes as a 20,000 hour remaining lifetime. I am fairly confident that this number exceeds my expected usage for this coil during my life time. :-) I am presently only about 4 hours into that 20,000 hour life time.
What I have learned is that the effect of VR is dependent in a highly non-linear fashion (exponent of 7.5) on the peak charging voltage compared to the maximum voltage rating of the capacitor. This is the dominant effect. It swamps everything else.
Steve White
Cedar Rapids, Iowa
----- Original Message -----
From: "David Rieben" <drieben@xxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Sunday, December 30, 2018 11:25:25 AM
Subject: Re: [TCML] Pulse capacitor spec question
Steve,
In lieu of Jim’s or Bert’s response, I believe I can answer that question. The voltage reversal is based upon the rated DC voltage of the capacitor. So, for example, if you are running a 50 kV (w/20% VR) rated pulse cap with only 10 kV peak for a coil, then even with 85% VR at 10 kV (8500 volts), that would still be < the 20% VR rating of the cap’s 50 kV rating. Of course, you would also need to figure in the 1.41x RMS conversion to peak value for making this calculation as well. At least that’s my understanding of this. I’m sure Jim or Bert can chime and correctly me if my reasoning is askew, though.
David
Sent from my iPhone
> On Dec 30, 2018, at 9:45 AM, Steve White <steve.white1@xxxxxxxxx> wrote:
>
> 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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