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Re: Maxwell capacitor 35Kv 0.03uf

Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi Bert, and Dave (Speck),

Thank you for your explanation, both on-list (Bert) and off-list (Dave).
I can always count on the Tesla list for good answers from well informed
individuals when I have a question ;^) I kind  of had an idea about VR
but didn't fully understand.

More enlightnened,
David Rieben

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, July 09, 2006 6:32 PM
Subject: Re: Maxwell capacitor 35Kv 0.03uf

Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Hi Dave,

The voltage reversal (VR) rating of a pulse capacitor refers to the rated peak-to-peak voltage excursion during an oscillatory discharge. It is commonly expressed as a percentage, and its added to to the DC voltage rating of the capacitor. For example, a capacitor that is rated for 20% voltage reversal and 20,000 VDC faceplate rating can handle 120% x 20,000 or a 24,000 volt maximum peak-to-peak voltage swing. In the case of a disruptive coil, this is the voltage swing between the initial fully charged voltage and the first reverse voltage peak.

High voltage reversals can induce partial discharges or treeing near the edges of the capacitor foil, or within voids or defects within the bulk dielectric. This causes progressive dielectric damage, eventually leading to premature capacitor death. Metallized capacitors are more resistant to this than film-foil caps. Although you can exceed the voltage reversal ratings, it comes at the expense of reduced capacitor life. Jim Lux's site has some very useful formulas drawn from a paper written by some Maxwell capacitor engineers:

The General Atomics site also has an excellent 11 page paper on the topic, "The Effect of Reversal on Capacitor Life". The paper contains a set of derating curves of lifetime versus voltage reversal. Interestingly, this paper also discusses derating as a function of ringing frequency for frequencies higher than 40 kHz (for their large type C capacitors):

When used as a Tesla coil tank cap, you can safely "overestimate" the primary Q by simply assuming that the voltage will have a 100% reversal. If we use the above 20,000 VDC capacitor in a Tesla Coil, we can only charge it to 12,000 volts and still keep the VR to under 24,000 volts when the gap fires.

A "quick and dirty" rule of thumb for polypropylene pulse caps is to divide the pulse capacitor's VDC faceplate rating by 3 to estimate the maximum RMS voltage from your HV source. Applying this rule to the popular 0.03 uF 35 kV Maxwell caps sold on eBay, we can predict that a single cap should work OK when driven by a 12 kV NST, but you should really connect two caps in series if you plan to use a 14.4 kV distribution transformer or 15 kV NST. Note that dividing the VDC rating by a larger factor, say 4 instead of 3, will provide additional margin and significantly extend capacitor life.


Tesla list wrote:
Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>
Hi all,
I'm going to have to jump up and show my ignorance
here but could someone please explain  the concept
of voltage reversals? I've noticed this effect men-
tioned a number of times in reference to pulse capacitors
and I am able to tell the the higher this rating, the better,
but that's about as far as my understanding of it goes ;^\
Thanks in advance for your answer(s).
David Rieben