Hi Grant,There are other compelling reasons to use multiple layers of thinner dielectric material. Somewhat paradoxically, the dielectric breakdown strength (in volts/mil) significantly increases as you reduce material thickness. Also, defects (present in all dielectrics) will lead to early breakdown. Although commercial capacitors use pristine, almost defect-free dielectric film, most Tesla Coilers do not have access to these materials. However, by using multiple dielectric layers, the odds of any two defects "lining up" are significantly reduced, thus permitting use of less than perfect material.
It is also important that mineral oil (or a similar dielectric fluid) be used to displace air trapped between PE sheets or between the sheets and plates in order to prevent corona. The combination of UV and the chemical reactivity of corona byproducts rapidly degrades and destroy polymer dielectrics. Commercial capacitor manufacturers are careful to design and rate their capacitors so that corona doesn't form during normal operation, thereby keeping voltage stress below the "corona inception" level.
BTW, be sure to use virgin polyethylene film, not recycled material, since the latter may contain small pieces of dirt, metal fragments, etc. that may lead to an early death of your cap.
Bert -- *************************************************** We specialize in UNIQUE items! Coins shrunk by huge magnetic fields, Lichtenberg Figures (our "Captured Lightning") and out of print technical Books. Visit Stoneridge Engineering at http://www.teslamania.com *************************************************** Grant Visser wrote:
<SNIP><SNIP><SNIP> Elated greetings to all from "Darkest Africa" :-) I see that Brian has beaten me to it and forwarded Jeff Behary's response to the list for which I am most grateful! I have appended a copy of his response to the end of this post for any who might have missed it! Just a quick note on why I wanted to add (one or more) "floating" plates into the capacitor when I could achieve the same by increasing the thickness of the dialectric....... With 2 plates; the dialectric material (of any thickness) between the plates would need to take the full brunt of the applied voltage. In the theoretical instance of a series stacked plate capacitor, I would be able to use much thinner dialectric (in my case 3 layers of 0.5mil polyethylene for a total of 1.5mils) between the plates because the applied voltage would be SPLIT (almost equally) across the series stacked plates. ie. Building a single capacitor stack to achieve the same as connecting many in series with external wiring thus reducing complexity and increasing overall reliability as a result of the simplification. A more compact system with fewer external connections should also have a very positive effect on the performance of the capacitor bank as a whole. My thanks to Brian for his initial comments and for pointing me in the right direction! My thanks also to Gary for his input on this topic and for pointing out that all dialectrics have a "Corona Inception Voltage" regardless of their thickness! Very interesting indeed! My thanks also to everyone else who took the time to comment and put their viewpoints forward especially considering how new I am to the list. My most grateful appreciation to you all! Happy coiling! Grant aka FreakyG Randburg, South Africa
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