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Re: Corona Loss? AAaaahhhh!!!
> Original Poster: "Yuri Markov" <wmondale-at-hotmail-dot-com>
>
> I'm confused, to say the least.
> I recently posted a message asking about capacitors, and what is and
> is not workable. About half the responces I got mentioned corona
> loss. What is corona loss? What causes it? Why is it a problem? And,
> of course, how do I prevent it?
Corona is produced when high voltage finds a sharp point or edge in your
conductive path. It will create a violet glow as the energy leaks out
of the conductor. Capacitors, with their thin sheets of plate, have
PLENTY of sharp edges and points to leak, unless you take steps.
The most common method of eliminating corona in a cap is to cut radiuses
on the corners of the sheets and to submerge the cap in oil. A sharp
corner on the sheet creates a point that concentrates the corona. The
oil does not allow the flow of ions nearly as well as air, preventing
breakout. The bug-a-boo there is to make sure that all the air is out
of the cap.
It doesn't take much corona to soften plastic. (In fact, it is common
practice in the manufacture of plastic [PE] tubes [toothpaste, skin
cream, shampoo, etc] to soften the plastic with a corona from a blade
just before the decoration is printed. This lets the ink penetrate the
plastic. Then they use UV light to cure the ink.) Once the PE or PVC
has softened, an arc will puncture the dielectric, and your cap has
become a doorstop. The hole, and possibly a carbon track, will continue
to allow the cap to discharge. You could even have one catch fire, if
you let it continue.
Glass is more spectacular when it fails. The uneven heating will make
the glass crack, or sometimes even explode.
Ron Beck, TCBOR charter member