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Re: My extended-foil cap failed

Gary,
I've always considered when designing home brew caps, to install a pressure
releif valve. If the cap fails or oil expands, the POOOM becomes a pooom. Less
mess. Also, I think it is important to leave an inch of space in the cap for
heating expansion rather than fill the cap to the rim. It therefore important
to keep these PVC tubing caps upright so that the actual capacitor foil and
poly is saturated in the oil. Laying the cap on it's side will expose the foil
and poly to the air space inducing heating problems and potential arc-though
failure.

I'm currently using professional caps completely sealed with no pressure
releif. But the caps have an arrow with the wording "This End Up". I imagine
the manufacturer is using the same method described above.

I ran my coil for a 5 minute run about a month ago. I was checking variac
temperature's with run time. Sure enough, the variac (as big as it is) did
heat
up very warm. My usual run time is about 30 seconds max. Thankfully, the 5
minute run didn't kill anything. That was a great night!!!! Vissions of ozone
in my head. BTW, never run a coil this long in an enclosed garage! This is an
outdoor activity.

Bart

Tesla List wrote:

> Original Poster: Gary Lau  10-Oct-1998 1204 <lau-at-hdecad.ENET.dec-dot-com>
>
> Hello All,
>
> I was eager to log some time on the extended-foil rolled-poly capacitors
> that I've documented on my new web site.  This capacitor is two units in
> series, each using .04" thick LDPE dielectric.  The caps were housed in
> 4" PVC SDR pipe and had no provisions for pressure relief.
>
> Last night I ran the coil for 1 minute, checked for any signs of cap
> heating, ran for 2 minutes and checked.  The ends were just slightly
> warm.  I ran for 3 minutes and checked.  The ends were again just
> slightly warm.  I was about 2 minutes into the next run when the roar
> diminished, then POOOMMM!  One of the PVC end caps burst, leaving about a
> pint of mineral oil on my garage floor and egg on my face.
>
> A post mortem disection revealed dielectric puncture under the long edge of
> the recessed foil, plus a slight discoloration of the poly elsewhere
> along the recessed edge of the foil, suggesting that the dielectric
> thickness may not have been adequate.  All poly and foil surfaces were
> evenly wet with oil.  Unclear is how long this cap would have survived
> had I limited run-time to a more conventional 30 seconds at a time.  Is
> running 7-8 near-continuous minutes something that is foolish even with
> the most robust capacitors?
>
> I will update my web site with this info, and in the future, recommend
> and use pressure release mechanisms on caps!
>
> Gary Lau
> Glum in Waltham, MA USA
> http://people.ne.mediaone-dot-net/lau/tesla/tesla.htm