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My extended-foil cap failed



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