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Cap Failure
All,
This message is intended to remind newer coilers that there are numerous
risks involved with coiling. Explosive capacitor failure is one of
these. Make sure that proper physical containment is used, not only to
protect you from ejected debris, but to also constrain the movement of
the high voltage lines going to the cap. Coiling IS an inherently
dangerous hobby! Experienced coilers each have their own set of "war
stories". Please take the time to (re)read the Tesla coil safety FAQ at
Chip's site http://bhs.broo.k12.wv.us/homepage/chip/safety.htm
Cap Failure:
============
Don Froula came over this evening to see my 10" coil in operation, and
also to try out his Condenser Products 0.025 uF 20 KV capacitor that he
had purchased as part of the last group buy, but had not had the
opportunity to try out. After a number of extended experimental runs to
demonstrate the coil, we hooked up Don's cap in place of the home-made
caps. The primary tap was readjusted for the slightly higher cap value,
the gap setting was reduced from 18 to 13 gaps (7 static, and 6 vacuum
for about 0.40" total), and the coil run at varying power levels.
Performance at this setting was quite good, with sparks easily reaching
48" long.
Several brief runs were made at varying input power settings. Because of
the smaller gap setting, gap saturation was occurring at higher input
power levels, and the PPS rate was probably in the 360 - 420 range. The
coil was shut down, and the capacitor checked for any signs of heating -
none. During the next run, power was adjusted to about 80% (about 112
volts) on the 15 KV 120 MA neon tansformer source, with good coil
output, when after about 20 seconds one end of the capacitor was
suddenly and explosively blown off with no prior warning. A loud "pop"
and a yellowish colored flash were heard and seen, and capacitor oil was
ejected in the immediate area.
As Ed Sonderman had previously described, these capacitors consist of a
number of identical rolled capacitors interconnected by soldered netal
straps. The metal looks to be tin(?), and these straps are soldered to
the end foil of the individual capacitor rolls as well as the capacitor
end terminals. The individual cap rolls look to be about 4" in diameter
and about 3-4" long, and are connected in series.
The PVC area around the plastic endcap and terminal was fractured, and
the endcap blown off with enough force that it apparently pulled off the
soldered connection between the metal strap going from the end terminal
to the end foil of the outermost capacitor roll. The last roll was
physically ejected from the capacitor housing, but was still
electrically attached via strapping to the next capacitor roll.
Under closer inspection, it was not obvious exactly what caused the cap
to blow. However, this appears to be the third cap in the earlier group
purchase that has failed in this manner, in this case at a relatively
low power level (2.5 KVA). Don will post some pictures of the cap so
that others can see the construction technique and failure mode. He's
also going to contact Condenser Products to get a replacement, since it
should be covered within the 1 year warranty period.
Thanks to previous posts on this conference, we were aware of the
possibility of explosive failure, and had taken precautions to orient
the cap so that it would do no damage if it blew (other than make an
oily mess :^)). It appears that there may have been a problem with some
of the CP capacitors in this lot, since I've not heard of any other
similar CP failures in a while, and none outside of this lot...
Safe (and hopefully non-explosive) coilin to ya!
-- Bert --