nesesu wrote in message
...
On Thursday, December 20, 2012 2:50:30 AM UTC-8, N_Cook wrote:
2x 250V , 2.2uF yellow epoxy sealed polyester? caps used as intermediary
high DC storage in an electric fence unit. Failed as one of these caps is
fairly consistently ohmic at 370 ohm. Now repaired 195V dc over them ,peak.
Used in the wettest part of the UK , Cumbria, so assumed due to condensation
getting inside the cap, no direct rain ingress traces seen. But removing the
cap casing and pulling back the leads , no trace of green/corrossion.
Heating , with low heat of a hot air gun , ohms drop to 350 and then back up
again on cooling. Failure due to damp or HV punch through? . This is PbF
solder but is RoHS a consideration inside such caps , ie dendrite formation?
If punch through then does the oscillator need knocking back a bit, 2
presets on the board. Replaced both of these caps and another one used in
the pulse shaping drive to the "pulse Tx" , the usual backwards mains Tx.
Thinking perhaps if unit is in direct sunlight perhaps the pump circuit
gives out 300V say. Schematic not seen but HV zeners in that area 270V + 16V
plus another , voltage not seen but probably 16V. I was inclined to drop the
value of the 270V one but I assume these are fudge values, "selected on
test" for nA leakage at less than combined knee voltage of about 300V and
has been working for 5 years , then presumably ok to leave as is , as these
zeners DVM diode test at least as normal.

As you say, there are several factors involved here. Indeed it could be
moisture ingress or it could be punch through. The punch through could be
from a spike or just a weak point in the dielectric that failed under long
term stress somewhat below rating. Since this is in a fence charger I would
also consider lightning. I have repaired a couple of fence chargers that
suffered from induced transients from nearby lightning, and in both cases
the caps had failed to high leakage. One unit was an electromechanical type
where the cap across the points had gone leaky, the other was similar to
what you have and the storage cap was blown.

Neil S.

++++


I've come across a near lightning struck one before and it knocked out a
custom PIC/ASIC with no work around. I would gave expected that more likely
than high V caps failing from spikes. This 2006 made unit oddly could have
be made in 1976 all discrete , assuming a Programmable Unijunction Transitor
for the SCR was available in 1976 . I had asked the owner about lightning
in area and he reckoned not.
I wonder what other test to determine if punch through or metalisation
creep/dendrite ? only for professional interest reasons, I'm reasonably
content to return, as is , without any mods. As lightning is "act of God"
cannot sensibly mitigate against that. One advantage of old circuits -
nearly always repeairable