Hi Bill,
Really an exciting story ;-)
I think once the TL084 got damaged it acts like a rather low impedant
part and will carry several mA or A, thus it smells after a long time
;-) It's a questing of the dissipated power and energy. The E field
probe was protected against overvoltages and as Roelof says, it is
rather immune to short impulses and protects itselfe a bit by becoming
conductive. The 1N4148 would have protected the amp input even if the
distance to the strike would be 1m i bet :-)
73, Stefan/DK7FC
Am 27.07.2011 19:24, schrieb Bill de Carle:
A couple of nights ago a thunderstorm came through here just after we
had gone to bed. We thought the storm had passed but then there was a
very loud bang. I heard the crackle and sizzle of electricity and the
boom almost simultaneously. A few minutes later fire trucks came so I
got up to investigate. Turns out the bolt hit a tall pine tree on my
neighbour's property. What remained of the tree was on fire. Our
telephone was not working but the mains power stayed on. I thought
for sure the next morning I'd have to replace blown out parts. At the
time of the lightning strike my antennas were disconnected (outdoors)
from the house because we knew the storm was coming. I was concerned
about a preamp for some VLF loops back in the woods and a
newly-installed mini-whip (J-310 front end) on the garage roof. I
measured the distance to where the lightning hit: 150 M from the
struck tree to the mini-whip and 200 M to the loop preamp. I checked
the preamp first: it was dead. Burned IC (TL084, I could still detect
the smell in the morning). When I tested the mini-whip I was amazed
it still worked fine! The loop has protective diodes (1N4148) across
the hi-Z ends and those diodes also survived the strike, which was
unexpected. Starting at the base of the struck pine tree one notices
a newly-made straight line trench in the ground (about 30 M long)
heading towards the nearby river. Evidently the current was making
its way to the water. I'm wondering why the mini-whip (closer to the
strike) survived but the TL084 did not. The preamp drives a buried
cable (about 50 M) to my radio shack. That cable (no DC path to
ground at either end) runs in a direction parallel to the trench made
by the lightning but displaced at least 100 M. Could enough voltage
be induced to burn up a TL084?
Bill VE2IQ
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