Hi Bill once you get outside the spec area some devices perform somewhat
oddly. Discrete transistors will take quite a lot of punishment if the
actual energy is not too great...i.e high spikes but very narrow. Even the
fine connecting wires (.001 in,) will carry amps for a very short period of
time. ICs on the other hand being a more complex stucture can be damaged by
quite low voltages. I think the TL084 is a bi-fet, and I am not very
familiar with that technology, but many ICs will fail if the inputs pins or
the ground pins at taken above the supply pin. On CMOS this can trigger a
catastophic Vdd Vee short by a kind of SCR structure switch on. I have seen
the same with 6116 static memories in a microcontroller attached to and
controlling a mains powered oven, The oven got a grounding fault and current
passed through the negative side of the thermocouple, mux, ADC and micro
card through the psu to ground. The only failure in all that was the 6116
CMOS static memory. The pressure inside the encapsulation was enought to
shatter the moulding a lift a small region of plastic above the chip. No
other devices needed replacing, and after making sure that we blocked the
"alternate" ground path through our controller (100R in series with the
t/c), the unit operated for many more years without trouble.
You didnt say whether your op-amps were powered? ..... I suspect not but the
generation of a large voltage across the loop might be enough to to lift the
input to a destructive level, and decoupling caps can store a lot of charge.
Alan G3NYK
----- Original Message -----
From: "Bill de Carle" <[email protected]>
To: <[email protected]>
Sent: Wednesday, July 27, 2011 6:24 PM
Subject: LF: Lightning Story
> 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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