Richard Rasker wrote in message
nl.net...
Hi all,
I'm using an HCNR201 opto-isolator device in a galvanically isolated
linear
signal transfer application. I have about two dozen of these in a not
completely unimportant application aboard sea ships, in a rather hostile
environment: the machine room, with heat (40 degrees centigrade),
vibration and moisture. For these reasons, I designed the whole thing to
be
very, very robust, and for a year or so, all was fine.
Recently, however, I got a complaint that one of those devices had failed,
and a bit of research showed that the opto-isolator was the cause: from
the
outside, the LED still behaves like a LED diode, but none of the two photo
diodes produce any output -- which strongly suggests that the LED doesn't
produce any IR output any more.
Now I'm a bit puzzled by this, as the whole input circuitry is designed in
such a way that the opto-isolator LED would be among one of the very last
components to break down in case of a voltage spike or such -- there are
zener diodes, low-ohm SMD resistors and an SMD opamp which would blow
first, and under no conditions, should the total LED current be able to
exceed 15mA (with 40mA absolute maximum rating). Destructive testing with
a
circuit here confirmed this: I managed to blow up a handful of parts --
twice -- but never the opto-isolator. Overvoltage, reverse voltage -- it's
all handled the way I designed it.
So my question: is this a simple case of "bad luck", or are there other
ways
a LED in an opto-isolator may fail in this weird way (current OK, yet no
light)?
Thanks in advance, best regards,
Richard Rasker
--
http://www.linetec.nl
I once had a failure in a HV isolation section of a scope, stored in a damp
environment. Cracking it open, mould had grown over the light tube causing
an electrically conductive path between both sides. If mould grew on the
face of the light guide it would block the light. Opto-isolator was
otherwise sealed , but only to non-military spec.
Try cracking one open one and look under a microscope for petri-dish-like
mould spots
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/