On Sep 9, 11:02*am, Richard Rasker wrote:
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

FWIW, many years ago were designed a high voltage control system that
needed 6000V isolation between the controller and the controlled
circuit. the intended environmental conditions were similar,
especially the high vibration. We chose the GE H11A1 optocoupler since
it was one of only a few at that time that was rated for 6000V
standoff. The system worked as designed but under a lengthy series of
environmental tests, we began to see failures of the optocouplers.
That was traced to a drastic reduction of the 'current transfer ratio'
of the device and by slicing several new and failed devices we were
able to track it down to the emitter which, as in your case, looked
normal electrically, but had much reduced IR emission [as low as 1% of
new]. It was finally determined as a process issue at GE and later
'improved' batches did not display such behaviour. The system was
released and worked well for many years with a low failure rate.

Neil S.