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)?


Both LED and photodiodes can die upon rather small reverse voltage
spikes. Without seeing your circuit it's hard to say which one is
vulnerable. I also don't know abs max because this miserable Acrobat
Reader crashed on that particular datasheet when scrolling.

--
Regards, Joerg

http://www.analogconsultants.com/

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