Richard Rasker wrote:
Joerg wrote:

Richard Rasker wrote:
Hi all,

I'm using an HCNR201 opto-isolator device in a galvanically isolated
linear signal transfer application.

...

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.

(I use xpfd; works very fast, never crashes)


Yeah, I have to get something better than this dreaded Adobe stuff.
Foxit doesn't work with all docs but maybe xpdf does.


The circuit is based on Figure 15A (page 11) of the datasheet
(a loop-powered receiver).
In my case, D1 is a 3.3V zener diode, R1 is 10K, and R3 is 10R.
Also, there's a 100R resistor in series with the LED, ...


I was going to say, the figure 15 schematic is pretty hokey there. A
recipe for ... phut ... *POOF*.


... and a 4.7V zener diode
across +Iin and -Iin, plus a small cap (0.1uF) parallell to D1 and PD1.

So any reverse voltage across the input is always kept below 1V, and in case
of severe spikes, the zener diodes and caps should (and do) limit voltages
to below 3.3 volts. In forward mode, with feedback from PD1 shorted out,
the forward LED current maxes out at 15mA. Any increase in input
current/voltage results in first frying the 10R resistor, then shorting out
the 4.7V zener.


That all sounds quite diligent and robust. If the layout is of same
quality the failures are probably more in the category of bad luck, or a
bad batch of devices although I have never had that happen with HP/Avago
in over 20 years.

Thanks to Propman, for posting a working link.

--
Regards, Joerg

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