On Thu, 10 Sep 2009 19:27:41 +0200, the renowned Richard Rasker
wrote:

Spehro Pefhany wrote:

On Wed, 09 Sep 2009 20:02:38 +0200, Richard Rasker
wrote:

Hi all,

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

[snip mysterious breakdown]

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)?

When you say that the LED behaves like an LED diode, you mean that Vf
is just what you'd expect from figure 9 of the datasheet?

Hmm, I get a Vf of 1.3V @ 10mA If, so that's rather low, but still just
within specifications.

Typical is 1.3V at 10uA, and 1.55V at 10mA. That's a rather
significant deviation! (3 orders of magnitude in If)

Have you compared a fresh one?

AFAIUI, some modern LEDs (Double Heterstructure) etc. are more complex
than simple diodes and it seems possible this one has been damaged but
still retains diode-like characteristics.

material.eng.usm.my/stafhome/zainovia/EBB424e/LED3.ppt

I'd look a lot closer at the possibility of something bad getting in
to that LED (have you shunted it with a Si diode in reverse?).



Best regards,
Spehro Pefhany
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