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Opto-coupler failure question (HCNR201)

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

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Reply to
Richard Rasker
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On a sunny day (Wed, 09 Sep 2009 20:02:38 +0200) it happened Richard Rasker wrote in :

Perhaps some part internal to the opto-isolater did break of due to vibration and blocks the light path? Did you open the defective one?

Reply to
Jan Panteltje

No, I didn't (yet) -- but isn't this very, very unlikely?

Richard Rasker

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Reply to
Richard Rasker

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? Is the reverse leakage close to typical values?

40mA is allowed for a maximum of 50ns (!).

Might just be bad luck.. but as someone else suggested, this sounds like mechanical damage of some kind to me if the diode appears electrically sound. No deliberate modification of the package such as bending of leads? Could the package be cracked at the leadframe or elsewhere due to inadequate support of the PCB?

Reply to
Spehro Pefhany

Also, check the reflow profile at the assembly line. Since it looks like this is European the higher temps of a lead-free process are much more taxing on such devices than the older (and better) leaded process is.

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Reply to
Joerg

It's incredibly likely compared to what seems to be the only alternative- an LED which acts exactly like a AlGaAs D but doesn't LE.

OTOH, an electrically damaged diode that measured something like a short would not be unusual at all. Could be something like lightning or RF damage. You don't have the opto in there because it's a benign environment, eh?

Reply to
Spehro Pefhany

On a sunny day (Wed, 09 Sep 2009 20:55:49 +0200) it happened Richard Rasker wrote in :

I could also imagine a bind wire coming lose of the photo transistor. Before doing any destructive investigation, perhaps you could also check the photo transistor. Vbe and Vbc diodes, if it has the base on a pin, multimeter + on collector,

- on emittor, and feed the base from the collector via say 100k Ohm. Should draw some current.

Reply to
Jan Panteltje

The HCNR201 isn't a normal opto-coupler; it doesn't have one photo transistor, but two photo diodes, one of which is normally used in a feedback circuit driving the LED. Both photo diodes behave the same, i.e. they don't respond to any current I send through the LED.

Richard Rasker

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Reply to
Richard Rasker

Somehow sounds like a busted LED.

BTW, it's best not to split groups and follow-up fields differently, it mangles your thread.

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Regards, Joerg

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Reply to
Joerg

Hm, I /did/ set the follow-up to sci.electronics.repair. I don't know what went wrong ...

Richard Rasker

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Reply to
Richard Rasker

No, what I meant was don't post in two NGs and then set the follow-up only to one. It splits the thread and also leads to double-efforts, like someone answering while another poster had given the same answer in the follow-up NG (which he hadn't subscribed to).

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Reply to
Joerg

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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Reply to
Spehro Pefhany

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