Opto-coupler failure question (HCNR201)

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.

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?

FWIW, this datasheet for the HCNR201:

.....displays fine with Foxit Reader(freebie PDF reader that takes less resources etc than Adobe Acrobat):

...

other

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

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, 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.

Richard Rasker

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

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?

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?

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

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

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.

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?

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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.

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I once had a failure in a HV isolation section of a scope, stored in a damp environment. Cracking it open, mould had grown over the light tube causing an electrically conductive path between both sides. If mould grew on the face of the light guide it would block the light. Opto-isolator was otherwise sealed , but only to non-military spec. Try cracking one open one and look under a microscope for petri-dish-like mould spots

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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.

Um, I'm afraid xpdf is of little or no use to you -- it's Linux only, and from your reference to Foxit I surmise you're running Windows.

No, in have about double the number of components in the primary circuit as the example in Fig. 15A -- and almost all extra components are safeguards and the likes.

I usually have good experiences with those optical devices as well -- that's also why I'm still a bit puzzled.

But anyway, thanks for you reaction.

Richard Rasker

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

I don't know what typical valueas are, but at Vr = 1V, I got about 1uA of leakage current, rising rapidly with higher reverse voltage. At Vr = 3 volts and up, the LED starts conducting whole milli-amps (current-limited to 2mA) -- so I guess that if the LED wasn't broken to begin with, it is now. But during all this, I monitored both photo diodes, and at no point did they produce any sigificant voltage into 10MOhm voltmeter inputs.

I know, and 20mA max is recommended. But as I said, the current is limited to some 15mA in several different ways. Under normal operating conditions (i.e. with working feedback circuitry), it can't exceed 5mA. And the rest of the original circuit is fine -- I stuck in another HCNR201, and it worked perfectly right away. So no other components have failed, at least not in any permanent manner.

Nope, I just bent the legs slightly inwards for normal assembly, as is usual with DIP cases. And I'm pretty certain that I didn't overheat it either during soldering.

The PCB is quite small (5x8cm, or 2x3 inches approx.) and very well supported. Also, the opto-isolator case doesn't show any cracks or other damage. But I guess I'll try to crack it open, although I seriously doubt if I can find anything -- if only because cracking it open will certainly disturb anything blocking the light path anyway.

Richard Rasker

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

Hehe, spot on -- this is a 24V ship's electrical installation, with heavy DC motors and other possible sources of interference. That's why I also use DC-DC-couplers (with a wide-range input) to supply the rest of the circuitry. That way, input, output and supply are all galvanically separated. But the LED still behaves as a LED -- in an electrical sense, that is. It's not shorted out or anything.

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.

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

Richard Rasker