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Opto-coupler failure question (HCNR201)

Discussion in 'Electronic Repair' started by Richard Rasker, Sep 9, 2009.

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  1. 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
     
  2. Joerg

    Joerg Guest

    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.
     
  3. propman

    propman Guest

    FWIW, this datasheet for the HCNR201:

    http://www.datasheetcatalog.com/datasheets_pdf/H/C/N/R/HCNR201.shtml

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

    http://www.foxitsoftware.com/downloads/index.php
     
  4. (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
     
  5. No, I didn't (yet) -- but isn't this very, very unlikely?

    Richard Rasker
     
  6. 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?
     
  7. Joerg

    Joerg Guest

    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.
     
  8. 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?
     
  9. N_Cook

    N_Cook Guest


    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
     
  10. 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
     
  11. [snip mysterious breakdown]
    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
     
  12. 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
     
  13. 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
     
  14. Joerg

    Joerg Guest

    Somehow sounds like a busted LED.

    BTW, it's best not to split groups and follow-up fields differently, it
    mangles your thread.
     
  15. Hm, I /did/ set the follow-up to sci.electronics.repair. I don't know what
    went wrong ...

    Richard Rasker
     
  16. Joerg

    Joerg Guest

    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).
     
  17. Joerg

    Joerg Guest

    Here is a short article about LED failure modes:

    http://www.emsnow.com/cnt/files/White Papers/DFRLEDFailures.pdf

    I am no expert on this but have done a fair bit of work with laser
    diodes, including ones in the >$1k class. It only takes microseconds of
    mishap and a $1k laser diode becomes a $1 LED. We also had cases where
    the diode looked quite normal electrically but only a miniscule or
    absolutely no optical energy was generated by it anymore.
     
  18. OK, thanks, this is quite interesting. For the time being, I'll just wait
    and see if this failure repeats itself (and try to be even more careful
    handling and soldering the devices).

    Best regards,

    Richard Rasker
     
  19. Joerg

    Joerg Guest

    Reminds me of a joke. No, no, this is _not_ meant to apply to your case.

    A bowling group returns from a road trip. Coming down a pass the brakes
    on the car fade. 40mph ...50 ... 60 ... some guys start to scream. The
    driver steers towards the guard rail, lots of sparks fly, some more
    screaming, a passenger faints, vehicle scrapes to a stop, all smoking.
    Everybody evacuates. One guy, an engineer, looks at the mess: "Tsk, tsk,
    tsk, unbelievable. Interesting. Hey, let's take it up back to the top
    and see if the failure repeats itself!"
     
  20. Joerg

    Joerg Guest


    Whoops, now it probably is. But it probably already was.

    What are the odds of both photodiodes failing around the same time while
    they are connected to very different parts of the circuit? This would
    also point to the LED as the culprit, at least from a Sherlock Holmes
    point of view.

    But you could then fire up the LED again, pulse it with something, take
    a photodiode from your parts bins, hook it up to the scope and see if
    stuff is received.
     
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