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Opto-coupler failure question (HCNR201)

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Bob Larter

Jan 1, 1970
0
Richard said:
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)?

Sounds to me like you were simply unlucky & opto failed by itself.
 
B

Bob Larter

Jan 1, 1970
0
Richard said:
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.

You can run xpdf in Windows under Cygwin, although it's probably too
much effort to install Cygwin solely for a PDF viewer. OTOH, Cygwin is
fantastic if you want to be able to use Unix tools under Windows.
I usually have good experiences with those optical devices as well -- that's
also why I'm still a bit puzzled.

Ditto. I don't think I've ever seen a failed optocoupler.
 
M

Mark Zacharias

Jan 1, 1970
0
Richard Rasker said:
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


Well, they won't respond by producing a voltage; the forward resistance of
the diode lowers when the input LED is lit, I would think. Any voltage is
from the load circuit to which the photo diode output is connected, yes?

I admit I've only loosely been following this thread - and I'm used to the
traditional opto-isolator using an LED on the input side and a
phototransistor on the output side.

Mark Z.
 
R

Richard Rasker

Jan 1, 1970
0
Joerg said:
Richard Rasker wrote: ....

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.

It's even simpler than that: any webcam and most cheaper digital cameras
can "see" an IR LED's output (this is also widely used to check remote
controls).

Richard Rasker
 
R

Richard Rasker

Jan 1, 1970
0
Mark said:
[snip]
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.
Well, they won't respond by producing a voltage; the forward resistance of
the diode lowers when the input LED is lit, I would think. Any voltage is
from the load circuit to which the photo diode output is connected, yes?

Actually, the photo diodes respond by generating a small current (typically
0.5% of the LED current); with a high-impedance voltmeter connected, this
results in a voltage in the order of one or two volts.
I admit I've only loosely been following this thread - and I'm used to the
traditional opto-isolator using an LED on the input side and a
phototransistor on the output side.

This particular device is suitable for galvanically isolated transfer of
analog signals with relatively high linearity. This is done by hooking up
one of the two photo diodes in the feedback of the LED drive circuit. With
both photo diodes closely matched, the other diode then delivers a current
accurately reflecting the input signal.

Richard Rasker
 
S

Spehro Pefhany

Jan 1, 1970
0
[snip mysterious breakdown]
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
 
J

Joerg

Jan 1, 1970
0
Richard said:
It's even simpler than that: any webcam and most cheaper digital cameras
can "see" an IR LED's output (this is also widely used to check remote
controls).

If you want to check remotes the photodiode on a scope is easier, heck,
even an LED will be sensitive enough as a photodiode. Then you can see
the transmission frequency and also all the codes. Makes it quite easy
to detect a short or bad button. I had that with the remote from an
80-year old. Pressed two different keys, same code came out. Opened it,
crud all over the board. Cleaned it, worked. "Oh, yeah, well, ahm, I
kinda did spill a can of V8 juice over it the other day".
 
N

Nemo

Jan 1, 1970
0
I can suggest a couple of things to check.

Opto couplers are complex mechanically, because they contain 2 different
types of component, which are then immersed in a resin which has to be
airtight, transparent to IR, have a thermal coefficient of expansion
matched to the rest of the device, and this is then covered by an opaque
layer to block out ambient IR. As a result of this compound construction
and all its failure modes, optos have a very high failure rate compared
to other components. You seem to be aware of this though as you mention
considering vibration, heat etc.

The manufacturers sometimes overlook one of these factors. There were
some cases of optocouplers a few years back (which used white plastic
rather than black bodies, I think) which didn't seem to do the last
function. They responded to ambient light / IR too. You'd think it was a
basic property of the device to NOT respond to this, but perhaps they
moved manufacture to a new factory and didn't emphasise the importance
of the resin type in the training. Or perhaps they were fake components.
If nothing else explains the problem, it may be worth checking the
supply chain to determine the provenance of these (expensive) devices.

The manufacturer's QA department should be able to comment on the batch
numbers written on the opto - a photo would probably be best if
possible, as this may give them other clues if they ARE fake like "all
our optos are in a different colour body".

When I had similar trouble a few years ago I asked the manufacturer's QA
department for failure rate information. It took a little while but they
produced it, which is when we realised how awful cheap optos were, and
switched to high quality ones (like the type you're using!). One of the
failure modes mentioned in the report was, IIRC 20 years later,
something about the resin going opaque as it aged! You should be able to
confirm that if you have an original circuit around, you could compare
its opto output with that of a new opto.

Does the circuit work OK if you drop a new opto into this position?

When all is said and done though, it sounds like your customers are
getting a bit overexcited about a single failure. As the application is
not unimportant (I assume this means it is for the defence industry), it
is a good idea to kick off an investigation to learn from the event and
improve the circuit if possible; but statistically a single failure in
one year conveys too little information to extrapolate.
 
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