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Opto-coupler failure question (HCNR201)

R

Richard Rasker

Jan 1, 1970
0
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
 
J

Joerg

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)?

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.
 
P

propman

Jan 1, 1970
0
Joerg said:
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.

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
 
R

Richard Rasker

Jan 1, 1970
0
Joerg said:
....


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.

(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
 
R

Richard Rasker

Jan 1, 1970
0
Jan said:
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?

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

Richard Rasker
 
S

Spehro Pefhany

Jan 1, 1970
0
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

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?
 
J

Joerg

Jan 1, 1970
0
Richard said:
(I use xpfd; works very fast, never crashes)

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

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, ...


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

... 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.

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.
 
S

Spehro Pefhany

Jan 1, 1970
0
No, I didn't (yet) -- but isn't this very, very unlikely?

Richard Rasker

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?
 
N

N_Cook

Jan 1, 1970
0
Richard Rasker 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)?

Thanks in advance, best regards,

Richard Rasker


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
 
R

Richard Rasker

Jan 1, 1970
0
Joerg said:
Yeah, I have to get something better than this dreaded Adobe stuff.
Foxit doesn't work with all docs but maybe xpdf does.

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.

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

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.
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.

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
 
R

Richard Rasker

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.
Is the reverse leakage close to typical values?

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

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.
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?

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.
Could the package be cracked at the leadframe or elsewhere due to
inadequate support of the PCB?

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
 
R

Richard Rasker

Jan 1, 1970
0
Jan said:
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.

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
 
R

Richard Rasker

Jan 1, 1970
0
Spehro said:
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?

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
 
J

Joerg

Jan 1, 1970
0
Richard 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.

Somehow sounds like a busted LED.

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

Richard Rasker

Jan 1, 1970
0
Joerg said:
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
 
J

Joerg

Jan 1, 1970
0
Richard said:
Hm, I /did/ set the follow-up to sci.electronics.repair. I don't know what
went wrong ...

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).
 
J

Joerg

Jan 1, 1970
0
Richard said:
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.

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.
 
R

Richard Rasker

Jan 1, 1970
0
Joerg said:
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.

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
 
J

Joerg

Jan 1, 1970
0
Richard said:
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).

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!"
 
J

Joerg

Jan 1, 1970
0
Richard said:
[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.
Is the reverse leakage close to typical values?

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.


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

But during all this, I monitored both photo diodes, and at no point did they
produce any sigificant voltage into 10MOhm voltmeter inputs.

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.

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.

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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