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).
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Here is a short article about LED failure modes:
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.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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
-- http://www.linetec.nl
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!"
-- SCNR, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Sounds to me like you were simply unlucky & opto failed by itself.
--
W
. | ,. w , "Some people are alive only because
\\|/ \\|/ it is illegal to kill them." Perna condita delenda est
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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.
Ditto. I don't think I've ever seen a failed optocoupler.
--
W
. | ,. w , "Some people are alive only because
\\|/ \\|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
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.
...
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
-- http://www.linetec.nl
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.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
-- http://www.linetec.nl
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
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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".-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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.
-- Nemo
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