I have heard it from multiple people over the last 5 years that Optoisolators are failure prone and should be avoided when able. I'm working on a project (for personal use) that I was going to put an optoisolator on to isolate a logic system from a 170V rail. I do plan to use this device for teaching and would hate for it to stop working because an isolator failed. I know there are magnetic coupling ones now that may be better?
The real question is that I have not been able to find any real data on the internet that confirms optoisolators fail easily or with respect to other solutions.
The only thing I found was a report by Sharp Found Here.
I'll also add all I need to do is move a square wave pulse of 100-500kHz from a logic supply that is floating to a -170V rail. So it can tolerate slow edges and such that a digital com system could not.
Answer
The best guide to component reliability I've ever come across is MIL HDBK 217 F. It details the reliability of components from the analysis of thousands of pieces of faulty/failed equipment gathered over many, many years.
For a simple, fixed value (< 1 Mohm) film resistor (section 9.2), the basic failure rate \$\lambda_b\$ is 0.001 failures per million hours at stress rate 0.5 and operation at 20 degC.
For an opto isolator (section 6.11) of the type with a photo diode the failure rate is 0.0025 times per million hours and, for a phototransistor it is 0.013 times per million hours.
So MIL HDBK 217F is suggesting that an opto might be about ten times more likely to fail than a simple resistor but it's still in the vicinity of once every 11,000 years!
Compare this with a diode (section 6.1) used for power rectification - it has a failure rate of 0.069 times per million hours - several times worse than an opto-isolator.
How many of these are you using in your design?
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