MK1 Steel router build log 1500x800x200

[Doddy]

Okay, a nice Sunday experiment.

With reference to https://www.renesas.com/en-eu/produc...tandard-p.html - which discusses the effect of reverse-biasing an opto-coupler (section 1.5).

I've hooked up a brand-new 4n25 opto-coupler, 1K between anode and +10V, cathode connected to either 0V (opto = on) or through a 10k resistor to 24V (opto = off, reverse biased, simulating the sensor behaviour). Collector to +10V, emitter through a 1k to ground. DVM across emitter and ground.

With the cathode connected to 0V (on), the voltage at the emitter raises from 0.0 to 9.06V.

Now, I connect the cathode to the 24V supply via the 10k resistor (simulating the sensor internal pull-up of 10k). Predictably, the emitter voltage drops to 0V as the opto-coupler turns off.

So, the real test - leave like that for 5 minutes, then disconnect the 24V supply and ground the cathode.

Result - emitter voltage rises to 9.06V

Conclusion - for a short-duration (5 minute) exposure to 24V (and a reverse bias of 14V) the performance of the opto isolator is not changed.

However, measuring the voltage across the 10k gives 0V (i.e. no voltage drop - no current flow) - the voltage hasn't reached the avalanche voltage of the opto's LED.

Since then, I've added in a second PSU giving me a range upto 60V, in place of the 24V supply. Cranking the voltage up it was evident that the avalanche region of the LED in the opto was around 50V. Removing the 50V showed an emitter voltage of 9.01V (a 0.05V reduction from previous). Replacing the 10K resistor with a 2.2k resistor (to increase the current flow during the reverse-biasing), and the emitter voltage dropped to 8.84V (further degeneration). Note, this effect does appear to be permanent, but not increasing (in the 5-10 minutes I waited).

So, what do I think? Reading random articles on the internet does suggest that reverse-biasing the LED will result in a deterioration of performance over a period of years. The article above suggests that even short duration reverse biasing can result in deterioration.

However, the avalanche voltage may be substantially higher than the datasheet (clearly you can expect the opto-isolator to tolerate a 6V reverse bias without damage - but the actual headroom that you have between that and the actual avalanche voltage is unknown, and likely variable between difference devices, age, temperature etc).

Having run this experiment I'm inclined to think that you're not likely to have a significant problem running the BoB with the inputs connected to a 24V-driven NPN sensor. If might be that, over time, the opto-couplers degrade, but I'm not convinced that this would result in failure. If it does, the optos are easily replaced (or the BoB, if these are still available after the failure).

BUT, I have decided that I'll be powering the BoB and the sensors with a common 12V supply - it's a personal choice but I will keep the reverse bias voltage to less than the maximum expressed on the datasheet.