Relays and Optocoupler - When is it required?

[fcb]

A discharging coil can have voltages many times higher than the energizing voltage . But the diode should be enough to resolve that .
Galvanic isolation also addresses any ground potential differences or ground looping which can cause errors to occur in an MCU .
Transients can also create line noise which could also have an effect .
It all very much depends on what components are being used and circuit design.

Should, yes. But what if the diode fails? Without isolation a failed diode could result in damage or unpredictable behavior to other parts of the system, with isolation that situation is very unlikely.

Show just one circuit (with a common ground) whose fault condition is protected by an optocoupler.  And with an equivalent number of components (or less) I'm pretty sure we (as in the forum) can come-up with a non-optocoupler circuit that it is smaller, cheaper and offers the same protection from your fault or faults.

[james_s]

Show just one circuit (with a common ground) whose fault condition is protected by an optocoupler.  And with an equivalent number of components (or less) I'm pretty sure we (as in the forum) can come-up with a non-optocoupler circuit that it is smaller, cheaper and offers the same protection from your fault or faults.

Missing flyback diode on the relay, NPN transistor switching the low side, could you not get a spike back through the transistor out through the base and into whatever delicate CMOS IC is driving it? Cracked solder joint on the emitter that is causing repeated spikes that are not shunted by the B-E diode? If there's an optocoupler then short of a spike of sufficient magnitude to flash across the IC this cannot happen. Also who says there has to be a common ground? In the case of a relay board for hobby/prototyping you never really know what somebody will connect it to. If it has optical isolation you don't need a common ground, you could have an entirely separate power supply over with the relays and the control circuit is only driving LEDs. The LEDs could even be multiplexed and drive a load of relays from a handful of IO pins. I know I'm grasping at straws here but there have got to be applications where it makes sense. Maybe not in some specific board where we've seen it done, but the basic concept of optical isolation being used in a relay coil driver.

[JeanF]

Thank you all for your insights and thanks to anfilt for asking. I actually had the same question when I encountered cheap Chinese relay boards and I also suspected a case of copy-pasting, but it's interesting to read the other design/safety considerations here