Optocoupler and mechanical relay failure modes / symptoms

I am considering the use two identical relays with their control "coils" in series.

(I am aware that the normal arrangement would be parallel)

My understanding is that with optocouplers the failure modes are generally in the load switching side (for various internal reasons) and it is unlikely that the control circuit fails open circuit. Hence if one of the devices goes faulty the other (which will be rated for the full voltage should the control short circuit) will be unaffected. Does this match with operational experience?

From an operational perspective, what are the common failure modes for optical and mechanical relay? Assuming a well-selected device, what kind of faults do control circuits see most frequently, or is it so rare (compared with the switching end) as to not be worth considering?

  • In my experience the normal opto-coupler failure in mains applications seems to be a dead short on the  load (contact) side, assuming the control voltages are correct (but  here they are in series, if one faults to short circuit does the 2nd one get cooked ?). If the LED part fails at all it is likely to be  short circut, as most semiconductors so the second relay then gets twice the voltage. If it was mission critical I may put parallel zeners across the LED ends that only conduct in fault and prevent it being over-volted. If it is an AC system then 2 Zeners in anti-series.

    Are your primary voltages sensible for such an approach, and is the load it is switching well defined ?


  • You'll need to check the range of voltages over which the relays are guaranteed to operate.  If you are wiring relay coils in series, each will only see half the supply voltage, and that may not be enough.

  • Thanks both.

    Yes, looking at multi-voltage devices so doubling (or halving) the voltage is okay.