Sergegsx:
The full design is here, as you can see there are already 2 x 1n400x at the begining as protection. however, previously I was told to put also this other diode across the optocoupler as it was a faster diode. Is this correct? should I leave it or remove it?
Without the zener diodes in the circuit, the 1N4148 in reverse-parallel connection provides the best protection. With the zener diodes added, you should either use a much higher value series resistor (so that the power dissipation isn't a problem even with the polarity reversed), or add back a single 1N400x diode as protection against reverse polarity. I would keep the 1N4148 as additional protection, because the capacitance of a 1N400x will let transients through.
Sergegsx:
Let me go through your calculations, CRT is new to me. After I read about it I will post again.
When doing calculations on the current transfer ratio, be sure to use the minimum guaranteed current transfer ratio, and check whether the Vce at which the current transfer ratio is measured is applicable to your circuit.