Quote Originally Posted by Warrier
...a 100k would be sufficeint as the voltage across the diodes is clamped to about +/-1.7volts.
Well, in general this is correct, but:
lets's assume the Opto's LED current is as low as 10mA,
even at that low current at a voltage of 120 or 240 V the resistor would have to be quite large (in size) and dissipate a lot of heat,
would you really like this?