I'm building a ac motor starter with a optocoupler (FOD4218) and a TRIAC (BTA16), one for each phase in a three phase 400V system. I have a question regarding the circuits that can be found all over the web, like this one: circuit http://www.electronics-tutorials.ws/articles/opto6.gif?81223b
What is really the purpose of the 100R resistor? Is it for voltage dividing or what? In my case where the circuit is between two phases, is it necessary? Some circuits do not have that resistor at all. I have planned to use 600R for the upper resistor.