robertferanec , 10-31-2018, 07:37 AM
I am not sure if someone really calculated it precisely. In this cases, very simple calculation like 3.3V / 100 OHM = 33mA is just enough .. as you said, just to be sure, that the current will not be bigger than 100mA.
PS: For safety, you may want to use the 3.3V as the maximum voltage value (even there will be some voltage drop on the transistor and resistor, it is just more simple for calculation), but you definitely need to count with situation when the transistor is open for a long time (e.g. mistake in programming the controller). Also, I would not use Vrms for calculation - when transistor is open, the full voltage will be applied, and the current at that specific moment needs to be limited.