I've been using transistors to switch on some LEDs. But what I never figured out was the resistor on base to the trigger. Is there some way to calculate that resistance for the load I want to trigger? Can I just use no resistor at all? I've tried no resister with a 2222 but I fried one that way, so I started using resistors from 250 to 1k ohms randomly... and so far they are working fine.