Hi all, a bit of a hypothetical for you. If I wish to drive a 3V LED at 20 mA from a 12V battery, I can use a dropper resistor. I subtract the LED's forward voltage from the 12V to get 9V, and divide this by the 20 mA, to get the value of resistor needed. R=V/I = 9/0.02 = 450 ohm. However, if I want to drive four 3V LEDs in series, the combined forward voltages will 'use up' the full 12V, so I have none left to drop across the resistor. I know I still need a resistor to limit the current, but how do I calculate its required value, if I want 20 mA? Again, a theoretical problem. I realise I could rearrange the LEDs in the real world, but it bothers me that I can't work out this simple 'thought experiment'.