Re: Question about LED voltage/mounting
It's even easier than that. The forward voltage of an LED stays relatively constant for a given operating current, so you don't need to know the equivalent resistance. The starting point is to choose the amount of current you want to run through your LED(s), which should be something slightly less than the maximum specified on the datasheet. Let's say you want to run your LED at 20mA and it has a forward voltage of 3V at that current. You have 12-3=9V left to drop, so you choose your resistor to drop 9V at 20mA.... 9/.02 = 450 Ohms.
If you ran three of these LEDs in series they'd drop 9V. With 3V left to drop, your resistor would be 3V/.02 = 150 Ohm.
You know this already but a reminder:
Series circuit: current is the same through each device
Parallel ciruit: current through each branch is added to the total
Let's say you have three parallel branches of three series LEDs. The voltage drop across all three branches is still 9V (same as one series branch), but you now need 20ma for each of three separate current paths, or 60mA. The series resistor here is 3V/.06 = 50 Ohms.
Last edited by dB; 05-22-2005 at 2:05 PM.