I am spinning my wheels here. How do you calculate for the correct resistor? For example I have an IR LED that runs a 1.2 forward voltage and 100mA forward current. My power source is 5v. From what I calculated I need a 41 Ohm resistor. My calculation is ((5v / 1.2v) / .1), problem is when I use lets say a 47 Ohm resistor and I check it with a multimeter I am getting about 2.2 volts, but when I use a 1k Ohm resistor I am getting below 2v on the measure.Any ideas?
My power source is 5v.