I am very new to electronics. Trying to go through the math myself I am already confused. Using Ohm's Law E = IR I try to find the resistance using the known voltage (9V) and desired current (20mA):The forward dropping voltage, or Vf, of an individual LED is very stable. Go below this voltage and the LED stops conducting. This LED is assumed to be 2.5V, pretty standard for a modern red unit. The target current is 20ma. Going though the math (using Ohm's Law) the resistor is 325Ω. Since 330Ω is the nearest standard resistor value 330Ω it is.
9 = .02 * R or 9/.02 = R, 9/.02 = 450
So why wouldn't you want to go with a common 470 Ω resistor in this circuit?
Last edited by a moderator: