I'm reading over a datasheet for some LEDs and in the forward voltage section it lists...
IF=20mA
What does this mean? Effective current (Ieff)?
Forward current
If is the current flowing through the diode when it is conducting (illuminated)
The Forward Voltage (usually Vf) is the voltage drop across the LED when it is conducting (illuminated), in this case with with an If of 20mA forward current flowing.
You could use the combination of values to estimate a resistor.
For example, if the voltage that you intend to connect is 5V, using the 'typical' value for forward voltage for green (-3301) as 2.1V:
R = V/I
R = (5V-Vf) / If
R = (5V-2.1) / 0.020 mA (20mA If) - typical
R = 145 ohm
Worst case Vf (forward voltage) is bigger which means a lower If, which is a safe direction to go.
Luminous intensity is specified at 10mA If, so that is a reasonable value to use to calculate the resistor.
HTH
GB
All of the above are correct.
Just remember that for an LED the forward current is a 'rating' and there are usually two of them. There is a maximum forward current (IfMAX) that you shouldn't exceed (like a speed limit) and a typical or recommended forward current (If) that will usually give the best tradeoff between emitted light and device longevity.
You should select the series resistance to give the desired If as described above. You will probably have to compromise a bit in order to be able to use a standard resistance.
The forward voltage is the value that you can expect to find across the LED after you have adjusted the series resistance to get If flowing through it. You probably won't get the exact value but it should be close.
You should see that LEDs don't work like incandescent lamps (we still have them in the US). With an incandescent lamp you fix the forward voltage and take what you get for the forward current. With an LED you fix the forward current and take what you get for the forward voltage.
Don