The data sheet for this LED gives the forward voltage range, at forward current of 20mA, of 2.8 to 3.8V. How can I choose a sensible current-limiting resistor value for this device, given such a wide range of possible forward voltages?
If I have a 5V supply and the forward voltage is 2.8V, I would need a 110-ohm resistor to give 20mA forward current (the rated value). But then if the forward voltage on a particular one was actually 3.8V, I'd only get about 11mA – and considerably less brightness. And of course operating from a 3.3V supply it's going to pretty random whether the thing switches on at all.
Am I misreading something?
Best Answer
If you just get a random LED, then you're right. Setting a "good" value will involve picking a resistor that will not cause too much current to flow in the worst condition (highest voltage rail, lowest resistance, highest/lowest temperature, highest/lowest LED Vf), and then live with the variations.
Note however that they are binned. Each bin has a span of 0.2 volts, making it easier to pick a value.
Depending on your LED source, they might already be binned, and the binning should be indicated on the distributor's listing.