The datasheet for some LEDs I'm considering lists the forward voltages as 2.7V typical and 4.2V max. (I'm looking at the "warm white" version.)
In the past, I've only ever looked at the typical forward voltage for figuring out the resistor value. But I've never used an LED where the maximum value may be so different from the typical.
What factors affect the actual forward voltage? Manufacturing variation? Temperature? Current? Something else?
Suppose I have a 3.3V supply and that I want to drive one of these LEDs at 20mA. Based on the 2.7V typical forward voltage, I'd use a 30 Ohm resistor. But if I end up with an LED that actually has a forward voltage of 4.2V, it won't light because 4.2V > 3.3V.
How do you design a circuit to accommodate such a wide range of forward voltages, or is there a reason I don't have to worry about the max forward voltage?
Best Answer
The mean value of the typical voltages are 2.55V and 3.14V for the maximum values. However, plugging in the same resistor value for a 2.2V LED vs a 4.0V LED would be very different currents and brightness levels.
The typical forward voltage means that the LED will be lit, how much depends on the LED itself and even the construction of the LED (viewing angle ECT) (not to mention that the human eye sees some colors better than others, so even two LED's with the same intensity will not be perceived at the same intensity).
The only way I've found to really get LED's with the right intensity are to ball park the resistors then fine tune the values based on the intensity, sometimes I'll get a few people to look at it if it's on a product.
The other way is to use a constant current driver, which is more complicated, but allows you to avoid a voltage drop if you can't afford it:
Source: https://hackaday.com/2012/03/08/led-tutorial-demystifies-several-control-techniques/