I have several hundred clear LED's of mixed colour and would like to sort them and determine their characteristics so I can use them. As far as my research tells me, there is no way to determine the forward current of an LED directly, and in order to determine forward voltage you need to know forward current. My question is, how do LED manufacturers determine these values, and is there any way I can do the same?
Answer
Just about all LEDs can take at least 20 mA. This is almost certainly true of anything in a discrete leaded case, like T1-3/4 or T1. Some small SMD parts may be rated for less.
I would use a 5V supply with a 330 Ω resistor in series. You are very unlikely to have LEDs that can be damaged by this. This will put at most 10 mA through the LED if you get it in the forwards orientation, or 5 V in reverse accross it for the backwards orientation. Neither should hurt ordinary LEDs. 10 mA will be enough for even inefficient LEDs to light up visibly on the bench (assuming typical indoor conditions).
Rig up a jig to show the voltage across the LED. Since the power voltage and resistor are known, you can calculate the current from this too. This will give you one datapoint for voltage and current, which should be good enough for most purposes.
If you don't see the LED light up in either orientation, then keep in mind it could be IR instead of dead. You can use most digital cameras to determine this because they can see the IR light.
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