I found a LED somewhere. How can I find out if it has a built-in resistor? As far as I can see, they look really equal to me. Measuring the resistance using the multimeter does show an infinite resistance - I don't know why. In case there is no built-in resistor, how can I find out the proper limiting resistor? How to find at what voltage the LED should be used?
Similar Q&A can be found here, but it does not oanswere the part about built-in resistors.
Answer
I'd suggest connecting the unknown LED together with current limiting resistor to the regulated voltage source and measuring its voltage-current characteristics.
simulate this circuit – Schematic created using CircuitLab
The voltage at which the "reasonable" current starts to flow may be considered the "on voltage" of the diode. The slope of the I-V curve above this voltage will allow you to estimate the R2 value (if it is mounted in your diode).
$$R_2=\frac{\Delta U_{VM1}}{\Delta I_{AM1}}$$
Of course you should watch out to keep the current reasonably low to not overload your diode. (You can also control how bright light does it emit). Unfortunately there is no method to state whet is the maximum allowed forward current for the unknown diode.
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