Why 3V LED still requires resistor when input voltage is also 3V

Audioguru again

Joined Oct 21, 2019
1,779
A blue LED might measure a forward voltage of 2.82V when fed 200uA from my digital meter. But when connected directly to a 3.0V CR2032 battery it might draw 300uA and still produce plenty of light at night, especially if its lens focusses all its light into a narrow beam. Then the battery will last for days.

But most little LEDs are used at a current of 20mA which is 67 times higher and the little battery will die soon.
 

MrChips

Joined Oct 2, 2009
21,687
As the battery voltage falls the LED current will drop faster exponentially. Hence the LED will be lit for a very long time, though progressively dimmer. This is a simple experiment as demonstrated by @Tonyr1084 .

You can try the same experiment with a 1F super-capacitor.
 
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