To illustrate these ideas, I conducted an experiment. I built a simple circuit consisting of a variable voltage supply driving an LED directly. I varied the voltage across the LED and measured the current that flowed. I had a 3000 mcd blue LED and a 5000 mcd white LED available to test, both 5mm. The results are in the graph below. It’s the most important thing in this article, and it’s worth repeating: a small change in voltage can produce a huge change in current. Note especially the portion of the curve between 3.2V and 3.4V. The current changes by a factor of 4 even though the voltage varies by 0.2V. While the specifics will be different for every LED, they all will have this sort of relationship. Overdriving an LED a little is going to degrade it substantially. Both the LEDs in the test were destroyed by the higher drive currents. They still lit up, but at a fraction of their original brightness.
Use my LED calculator to select the correct current limiting resistor.
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