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Another way to ask this question would be what is the best drive current for my LED? What I encounter frequently is people using the maximum ratings provided by the manufacturer. But with high-brightness LEDs, drive currents well below the maximum, and well below The typical rating provide a very useful level of brightness.
So lets review whats typical: for 3mm and 5mm indicator-type LEDs, datasheets often list a typical forward current of 20mA. Forward voltage will depend on the junction material of the LED, but lets use a white LED as an example. 3.3V would So applying the series resistor calculator , we get a curent-limiting resistor value of 85 ohms (100 is the next largest standard value), and a power dissipation in the LED of (3.3V * 0.02A) =) 66mW.
So for this experiment, I constructed an array of sixteen LEDs in parallel across a 5V source, each with a different current-limiting series resistor. I recorded the current through and forward voltage drop across each LED, and calculated the power dissipated by the LED Lacking a light meter, I have taken photographs to allow you to evaluate the relative brightness. Measurements were made with a Velleman digital multimeter. The resistors I used were 5% 1/4W carbon-film resistors, so readings are given to two significant Figures.
Current varied from 6 mA at left to 24 mA at right
Variations in brightness are noticeable, but above 10 mA its very difficult to discern differences 