How is LED brightness related to current?

You might expect that with the drive current varying by four to one over this range, that the brightness would vary proportionally. But between 10 and 20 mA very little difference in brightness is noticeable.The photo shows the LEDs in the experiment simultaneously lit, and arranged in order of increasing drive current from left to right. Since these are high-brightness LEDs, it’s notable that even driven at 6mA, they are uncomfortable to look at directly.

Table 1 lists the values for each series circuit.

        Table 1
R(ohms) Id(mA)  Vf(V)   P(mW)
68      24      3.1     74
82      20      3.1     62
100     18      3.1     56
110     16      3.1     50
120     14      3.1     43
130     13      3.1     40
140     13      3.0     39
150     12      3.0     36
160     12      3.0     36
170     11      3.0     33
180     11      3.0     33
190     10      3.0     30
200      9      3.0     27
220      8      3.0     24
270      7      3.0     21
330      6      3.0     18

Conclusions

There are several important lessons to be drawn from this experiment. First, overdriving LEDs gives minimal additional brightness while shortening the useful life. This experiment didn’t address service life directly, but the increased power dissipated in an overdriven LED heats the junction and degrades the LED. Second, rounding up to the next largest standard resistor produces imperceptible brightness reduction. Conservative choice of current-limiting resistors has a minimal downside. Third, there’s no need to race to the store for a resistor if you have a supply of slightly larger resistors on hand. And slightly larger can include values up to double. Fourth, dimming resistors by current control is challenging because of the non-linear relation between brightness and drive current. As cpemma would put it, “all the action is at the bottom end.”

Next steps

I intend to investigate this further once I get a light meter. Being able to plot a curve for brightness vs. drive current would be an important enhancement to this experiment.

The experimental setup consisted of LEDs and resistors

Acknowledgements

Special thanks to cpemma for the original suggestion for this experiment.

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