In actual practice, I've found the values in the chart to be ballpark figures, usually a bit too high. I usually measure the actual current and adjust the resistor until I get 20 mA. For 12 Volts, this usually works out to be a 470 Ohm resistor, which is the value I use for my LED light chaser strings. Here's an example of current measurements I made using 12 Volts and 470 Ohm resistors, according to the LED color. As you can see, 470 Ohms keeps the current fairly close to 20 mA regardless of the LED color. As expected, the LEDs that require the least voltage draw the most current.
For 5 Volt operation, my resistor value of choice is 150 Ohms. For a red LED, the current is pretty close to 20 mA, but for a blue one, it is more like 10 mA. The lower voltage makes it more difficult to maintain the same current through different color LEDs than is possible at 12 Volt operation. As you can see from the chart above, at 12 Volts, the differences in current between LED colors is only 21.3-17.4=3.9 mA. But at 5 Volt operation, the difference is 10 mA (twice the current through a red one than a blue one. This is because at 5 volts, the difference in the supply voltage - the voltage required by the LED is much greater than at 12 volts.
There are some cases where you do not to want the LED to draw 20mA,
even if it can handle that much, like when using them with a device that
cannot source or sink that much current, such as a PIC microcontroller
chip. Although a single output should be able to supply that much current,
you could get into trouble if all outputs are expected to supply that much
at the same time. In that case, the recommended value is 680 ohms, according
to the books published by Square 1. Another
reason to supply less than the rated current of the LED is to conserve battery
Copyright © 2002, Colin Pringle (email@example.com)
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