Hobby Issues

[Al Skierkiewicz]

James,
The value I calaculated was based on 40 ma, just a guess on my part as max current through your LED. The actual recommended is the 30 ma which is the value you used. The closest standard value resistor is either 470 or 560. The 470 should be just fine. Power=I^^2 * R or V^^2/R or V * I. Since you have already calculated the voltage drop for the resistor, 14.62 and the current is 30 ma, then the power in the resistor is 14.62 * .03=0.4386 watts. A half watt resistor should then be fine unless you severely limit the airflow around the resistor. A 1 watt resistor would work fine as well. If you are unable to find 1 watt resistors, you can use two 910 ohm, 1/2 watt resistors in parallel, to form a 1 watt, 455 ohm resistor. Resistor wattage is more often known by it's size or the package markings. Less frequently it might be printed on the resistor body. I believe Radio Shack has a few 1 watt resistor choices, but many half and quarter watt choices.
To not confuse terms, the 18.32 volts is the Peak voltage developed by the transformer which is the highest voltage available for the LED during the peak of the sine wave. Real is a term used in complex AC circuits in which there exists reactance (capacitors and inductors) that affect calculations using vector analysis.