> jeez, why didn't i pay attention more back in high school? > > question: > > i have a super bright , jumbo LED that reqires 2.6v @ 50ma > > how do i figure the resistance of the dropping resistor for a 12v > source? > > frustratedly yours, > -dan > V = IR (?) You actually are running about 14v in the car, and normal LED's have about that 2.5v drop across them, so you want a resistor that will pass 50ma at 11.5v. If you round to 12v (after all, resistors are commonly only 10% accurate), this works out to 240 ohms -- 12 = (.050)x(240). Notice that by doing this though, you mostly heat up the resistor rather that produce light. You might want to series a bunch of these (like 5) which will give five times the light for the same current draw. malcolm
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