So I was working on the switch backlighting for A switch plate on my boat. There is a single led mounted in a light bar that mounts behind the engraved legend plates.
I bought a replacement led at RadioShack. It is a red 800 mcd LED. The package says 1.7V - 20mA.
I needed to calculate the voltage drop resistor. So I took incoming voltage of 12v - 1.7v= 10.3v.
10.3v / .02A = 515 ohm resistor. Right?
So I buy a 560 ohm resistor and wire it in series with my led. Luckily I am able to bench test with a variable power supply and a volt meter. As I bring the supply voltage up to 12v, the voltage at the led rises over the led 1.7 volts. Why?
What I ended up doing was wiring 2 - 560 ohm resistors in series and that brought the led voltage to where it needed to be. Why did Ohms law not work? What did I do wrong or not account for? Could the LED current be labeled wrong on the package?