Originally Posted by David_Hart
Yes and no... You can buy a LED bulb, but you also need to install a resistor on each light. The car computer has a function that checks the power use for each bulb. If the power drops too low (i.e. blown bulb, no power use) then you get a warning on the console display and you get a fast blink for your turn signals. As you probably know, LEDs use less power than bulbs. So, to avoid the bulb out problems, you need a resistor to increase the power being used at the same level as a bulb.
Well...not quite. The purpose of the resistor is current LIMITING...not to increase the power being used.
Incandescent bulbs are resistive elements. You're getting light out of them because the current moving through the filiment heats it -- causing it to glow incandescently. As it heats, it's resistance grows, giving you a nice current limited solution. However, the constant heating and cooling causes material fatigue, which is why these bulbs eventually fail.
LEDs on the other hand are semiconductor devices. They don't really have a "resistance" per se. Current through them will rise exponentially as voltage is increased -- it's not really a linear function like with a resistive element. Anyway, LEDs are rated to have a maximum brightness at a given current--above that current, no real increase in brightness occurs; you're just wasting it. In addition, as you increase that current, the LED junction heats. So, just dumping current into an LED unchecked can cause junction breakdown and premature failure. The resistor used with LEDs is sized to provide a specific voltage drop given the driving source, and therefore limiting the current to the desired amount.
The calculation isn't entirely straightforward because of the way LEDs work. You really need to know the forward voltage and current of the LED -- the manufacturer usually specifies the forward current and voltage at a specific brightness up to max -- and then use that to figure the size of your limiting resistor.