LEDs and Resistors?

Different people will tell you different things, but its good practice to put a resistor on EVERY LED.  The resistor will merely limit current, thus protecting the LED.  The only problem is determining the optimal resistor for your application.  You should be able to find a current rating for your LED.  You should also know your voltage (max battery voltage ~13.8 - 14volts).  You can then use the forumula R = V/I  (R = resistance in ohms, V = volts, I = current in Amps).  The result will be your optimal resistor to run the LED at full brightness.

Now, if you are using a bunch of LEDs things will get more complicated.  If you search the 'net you should be able to find an LED resistor calculator that will tell you what resistor value to use, what wattage rating, and how to wire the LEDs (parallel, series, or both)

Hope this helps!

KPierson:  Thanks for the info.  What normally happens if resisters aren't used?   I also did a search as you suggested and found numerous calculators with some listing the specs for each color.  However, not every site's specs were the same.  For instance, I'll be using blue LEDs and different sites give different voltage draw on them for the same size LEDs.  What gives with that?  Would them generally be the same?  I did use the calculators using the different parameters I found on different sites and the resistors range from 220ohms up to 660.  How critical is this and should I use the addage - if in doubt, be safe and go high?

The higher the resistance, the lower the current, so higher would be the safe route.

If it were me, I would go with the 660 ohm first, and see how bright the LEDs are.  Then, if they arn't brighter, gradually lower the resistance until you are happy (while staying above the lower limit given).  For the super bright LED shift light that KPtechnologies sell comercially we use a 470 ohm resistor inline with a super bright 3mm LED.  Although we have 5 different colors, we use the same resistor for each one (simplifies manufacturing).  The 470 ohm resistor is powered from a regulated 5VDC source, so the total current is roughly 10mA (not much at all, and they are blinding if you look directly at them). 

LEDs parimeters can vary drastically from manufacturer to manufacturer, and even from LED to LED in the same batch.  Always assume the parmimeters given to be averages, and not necesarrily exact for the LED you hold in your hand (another good reason to use a resistor even with 12vdc LEDs).  This includes the voltage draw, as all LEDs are NOT created equal.

We had a customer recently who was having problems with his shift light.  While trying to troubleshoot the problem with me he told me he was absolutely positive his LED was bad, and that was the problem.  I asked him how he was so sure, and he told me because he hooked it up directly to the car battery, and it never came on!  This is typical of low voltage LEDs when you hook them up directly to 12vdc.  Even though he said it never lit up, I would assume it flashed VERY brightly for 1-2 seconds, and then was permanently damaged.