Hey guys, I'm new to the forum and I have a simple question.
I've made a third brake light for my truck, and it incorporates about 120 red LEDs. I just want to know what resistor I should use. They're wired in parallel so it's going to be 12ish volt to 3ish. I know if I use 1 LED I need a 470ohm resistor. Should I just get a 470ohm that can handle 2 watts? Or do I need something different all together?
THANKS!!
rjb

Oh yeah, the LEDs run at 20mA

 

It would help the process along if you could provide us with a part number for the LEDs that you will be using.

hgmjr

 

All wired in parallel I assume? 2Watts is good for 100 at nominal brightness, and 'might' overheat and smoke with 120leds on long operational on times. I would make 2 sets of 60leds (redundancy if you blow out 1 side) Use the R=E/I formula and maybe add 10% to R value to ensure longer led life (R=12/.020) (or .030...) I did a 8' by 12' Unit status 'mapboard' for our 2xx odd comm sites in Germany, and used 1/8w 470 ohm on each indiv LED, cuz the commander didn't tolerate ANY LED downtime. Those LED had to be ON, Blinking slow or blinking fast ;-)

 

Here is a small sample of what you would need to do...

I'm figuring on 20ma at 13.7V if each diode dropped around 2.5V Vf (an assumed Vf for red). Diodes vary considerably, if it were blue or white you could only have 3 per chain. The Vf is an assumtion, yours could be different. The resistors can be ¼W each. At 12V you would have 11ma per chain. If there were 30 chains at .02A each it would add up to .6A, and it would be extremely bright (understatement there).

You shouldn't put chain of diodes parallel to each other without separate resistors, as variation can cause premature failure of one chain, forcing the rest to absorb more current. This in turn can start a chain reaction resulting in diodes popping like corn. The individual legs prevent this, and add a level of redundancy besides.

 

I didn't even THINK of one or two or more blowing and all the others absorbing the rest of the current. Thanks for the info! A redesign is in order. I'll keep you posted and maybe even show a pic!
thanks
rjb

 

If one or two blow and OPEN, all in that chain will go out too. If on the other hand they FUSE, all other will get really bright and blow too - or the series resistor will and the remaining lights will be saved. Either way it is a pain. Use 2x redundancy and several strings of LED so you will have light reguardless what happens