Author Options:

wiring LED's to motorcycle. need help!? Answered

so i am planning on putting green led's on my motorcycle
wired 3 led's in a series and tested them on the 12V battery. everything looked good. i undid and redid the leads to the battery and everything was good. as it was sitting there and i was walking around the bike the leds went out. i cut each wire between the leds to check each individual one. they're all 3 dead. there's not way any wires could have crossed or touched because the connections (other than the battery leads) were sealed and waterproofed.
why did this happen?

feel free to correct me because i'm a complete noob with this but i believe it was the amperage. i'm not running any resistors. and resistors control amperage i believe? i am planning on running a fuse in the line but this has nothing to do with controlling the amperage. 
the 12V battery was showing a constant 11.6V on my meter. so this means each led was running at about 3.9V per led. is this correct?

(these are 3mm leds i believe) the brightness was perfect as they were before they blew. 

my question is what did i do wrong? and how do i keep them from blowing. i need this to be reliable because after i install them it will be permanent and i can't undo them to rewire anything.
if you suggest using resistors please explain details including where in the line to wire them, as i know nothing about resistors. also i would like to keep the same brightness i had.


+1 to what steveastrouk is saying.

You should have gotten something with your LED's to show what they are rated at but 2V max and 20mA is a fair rule-of-thumb. The equation looks like the pic. Since they are in series the current is constant as 20mA (or whatever you're are rated at) and the Voltage required for each gets added together. You can add more LED's until the equation works out to almost 0 or put a resistor of close to the calculated value in series before the LED's. Slightly bigger is safer when choosing a resistor.


So assuming they're rated at 2V and I work out the resistor value... Will putting a flighty bigger resistor affect the brightness of the LEDs? Also, what exactly blew the LEDs? Was it the current or the voltage? I can't imagine it was the voltage because I had them running at a similar voltage before without problems.
And does a resistor control voltage, current, or both?

The current blew them, there was nothing to stop it being as high as it wanted to go. The resistor stops it. The resistor LIMITS current. Think of it as a restrictor in a pipe.

So why didn't the current blow them when I had them hooked up to the AA's? Is there a lot more current in a 12V motorcycle battery than there is in a AA?

Also where,other than radio shack, sells 3mm LEDs? I need to buy new ones because I don't know anything about the ones I have.

You didn't mention using AAs in your question, but either you didn't have enough to make a 12V stack of AAs or the internal resistance of the AAs helped you a bit.


i originally had them hooked up to a few AA batteries to test everything out before i put them on the bike.

i wasn't sure what the voltage drop of the LED's actually was so i tested it today. when i hooked up the LED's parallel to about a 4V power source they each read about 3V on my meter. this means this is their voltage drop correct?
so with a 12V power source i'm using 3 of these LED's in a series. i'm also ASSUMING the "reccomended current" is 20mA. hopefully this is right. with all that said this is what i work out.
[12-(3x3)] / .02 =150 ohms
so with my math i should use a 150 ohms resistor.

Did you have a resistor in series when you tried it ?

If you just put the LED across the supply, you're forcing the voltage across the LED to be the battery voltage, and that forces a high current through them.

Adding the resistor gives us something to waste a bit of energy in, and limits the current in the diode.


i didn't have a resistor when i was testing the voltage drop of the led's.
i had 3 AA's powering the led. and i know this would be putting about 4.5V through the led but the led still read 3V even without any resistor.

You're relying on the battery to limit the current !

Whatever, use the resistors we've already calculated and you'll be good to go.


You've burnt the LEDs out, they'll have been very bright for a few minutes, then they've fried.

You need to know what LEDs you've used, to calculate the resistor needed. 3mm leds typically only take 20mA, and I'd guess at 1.5V each, so you have 4.5V across all three when running correctly. THat means you need a (12-4.5)/20mA or
380 ohms - try a resistor as close to that as you can manage. 1/4Watt rated.

you say they should be running at 1.5V? or 4.5V?
i believe the "forward voltage" is 3.3 according to this site http://led.linear1.org/led.wiz

putting that 3.3V value into the calculator along with 20mA and a 12v power source it tells me to use a 120ohms resistor.

would you agree with this? or do you still suggest using a 380 ohms resistor?

3.3 V sounds high for 3mm green - assuming they're not super-brights ? Otherwise, yes, (12- 3.3 x 3)/20mA - try 120 Ohms.


Is it unusual for all three LEDs to die at the same time? I would think the first death should break the circuit.

Totally usual, they can die short circuit rather than open, particularly when they die from being cooked.

And after one shorts, the remaining 2 pull more current through the shorted one until the next one is destroyed.