Built B16A

im working on a custom LED license plate light for my street bike.

the plan is to use 6 LED's. each are rated at 3.6V, 4V MAX. so if i run 2 circuits in series.. everything should work right? or should i rate the max voltage for 13V with the alternator?

nsxxtreme

The important thing is to spec your LEDs for current not voltage. Unless your LED has an internal resistor so someone has done it for you.

An LED will sink every bit of current you throw at it (until it burns up). Its just a diode that emits light. So its a basically a short. 3.7v LED sound like it has resistors but I don't know. You still need to check the current because if they are rated for 3.5v for their max current rating then you can burn them out with a higher voltage.

For instance a normal LED has a forward voltage of .7v this means it takes .7v to turn the diode "on". So assume this diode is capable of 30mA
(14.4-.7)/R=.030 solve for R gives you the resistor you need. You can see that as the voltage changes so does the current.

xTimmyx

here's a super stupid easy way to tell what you need for resistors.

http://www.besthongkong.com/in...id=12


different colors have different rating for each of them.

nsxxtreme

I wouldn't use that chart. It doesn't take into consideration current requirements for a specific LED.

different LEDs have different requirements.
Post up the specs on the LEDs you bought.

Built B16A

thats the product, and thats the conversion chart i found online.

now.. since vehical systems are 12V.. but one the engine is running, and the alternator is charging the battery, its prolly common for the electrical system close to 13V... should i base my source on 13 volts?

the difference in 12 to 13 volts is a 1/8 WATT, 100 ohm, and 150 ohm. i just don't want to finish, soldier these in, and blow my circuit cuz it spiked a volt.

DDEVIANT1

vehicle electrics run about 14.4 volts when charging...

vigor5spd

Go to Radio Shack and buy you a Voltage Regulator.

LM317T
R1 = 150 Ohms
R2 = 270 Ohms

You will drop the 12V down to 3.5V, wire all the LED's in parallel. If you wire in series and one goes out then all of them will go out.

http://www.reuk.co.uk/LM317-Voltage-Calculator.htm

Built B16A

man, thats some complicated stuff.. lol, i wish i was better at math, ohms law, and electrical.

anyways.. here's my finished result.

3.3 V LED's, 3 in series.. a 150 ohm 1/8 watt resistor. the resistor is rated for a 13V system, so i shouldn't have a problem with it running and recharging voltage.










it was prolly over kill. im sure i could of got away with prolly 3 lights total. lol

nsxxtreme

(14.4-9.9)/150=30mA
Either one of two things will happen.
1) you'll be fine
2) you'll burn your LEDs out early.

I would have used a 180 ohm resistor.

White Smoke

I would have gone with a bit larger resistor but you will probably be fine. Make sure you cover up those wires as well.