10mm LEDs as turn signals on motorcycle

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Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=53799
Printed Date: May 13, 2025 at 11:44 AM

Topic: 10mm LEDs as turn signals on motorcycle

Posted By: maxreps
Subject: 10mm LEDs as turn signals on motorcycle
Date Posted: April 12, 2005 at 7:15 PM

I have 2 5000mcd LEDs rated at 2.4v - 20mA and want to install 1 on each turn signal pos & neg wire which supplies 13v. I can't get thoughts straight on the resistor value needed to drop the voltage from 13 to 2.4v. I've got a standard led with a 1600 ohm resistor and the LED sees about 2v so logic would indicate that 1600 is close. I ascertained that 1600 ohms caused an 11 volt drop so every 100 ohms of resistance drops .69 volts so 1550 ohms should let the LED see about 2.34 volts.

If anyone can shed "formula" related light on this please do. Any help is appreciated...

I know I could possibly find 12v LEDs somewhere but these were local and very bright so if I can figure out the resistor issue.

Replies:

Posted By: NowYaKnow
Date Posted: April 12, 2005 at 8:12 PM

Just a quick copy and paste from one of my previous posts on this:

R = (Source Voltage - LED voltage required) / (current in milliamps) * 1000

So say you have a 3.6v led that needs 30mA, and we'll assume your car charges to about 13.5v with the car running.

R = (13.5v - 3.6v) / (30) * 1000

R = (9.9 / 30) * 1000

R = .33 * 1000

R = 330 ohm resistor needed in SERIES.

You'd want to use a 1/2 watt or 1/4 watt resistor depending on what led your using. There's your formula just plugin your LED specs and see what you come up with..Luckily LED's are very tolerant so you don't have to get them dead on or anything just close enough will be fine.

Running your values through I come up with a 555 ohm resistor. A 560 ohm should be an easily available value at radio shack. Hope it helps,

Mike

Posted By: maxreps
Date Posted: April 13, 2005 at 7:07 AM

thanks Mike, that is what I was looking for... I appreciate that and I actually had done a search before posting but didn't see that info. Thanks again!

Chris