LED resistor type

Printed From: the12volt.com
Forum Name: Lights, Neon, LEDs, HIDs
Forum Discription: Under Car Lighting, Strobe Lights, Fog Lights, Headlights, HIDs, DRL, Tail Lights, Brake Lights, Dashboard Lights, WigWag, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=24002
Printed Date: May 20, 2024 at 5:15 PM

Topic: LED resistor type

Posted By: fliggah
Subject: LED resistor type
Date Posted: January 05, 2004 at 7:25 PM

i pretty much have an idea how to go about getting the right resistor to a certain type of led.. now my question is how do i determine what type of resistor I need if I have a series of leds of the same type..is there a calculation formula...to better get you an idea of what I'm doing is that 'm trying to replace my brake lights with LED lights, probably using about 8 LEDs.

thanks

Replies:

Posted By: mindctrl
Date Posted: January 05, 2004 at 8:12 PM

If you want to replace your brake lights with LED lights, they make cluster bulbs that come preloaded with LEDs and they fit into your old light socket, works just like a regular bulb only brighter and faster. Find them at www.cardomain.com or any local auto accessories store. Hope this helps.

--mindctrl

Posted By: xetmes
Date Posted: January 05, 2004 at 8:18 PM

Do you know the specs of the LEDs? You will need to know the forward voltage around 1.7V for red usually. Then determine how many you will have in series, find the total voltage drop from all the LEDs and the find the difference of that volatage from the supply, resistor will dictate the current (I = V/R)

Sorry if this sounds vauge but I know this has been covered many times in these forums...

Posted By: fliggah
Date Posted: January 05, 2004 at 8:55 PM

let's say I have 8 red leds that will be connected in a series and each one has a 1.7 forward voltage...now my power supply is 12v how do i find the right resistor...BTW how do I find the total voltage drop... I know 'm askin' a lot of questions, I could easily go and just buy pre-made ones but I think it's better to learn..

thanks again

Posted By: xetmes
Date Posted: January 05, 2004 at 9:32 PM

if you have 8 in series with 1.7V each it will be 8 * 1.7 = 13.6, so it really shouldnt be done since the supply is only 12. Lets say you use 6 instead...

6 * 1.7 = 10.2 V

so the voltage across the resistor is going to be 12 - 10.2 = 1.8V

I will assume you want 20mA through the LEDs (common value)

R = V / I

R = 1.8V / 20mA = 90 ohms, you would use a resistor close to 90 ohms

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now lets say you hook the other 2 up alone (ideally I would use 4 and 4)

2 * 1.7V = 3.4V

therefore the voltage across the resistor is going to be 12V - 3.4V = 8.6V

so the resistor value you are going to need is... (assuming 20mA again)

R = V / I

... ahh you calculate it posted_image what should it be assuming the perfect 12V source?

Posted By: fliggah
Date Posted: January 05, 2004 at 10:10 PM

A PUBLIC THANK YOU FOR XETMES posted_image