XbStorm

Hello All.. wantded to know the size of resistor needed for a/c heater led's i have the leds but cant find the post on the size. read a post that said to use 470 ohm but not sure of the watts? thanks XbStorm.....

987KEITH

I belive it's 470. I actually bought some 4" cathodes from oznium.com I'm going to give those a shot first.

Keith

Dr_No

You can also google it like I did. If you know the specification of the LED then it will give you the required resistor.
This is the one that I used.

http://ourworld.compuserve.com/homep...Bowden/led.htm

It is helpful when you need to wire more than one LED in series (end to end)

Tialian

Not sure what you mean by watts...??? You got 12v coming from the car and you want a 470 ohm resistor for each 5mm LED so you don't blow them up.

peteski

Here is how you calculate it:
I'm assuming that you have a single White (or Blue) LED in series with each resistor.
Supply voltage=13V (roughly - when car is running).
Voltage across the LED=3.2V (assuming a White or blue LED).

Voltage across the resistor: 13-3.2=9.8V
Current flowing through the resistor: 9.8/470=0.021A
Wattage dissipated by resistor: 9.8 x 0.021 = 0.21W

So, a 470 ohm, 1/4 Watt (or larger) resistor will work.

This is an example of where we actually utilize all that basic math we learned in school... :D

XbStorm

Thank you Everone this helps a great deal. I will let you know how this turns out...

kkawana

haha peteski...gotta love it when that phrase "when am i gonna use that in real life?" comes back and bites us huh??

peteski

Yup.... it is true... we do sometimes use knowledge we've picked up in school.
Who would've thunk... :D