# correct resistor help

#### gizmo13

Apr 20, 2010
2
Hello all,

I am making a chaser light unit for my car and I am planning on using a 3w led.
I have used a website called ledz.com which has an automatic resistor calculator.
I typed all the info: 14v, 2.2 voltage drop with 700 mA and it gave me a resistance
calculation of 16/857 ohms.
Is this correct??????

Craig

#### Laplace

Apr 4, 2010
1,252
The formula is R=E/I = (14-2.2)/0.700 = 16.857142

#### nbw

May 8, 2011
48
Just be aware that if your battery voltage is down a bit; e.g. you have lights on, or the battery is just getting old - you won't be seeing 14.4V but a little less. Might also be worth considering some form of protection in case the odd higher voltage spike comes through...!

#### BobK

Jan 5, 2010
7,682
Hopefully, the calculator also told you that the resistor will dissapate 8 Watts. It's going to get hot, and if you use less than a 10W resistor, it's going to melt.

Bob

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Using a resistor to drive a high powered LED is generally not the best idea. You are far better off finding a constant current driver, and preferably a switchmode device so that you can get high efficiency as well as constant brightness as the voltage fluctuates.

#### gizmo13

Apr 20, 2010
2
Thanks all.
Will do some more research.

Craig