3w led light

[iroc_03]

I am building a light using five 3w leds. The leds are 3.2v / 700ma and will be used on a 12v car power supply and need to know what resistor, etc i need to reduce the voltage to them. They will be wired in parallel. I have used a led calculator to find the proper resistor but am new to electronics and am not sure if the resistor is all I would need. I don't want to burn out my lights. Thanks for any help.

 

[KMoffett]

Wiring LEDs in parallel is a bad practice.

If you have three LED/resistor series-combinations in parallel it will work. But, you will be wasting a lot of power (heat) in the resistors by wiring the three combinations in parallel:

One series LED/resistor set (12v-3.2V)/0.7A=~12Ω....(12v-3.2V)*0.7A=~6W...6W*5sets=30W of heat in the resistors.

A better way is one set 3 LEDs and one resistor all in series, and another set of 2 LEDs and one resistor all in series:

One set of 3 LEDs and 1 resistor in series (12v-9.6V)/0.7A=~4Ω....(12v-9.6V)*0.7A=~2W...2W*1set=2W of heat in the resistor.
One set of 2 LEDs and 1 resistor in series (12v-6.4V)/0.7A=~8Ω....(12v-6.4V)*0.7A=~4W...4W*1set=4W of heat in the resistor.
Total of 6W with 2 resistors as opposed to 30W with 5 resistors.

ken

 

[audioguru]

The absolue max allowed current into one of those LEDs when it has a huge perfect heatsink is 700mA. Your car battery is almost dead when it is only 12V and might charge at 14.4V.
Therefore your LEDs will burn out very soon.

 

[berty_a]

It works better with a dc/dc converter,you can have just 3W power loss.