Electrical – 10x 3w led lights to run off 12v battery

12vautomotivehigh-powerledwatts

i have ten 3w leds i need to build an array to run from a 12v car battery.

Led (forward voltage 3.6) (forward current 700 ma )
(Source voltage 12-13.9)

Will this work and if it will what wattage does the registers need to be?
if not can you help with a design that will. Thank You

schematic

Best Answer

Simple Ohms Law. V = IR = .7 Amps * 10 Ohms = 7 Volts. Then P = V * I = 7 * .7 = 4.9 Watts. So you need more than that to make sure they don't fry themselves. 6 to 10 Watts each, and a heatsink for both the resistors and the leds. A fan may be a good idea.

Of course, that's ideal numbers. Your resistor may not be exactly 10 ohms, your battery not exactly 12 Volts, the leds not exactly 3.6V at 700 mA, so the resistor voltage and current change, etc. 3.6v + 3.6v + 7v is 14.4 volts. That's only on a car battery if being charged.

If you could do 9 leds instead, it's typically done with 3 leds in series with the resistor. Then you only need 1.2V across the resistor and then it's much less power wasted. 1.2 * 0.7 = 0.84 Watts, a 1 W resistor would be fine but use a 2 W just in case.

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