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electronics circuit designing question? Answered

so i am pretty good in electronics, im curently modifing a circuit and just found the value of the resistors with the ohms law. i have to lower from 12volts dc to 3.5 volts dc to a 300mA consuming high powered led with the cathode to the ground. if my calculations are right i would need to use a 30 ohm 3w resistor, should i take a 4w so the heat is not too intense? the led is not going to be light up always, its going to oscillate on music(color organ).

are my calculation right?



Best Answer 7 years ago

Your calculations are correct. A 4W resistor would probably be enough, considering that it won't be on 100% of the time. If you can't find a 4W resistor, move up to 5W as they are quite common and plentiful.

just bought a 33Ω 3w, both the resistor and high power led will be on a heatsink, hope nothing catches fire lol

Should be fine. Mount with heatsink compound, and monitor the temperature for the first little while until you're confident the temperature is stable.


7 years ago

You want to run a 300ma 3.5V LED on 12VDC.
12-3.5 =8.5VDC
Resistance = V / I  =  8.5 / .3 = 28.3 ohms
Resistor P = V * I  = 8.5 x .3 = 2.5 watts

So a 30 ohm 4 watt Resistor will work...