# electronics circuit designing question?

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?

5 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.
pbergeronronronron (author)  jeff-o5 years ago
just bought a 33Ω 3w, both the resistor and high power led will be on a heatsink, hope nothing catches fire lol
5 years ago
Should be fine. Mount with heatsink compound, and monitor the temperature for the first little while until you're confident the temperature is stable.
iceng5 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...

A