i know how much current is coming into the circuit and the voltage supplied. I also know the power of the led im goin to use. How do i calculate the voltage drop needed and how much resistance
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It is important you know what the LED requirements are!
You need its operating voltage and current, or its operating voltage and wattage (power), which you say you have, but have not supplied here!
If you have voltage and wattage, divide wattage by the voltage, answer will be the current. So you then have LED operating voltage, and current.
Next, you subtract the LED operating voltage from the supply voltage, e.g. (12V - 3V) = 9V. This is the voltage you need to DROP across the resistance.
Now using that voltage and the LED current and Ohm's Law, you calculate the resistance. e.g. if the LED operating current was 50mA (.050A), then resistance in ohms = (voltage / current), = (9/.050), = 180 ohm resistor required.
You also need to check the resistor you use can handle the heat dissipation. Using the above figures again, and Watt's law, W = (voltage x current), = (9 x .050), = 0.45W.
So you would obtain a 180 ohm, 1/2 watt resistor if the above example LED was used.
You need to substitute the actual voltage/ current for your LED, and rework for resistance and wattage in a similar manner to above.
You need its operating voltage and current, or its operating voltage and wattage (power), which you say you have, but have not supplied here!
If you have voltage and wattage, divide wattage by the voltage, answer will be the current. So you then have LED operating voltage, and current.
Next, you subtract the LED operating voltage from the supply voltage, e.g. (12V - 3V) = 9V. This is the voltage you need to DROP across the resistance.
Now using that voltage and the LED current and Ohm's Law, you calculate the resistance. e.g. if the LED operating current was 50mA (.050A), then resistance in ohms = (voltage / current), = (9/.050), = 180 ohm resistor required.
You also need to check the resistor you use can handle the heat dissipation. Using the above figures again, and Watt's law, W = (voltage x current), = (9 x .050), = 0.45W.
So you would obtain a 180 ohm, 1/2 watt resistor if the above example LED was used.
You need to substitute the actual voltage/ current for your LED, and rework for resistance and wattage in a similar manner to above.