The resistor needed for an LED connected to a 12V power supply depends on the LED's forward voltage and forward current. Without that specific information, we can provide a generalized method to calculate it, and an example with typical values.
Calculating the Resistor Value
To determine the appropriate resistor value, use Ohm's Law and the following formula:
Resistor (R) = (Voltage Source (Vs) – LED Forward Voltage (Vf)) / LED Forward Current (If)
- Vs: The voltage of your power supply (in this case, 12V).
- Vf: The forward voltage of your LED. This is the voltage drop across the LED when it's lit. It's a characteristic of the LED, and you can find it in the LED's datasheet. Common values are around 2V for red LEDs, 3V for green or yellow LEDs, and 3.2V - 3.6V for white or blue LEDs.
- If: The forward current of your LED. This is the amount of current (in Amperes) the LED needs to operate at its intended brightness. This is also found in the LED's datasheet. Common values are 10mA (0.010A) to 20mA (0.020A).
Example Calculation
Let's assume:
- Vs = 12V
- Vf = 3.4V (typical for a white LED)
- If = 0.010A (10mA - a common forward current)
Then:
R = (12V - 3.4V) / 0.010A = 8.6V / 0.010A = 860 Ohms
Therefore, in this example, you would need an 860 Ohm resistor. However, standard resistor values rarely match calculated values exactly. You should use the next highest standard resistor value available (e.g., 860 ohms is not a common value, so use 870 or 910 ohms).
Important Considerations
-
LED Datasheet: Always consult the LED's datasheet for its specific forward voltage and current requirements. This information is crucial for accurate resistor selection. Using incorrect values can lead to under-driving the LED (dim light) or over-driving the LED (potentially damaging it).
-
Power Rating: The resistor also needs to have a suitable power rating. Calculate the power dissipated by the resistor using the following formula:
*Power (P) = (Vs - Vf) If**
In our example:
P = (12V - 3.4V) * 0.010A = 0.086 Watts
Use a resistor with a power rating greater than this calculated value. A 1/4 watt (0.25W) resistor is typically sufficient for low-current LEDs, but higher-power LEDs may require larger resistors.
-
Series Resistor: The resistor must be connected in series with the LED. Connecting it in parallel will short circuit the power supply.
Summary
The resistor value needed for an LED on a 12V supply is calculated using Ohm's law and depends on the LED's forward voltage and forward current. Always consult the LED's datasheet and choose a resistor with an appropriate power rating.
