How to Choose a Resistor for LEDs
- 1). Read the technical data sheet or packaging for the LED to determine the correct operating voltage and the recommended maximum current. The operating voltage is also known as the forward voltage, or Vf, For a red LED the Vf is around 2.0 volts. Blue and white LEDs have a Vf of approximately 4.0 volts. The recommended current for an LED is typically around 20 mA.
- 2). Use a digital multimeter to measure the voltage at the point in the circuit where the LED will be attached. Subtract the LED Vf from the circuit voltage to calculate the voltage drop required from the resistor. For example, with a 12-volt supply and a Vf of 3.4 volts, the resistor needs to drop 8.6 volts to provide the correct voltage to the LED.
- 3). Calculate the value of the resistor by substituting the voltage and current values into Ohms Law. This states that voltage = current x resistance. Rearranging the formula shows that dividing the voltage by the current gives the value of the required resistor. In the example (12-3.4) / 0.02 Amps = 430, so the resistor value would be 430 ohms.
- 4). Choose a commercially available resistor rated as near as possible to the calculated value, or combine several smaller resistors in series to achieve the correct value.
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