LED Circuit Resistor Value Calculator
We often have an LED circuit we want to create and we need to figure out what value of resistor to use.
We often know three facts:
- The voltage of the circuit (source voltage)
- The voltage drop across the LED (a property of the LED). This is typically 1.7V for a red LED and 3.0V for a blue or white LED.
- The amount of current we want to flow through the resistor - typically 2 to 20 milliamps depending on the brightness we want.
To calculate the resistor needed, we subtract the voltage drop across the LED from the source voltage and apply Ohm's law. So our formula is:
Resistance = (Vsource - VLED) / Current
Here are the voltage drops for different colors of LEDs:
LED Color | Voltage Drop | Notes |
---|---|---|
Red | 1.8 - 2.2 | Used in indicator and alarm applications |
Amber | 2.0 - 2.1 | Used in automotive and signal lighting |
Yellow | 2.1 - 2.2 | Common in display panels and traffic lights |
Green | 2.0 - 3.5 | Bright, used in displays and decorative lighting |
Blue | 2.5 - 3.5 | High-brightness, used in various applications |
White | 3.0 - 3.5 | High brightness, used in general illumination |
The electronic component manufacturer Vishay has detailed data sheets on individual LEDs. Here is a web page of their red LEDs. Note that in the data sheets for any given component, the voltage drops are not specified exactly. Due to variations in the manufacturing process, only a range of voltage drops is specified with a min, average and max value.
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