LED Current Limiting Resistance Calculator (with Practical Example)

This calculator can help you determine the appropriate resistor value for a LED forward voltage drop and power supply. The resistor is used to limit the amount of current into the diode.

Calculator

To find the resistor, enter the following for the LED

  • Supply Voltage (V)
  • Forward Voltage (Vf)
  • Current (I)

LED Current Limiting Resistor Formula

R = (V-Vf)/I

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A LED current-limiting resistor is a resistor used in series with a light-emitting diode (LED) to prevent too much current from flowing through the LED. LEDs are sensitive components that can be damaged or destroyed if they receive more current than they are designed to handle. The resistor ensures that the current flowing through the LED remains at a safe level, extending the LED’s lifespan and ensuring proper operation.

Why Do LEDs Need a Current-Limiting Resistor?

LEDs have a forward voltage drop, which means they only require a specific voltage to operate. However, without limiting the current, an LED can draw more current than it can safely handle, resulting in overheating, reduced lifespan, or even permanent damage. A current-limiting resistor helps control the amount of current passing through the LED.

Example Calculation

The data sheet for a diode can be used to find its forward voltage and current.

From the diode current voltage plot below Vf = 2 V and we use I = 5 mA

Use these values in the calculator above to find a resistor value of 600 Ohm. Approximately 15 mW of power is dissipated in the resistor.

Referring to this datasheet, any of the resistors can be used based on the power rating information provided. For instance the smallest 0402 resistor has a rating of 63 mW which exceeds the calculated dissipation by a factor of 4.

If you cannot find the data sheet for the LED here are a couple of other methods to find the forward voltage:

  1. Examine the LED:
    • Look for any markings on the LED or its packaging. Some LEDs have the forward voltage printed on them.
  2. Use a General Guideline:
    • In the absence of specific information, you can use a general guideline. For example:
      • Red LEDs typically have a forward voltage of around 1.8 to 2.2 volts.
      • Orange and yellow LEDs typically have a forward voltage of around 1.9 to 2.2 volts.
      • Green LEDs typically have a forward voltage of around 2 to 3.1 volts.
      • Blue and White LEDs typically have a forward voltage of around 3.0 to 3.6 volts.

Note that these are approximate values, and the actual forward voltage can vary depending on the specific LED model and manufacturer.

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