This is a simple calculator that will help you find the nearest/best value of resistance (R measured in Ohms) and power (P measured in Watts) of resistor (E24 Standard Resistor Series) for different types of LEDs. To calculate the resistor for a simple LED circuit like above, enter a data into a form and click the button.
Here are the equations used in calculator.
, , where
is Forward Voltage of LED.
The voltage drop when we supply the rated forward current. You can find this data in the LED datasheet, but is somewhere between 1.3V to 3.5V depending on type, color and brightness. If you are still unable to find the forward voltage, simply connect the LED through 200Ω with 6V battery. Now measure the voltage across LED. It will be 2V and this is the forward voltage.
is a Forward Current of LED.
This is the amount of maximum current that LED can accept continuously. It is recommended that provide 80% of LED forward current rating for long life and stability. For example, if the rating current of LED is 30mA, then you should run this LED on 24mA. Value of current over this amount will shorten LED life or may start to smock and burn. If you are still unable to find the LED forward current in the LED datasheet, than assume it 20mA because a typical LED’s run on 20mA.
is supply voltage.
It’s important to use suitable resistor because your LED will begin to draw a bunch of current (diode has a characteristic I-V curve) and in some cases, burn out. A resistor is used in series with the LED to keep the current at a specific level called the characteristic (or recommended) forward current.