How to Use It?
To use the calculator, simply enter the supply voltage, forward voltage, forward current, and number of LEDs, and it will calculate the resistance required to drive those LEDs at a specified current level.
You can specify the forward voltage of the LED by typing a value directly into the forward voltage input field or by picking an LED color.
As a bonus, it also calculates the power consumed by each LED, all LEDs and the resistor.
LED Voltage and Current
LED stands for Light Emitting Diode. It is a special type of diode that produces light when a current flows through it in the forward direction. LEDs are available in a wide range of colors with the most common being red, green, yellow, blue, orange, white and infrared (invisible) light.
Every LED has a characteristic forward voltage drop (Vf). For most low-power LEDs, the typical voltage drop is from 1.2V to 3.6V for currents between 10mA to 30mA. The exact voltage drop will, of course, depend upon the semiconductor material used, color, tolerance, forward current flowing through it, and other factors.
Here’s a table that shows the typical forward voltages of LEDs of various colors.
| Color | Forward Voltage (V) |
| Red | 1.8 to 2.1 |
| Amber | 2 to 2.2 |
| Orange | 1.9 to 2.2 |
| Yellow | 1.9 to 2.2 |
| Green | 2 to 3.1 |
| Blue | 3 to 3.7 |
| White | 3 to 3.4 |
As the LED is basically a diode, its IV characteristics curves can be plotted for each color as shown below.
Unless otherwise specified, you should consider a nominal drop of 2V and forward current 20mA.
The Current Limiting resistor
If you connect an LED directly to a battery or power supply it will try to dissipate as much power as possible, and, it will destroy itself almost instantly.
Therefore it is important to limit the amount of current flowing through the LED. For this, we use resistors. The resistor limits the flow of electrons in the circuit and prevents the LED from trying to draw too much current.
The current-limiting resistor is placed in this way:
In the above circuit, the resistor has a node voltage of Vs on the left and a node voltage of Vf on the right, the voltage across the resistor is the difference between the two voltages.
By applying Ohm’s law, the current-limiting resistor is calculated as:
If you have multiple LEDs connected in series, you should consider the total voltage drop across all of them, which is the forward voltage drop of each LED multiplied by the number of LEDs.
Modifying the above equation, we get:
Power Consumption
To calculate the power consumption, you can use the formulas listed below:
Power consumed by each LED:
Power consumed by all LEDs (total):
Power consumed by the resistor:
The last equation helps you determine the power rating of the resistor, which is the maximum amount of power that it can dissipate without overheating.
Example 1
Consider a red LED with a forward voltage drop of 1.8V connected to a 5V DC power supply. Calculate the value of the current-limiting resistor required to limit the forward current to approximately 10mA.
Using the formula above, the current-limiting resistor is:
This suggests that we will need a 320Ω resistor to limit the current to 10mA.
Example 2
Consider you want to power three red LEDs with a forward voltage drop of 1.8V and a forward current of 20mA using a 9V supply.
You can find the value of the current-limiting resistor as follows:
This suggests that we will need a 180Ω resistor to limit the current to 20mA.
