What is a LED Resistor : Circuit, Working & Its Applications

The resistors and LEDs are the most essential electronic components which forms the basis of most of the electrical or electronics circuit. The main function of an LED or Light Emitting Diode is to emit light once electric current is supplied through it whereas a resistor is used to restrict the flow of current. These components are more frequently used in  to build mini electronics projects like LED glowing using a battery and a resistor.

So this article discusses an overview of a LED resistor and its working with applications.

What is a LED Resistor?

When a resistor is connected in series with an LED in a circuit to protect the light-emitting diode by restricting the flow of current through them is known as an LED resistor. In a LED circuit, a small change within voltage supply can cause the LED to glow very brightly or dim or they can even damage also. So a series resistor is used to avoid the flow of current effect through LED slightly.

LED Resistor Circuit

The LED resistor wiring diagram is shown below. So, the required electronic components to build this circuit are an LED, battery, and resistor. Before connecting this circuit, we have to know the polarity of each component so that the connection of these components can be done very easily.

LED Resistor Circuit
LED Resistor Circuit

An LED has two terminals a positive & a negative where the positive is known as the anode and the negative is known as the cathode. The LED schematic symbol is similar to the diode apart from two arrows outwards. The anode is marked with a triangle & the cathode is marked with a line.

Generally, the two terminals of an LED include one longer terminal and one shorter terminal where the longer term is the anode and the shorter terminal is the cathode. Resistors have no polarity so they can be arranged in any way so they work fine in any direction.

The polarity of batteries can be determined very easily because the two terminals of the battery the positive & negative will be marked through a positive (‘+’) and a negative (‘-‘) symbol.

The connection of this circuit can be done as per the simple circuit diagram shown below. Here, the resistor is connected in series with a battery and voltage source.

How Does a LED Resistor Work?

Once an electric current flows through an LED then it emits light. This simple circuit uses a voltage source to power an LED through a resistor which is connected in series with the LED. The resistor used in this circuit is frequently known as a ballast resistor. The purpose of the resistor in an LED circuit is to limit the current flow throughout the LED and avoids excess current otherwise that can damage the LED. If the voltage source in the circuit is equivalent to the voltage drop of the LED, then no resistor is necessary.

The resistance of this resistor can be calculated very easily through Ohm’s law & Kirchhoff’s circuit laws. The LED voltage (VLED) can be subtracted from the voltage source (VLED), after that it can be divided through the required current (I) for LED operation. So the equation for resistance can become as

R = V – VLED/I

So this circuit can be used in a monitor of a computer, DVD player like a power-on indicator. Even though, this circuit is extensively used in consumer electronics.

How to Calculate the Value of Resistor for LED?

The example problems of LED resistors for the single resistors and multiple resistors are solved below.


The simple LED circuit diagram is shown below which includes 3V of voltage & 35 mA of current that should be connected to a 9 V supply. Calculate the resistance value?

Single LED Resistor Example
Single LED Resistor Example

We know that VLED = 3V, V = 9V and I = 35mA

The formula to calculate the resistance value for a single resistor in the above circuit is;

R = V – VLED/I

Substitute the values in the above equation to get ‘R-value.

R = 9 – 3/0.035 = 171 ohms

In the above circuit, the resistor must have 171 ohms of resistance. If the exact value is not accessible, select the next higher resistance value to maintain the current under the LED limits.


In the following circuit, two LEDs are connected in series with a resistor and voltage source 9V like Red LED & Blue LED. The Red LED voltage value is 3V & blue LED voltage value is 4V where both LEDs have 30mA of current. Please find the resistor value for the given circuit.

Multiple LEDs Circuit
Multiple LEDs Circuit

We know that values like V = 9V, VLED1 = 3V, VLED2 =4V and I = 30mA or 0.03A.

The formula to find the resistance value when two LEDs are connected in series with a resistor is as;

R = V – VLED1 – VLED1/I

R = 9 – 3 – 4/0.03 = 2/0.03 = 66 Ohms

R = 66 Ohms

LED Blinking with Arduino

One of the beginner projects designed with Arduino is LED blinking. In this project, a resistor is used to restrict the current flow throughout the LED. The required components to build this LED blinking circuit mainly include Arduino Uno Rev3, Breadboard, Jumper Wires, and a 220Ω resistor.

Arduino based LED Blink
Arduino based LED Blink

The connections in the above circuit are made like this. A voltage supply from Arduino is given to an LED through a resistor. So a breadboard and connecting wires are required to connect an Arduino board with the components like an LED & a resistor.

In the above connection diagram, the voltage supply from Arduino is given to one of the legs of the resistor whereas the remaining leg is connected to the LED. So we can observe that the LED is directly connected to both pin 10 & GND terminals of Arduino. A 220 ohms resistor is connected in between pin 10 of Arduino & the LED. Once the program is uploaded then the LED will turn ON & OFF.

Arduino Sketch that blinks an LED every second
int pinLed = 10; // LED is attached to pin 10
void setup()
// set pin 10 as an output pin
pinMode(pinLed, OUTPUT);
// turn the LED off at beginning
digitalWrite(pinLed, LOW);
//start serial connection
void loop()
// turn the LED on
digitalWrite(pinLed, HIGH);
// wait for 1 second
// turn the LED off
digitalWrite(pinLed, LOW);
// wait for 1 second

Thus, this is all about an overview of LED resistors with examples. Generally, every LED has a current limit that it can carefully handle. Once the current supply is increased above the fixed range, then LED will damage. So, restricting the flow of current throughout the LED using a series resistor is a simple practice. But this technique is not applicable for high current LEDs because they require a more reliable switching current regulator. Here is a question for you, what is the Arduino board?