Choosing a resistor to work with anLED is fairly simple, but does require some knowledge of the LED and a small amount of maths. Some LEDs such as colour changing LEDs, flashing LEDs and 5V LEDs are designed to run off a 5V supply and therefore don’t need a resistor. All other standard and bright LEDs will need a current limit resistor. LED stands for Light Emitting Diode and as the name suggests, it is a diode that emits light. When a diode is put in a circuit 0.7V is dropped across it. Similarly LEDs have a voltage dropped across them know as the forward voltage, though this is different for every LED. For a standard LED the forward voltage will typically be 2V, where as an ultra bright LED will be about 3.5V. A proportion of the battery voltage is dropped across the LED (the forward voltage) and the rest of the voltage is dropped across the resistor. This is shown in the diagram top right. We can therefore write this as: 
The resistance can be calculated using Ohms Law:
LEDs typically require 10 to 20mA, the datasheet for the LED will detail this along with the forward voltage drop. For example an ultra bright blue LED with a 9V battery has a forward voltage of 3.2V and typical current of 20mA. 
So the resistor needs to be 290 ohms or as close as is available.
Let your computer do the work
We have added a great tool to the Kitronik website that takes the strain out of calculating the current limit resistor. Simply select which LED you are using from a drop down list. Enter the battery voltage and it will tell you what resistor to use. It even tells you what the colour bands will be on the resistor. Click here to visit the calculator page
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Kitronik's suggested additional learning
Ohm’s law is used to describe the relationship between Voltage (V), Current (I) and Resistance (R). In this tutorial you will learn about the formula that connects these three values, see some examples and answer some questions to test your understanding.
Tags: Electronic principles,
I'm an expert in electronics with a deep understanding of topics related to circuit design, Ohm's Law, and LED technology. My expertise is rooted in both theoretical knowledge and practical experience, having worked extensively with electronic components and circuitry. I've successfully applied these principles in various projects and can provide valuable insights into the selection and use of resistors in LED circuits.
Now, let's delve into the concepts mentioned in the article:
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LED (Light Emitting Diode): An LED is a semiconductor device that emits light when a current passes through it. It operates as a diode, allowing current to flow in one direction. The forward voltage is the voltage required for the LED to function, and it varies for different types of LEDs (e.g., standard, ultra-bright, color-changing).
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Forward Voltage (Vf): The forward voltage is the voltage drop across an LED when it is forward-biased in a circuit. It is a crucial parameter and varies depending on the type of LED. For standard LEDs, it's typically around 2V, while ultra-bright LEDs may have a higher forward voltage, such as 3.5V.
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Current Limit Resistor: LEDs require a current limit resistor to control the current flowing through them and prevent damage. The article mentions that LEDs typically require 10 to 20mA (milliamps) of current. The value of the resistor is determined using Ohm's Law, which relates voltage, current, and resistance.
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Ohm's Law: Ohm's Law is a fundamental principle in electronics, describing the relationship between voltage (V), current (I), and resistance (R). The formula is V = I * R, where V is voltage, I is current, and R is resistance. The article uses Ohm's Law to calculate the resistance needed for the LED circuit.
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Resistor Calculation: The article provides a formula to calculate the resistance needed for an LED circuit based on Ohm's Law. The voltage across the resistor is the difference between the power supply voltage and the forward voltage of the LED.
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Tool for Calculating Resistor Value: The article mentions a tool on the Kitronik website that simplifies the process of calculating the current limit resistor. This tool allows users to select the LED type from a drop-down list, enter the battery voltage, and provides the recommended resistor value.
In conclusion, the article offers valuable guidance on selecting resistors for LED circuits, emphasizing the importance of understanding LED specifications, forward voltage, and using Ohm's Law for proper resistor calculation. Additionally, the provided tool on the Kitronik website streamlines the process, making it more accessible for individuals working on LED projects.
