So you’re starting out in electronics and you need a fast way to calculate the size of resistor you need for your circuit?
TL;DR Calculate the current of the circuit, voltage supply and the voltage drop across your LED load. Use Ohms law V = IR to calculate the required resistance for your LED. Follow this formula R = ( V supply - V led) / I (current). A video and example circuit is provided below.
LEDs are undeniably fun to play with when you are starting your electrical learning journey. However, unless you’re using the kind designed to run off high voltage, you will need to use resistors to light them up. Or else you run the risk of breaking burning them out.
Calculating the resistance LEDs need to work is part of the fun of working with them. And thankfully, it’s not complicated at all.
This post will teach you how resistors work and help you learn how to choose the correct resistance value for your LEDs.
What is a resistor?
A resistor is a device that resists the flow of electrical current. It does this by forcing more potential difference (voltage) across it than exists in the circuit.
The resistance of a resistor is measured in Ohms (Ω) and is given by the following equation:
R = V / I
R: Resistance in Ohms,
V: Voltage across the resistor, and
I: Current through the resistor.
We will first learn how a resistor works and how to read a resistor, and then we will see an example of wiring an LED with resistors.
How a resistor works?
When resistors are introduced into a circuit, they restrict the flow of current in the circuit. While there are many different kinds of resistors, they can only reduce current flow in three ways:
- Using a material that does not conduct electricity well
- Making the conductive material thinner
- Extending the length of the conductive material
Wire-wound resistors are the most common kind of resistor since they tend to be affordable. These have an insulating center with a conductive wire (typically copper) wound around them.
There’s also a second kind of resistor called a carbon-film resistor with a spiral of carbon around the center rather than a wire winding.
Wire-wound resistors are preferred over carbon-film resistors since they tend to be more precise. Further, the resistance they offer only depends on the thickness of the wire and the number of turns.
Wire-wound resistors are also stabler than other kinds of resistors at higher temperatures, making them seem like a superior option in many ways.
How to read a resistor?
While resistors look almost identical from the outside, there is a way to tell the difference between them by learning to read them.
The typical resistor has three rainbow-colored bands on it, after which there’s a little space, then a fourth band that’s usually colored brown, red, gold, or silver.
These colored bands allow you to identify the resistance of a resistor. Here’s how you read a resistor’s value:
- Hold the resistor so that the rainbow-colored bands are on the left.
- The first two colors of the bands represent the first two digits of the resistor’s resistance value.
- The third band is a decimal multiplier representing the power of ten that needs to be multiplied with the first two digits to calculate the resistance of the resistor. In other words, the third band represents 10x and the color of the band represents the value of x.
- The fourth and final band on the resistor represents the tolerance of the resistor. The tolerance gives you an idea of how accurate the resistance value is.
Let’s say the first three bands of a resistor are brown, black, and red, respectively. This would indicate that the resistor has a resistance of 1000 ohms.
If the resistor’s fourth band is gold, it means that the tolerance is 5%, and the resistor’s value can be anywhere between 950 and 1050 ohms.
You may find a resistor with five bands rather than four. In this type of resistor, you must treat the first three bands as the first three digits of the resistance, the fourth band as a multiplier, and the fifth band as the resistor’s tolerance.
You can refer to the color code values using the chart below:
|Resistance Values||Tolerance Values|
How to choose the correct size resistance value for LEDs?
We can use the simple Ohm’s Law to calculate the correct resistance value for LEDs.
But before you can use Ohm’s Law, you must determine the correct voltage and current values.
To derive the correct voltage (V), note down the voltage of the power supply and the voltage of the LEDs. For the sake of explanation, let’s assume you’re using two AA batteries in a series. The power source has a voltage of 3V.
LEDs have a unique characteristic called forward voltage (Vf) which denotes the amount of voltage lost across the LED when operated across a specific reference current. The reference current is typically assumed to be 20 milliamps.
But the difficulty with forwarding voltages is that they vary depending on the color of light the LED emits. Sometimes, the forward voltages of the same color of LEDs can also vary.
Let’s assume that you’re working with a standard Yellow LED having a forward voltage of 1.8V.
Therefore, the voltage across the LED will be equal to:
Substituting the values:
3V - 1.8V = 1.2V
The voltage across the LED is 1.8V, and the drop across the resistor is 1.2V. We can calculate this through the Ohm’s Law equation.
Your LED needs 1.8 volts to operate. It is the voltage across the LED.
But your battery provides 3 V. You cannot apply 3 V to your LED. So, you use a suitable resistor that will drop some voltage across itself so that only 1.8 V is left for LED.
So, 3 V are split into two parts. 1.2 V goes to resistance, and the rest of the 1.8 V appears across the LED, which is the suitable operating voltage for the LED.
Finding the value of the current is easy. The current rating of the LEDs is typically listed as If or Imax in the datasheets. It typically ranges between 25 and 30 mA.
Let’s assume the current is 25 mA.
Substituting the values in Ohm’s Law:
R = V / I R = 1.2V / 25mA R = 1.2V / 0.025A R = 48Ω
We can shorten the process of calculating the resistor value by writing the equation this way:
There’s no need to understand a whole other approach to learn how to calculate resistance for an LED circuit strip.
To work out the resistance for a strip of LEDs, all you have to do is make a slight adjustment to the formula above, like so:
There are a few ways to find out what resistor size you will need for your LEDs or any load for that matter. I would strongly suggest having a ground-up understanding of voltage drop and Ohm’s law. Once you have a fundamental understanding of electricity, everything becomes so much easier!
But if you don’t want to calculate the resistance by hand, you can plug in the values you know into an LED resistor calculator. A simple Google search should bring up several online tools you can use.
Still got Questions?
Hit me up in the comments below, and i’ll do my best to help you out!