Why connect a resistor to an LED?


Table of Contents
0:00 Opening
0:18 What is LED in the first place?
1:25 A Look at LED Products Around the World
2:24 Why connect a resistor to an LED?
3:18 Summary

Why connect a resistor to LEDs|Introduction to Practical Electronic Circuits with LTspice

I get what you're saying about resistors, but I don't get why you'd connect a resistor to an LED.

Let's think together about the characteristics of LEDs and why we connect a resistor to an LED!

For those who are interested in this topic, we will talk about "rethinking why we connect resistors to LEDs.

The rest of this video is explained in the CQ Publishing newsletter.
Please also see here.

Please read this article to the end so that you will understand why resistors are connected to LEDs.

What is LED?

First, a brief explanation of what LEDs are is given as a preliminary knowledge.

What is LED?
LEDs are a type of diode with the property that current flows only in one direction.

The structure consists of a hole-rich P-type semiconductor and an electron-rich N-type antiferromagnet, which is called a "PN junction.

P-type semiconductor and N-type antiferromagnetPN Junction

This PN connection is normally electrically balanced, so no electrons are transferred.

PN Junction

However, when voltage is applied from the outside, the balance is disrupted and electrons gradually move and current begins to flow.

PN Junction

And there is a characteristic that the current flows rapidly after a certain threshold.

PN Junction

If the horizontal axis is the "magnitude of voltage applied from outside" and the vertical axis is the "flowing current," a curve similar to the graph in the following image is drawn.

Voltage vs. current graph

This characteristic is sometimes called the "forward characteristic" or "Vf characteristic" of the LED.

Please be sure to keep in mind that LEDs have these curve characteristics, as it is very important to consider the operating point, which is also explained in the newsletter!

And LEDs are always connected to a set of resistors.

LEDs and Resistors

A look at LED products in the world

Let's pick up some LED products in the world and check them out.

First, let's look at the LED tape we bought at Akihabara.

As shown in the image below, LED tape is a product with many LEDs on a long, thin wire in the shape of a tape, which can be plugged into a 5V USB port to turn on many LEDs.

LED tape bought at Akihabara

As shown in the following image, a closer look at the contents reveals that resistors are connected between LEDs in equal-sensory alignment.

Finally, let's look at the LEDs that came with the "AgIC" conductive ink starter kit.

AgIC" is a product that makes it easy to enjoy electronic construction by allowing users to create circuits as if they were drawing on a piece of paper.

As shown in the following image, LEDs could be easily attached.

And this circuit also looks like only the LEDs are connected, but if you zoom in on the LED board, you can see that there is a resistor firmly on it.

As shown above, it was confirmed that products in the world always include a resistor in combination with the LED.

Reasons for resistor connection to LEDs

Finally, here are the reasons for making resistor connections to the LEDs.

In conclusion, there are three reasons to connect a resistor

Reason for connecting a resistor
  1. LED Protection
  2. Stabilization of LED light intensity
  3. Power Supply Protection

In many cases, only the first reason is explained on the Internet, but actually the second and third reasons are also important.

We will take a closer look at the reasons for connecting resistors.

Reason for connecting a resistor (1)
LED Protection
To limit the amount of current flowing to the LEDs so that the allowable current of the LEDs is not exceeded.
Reason (2) for connecting a resistor
Stabilization of LED light intensity
It is used to stabilize the alternating current of the LEDs even when fluctuations in supply voltage or ambient temperature occur. The concept of operating points explained at the beginning of this section is easier to understand.
Reason for connecting a resistor (iii)
Power Supply Protection
The reason we need "power supply protection" is on the power supply side. If a coin cell battery or linear regulator is used as the power supply, the allowable current is low, and one of the reasons for connecting a resistor is to prevent the current from exceeding that magnitude.


In this article, we have introduced the reasons for connecting a resistor to an LED.

The information discussed here is up to an overview level, so if you want to know more details, please take a look at the newsletter as well.

The newsletter explains a more detailed concept of the operating point and how to analyze the operating point in a simulation.

If you are just getting started with electronics, be sure to check out our other articles and videos!