The Light Emitting Diode, also commonly referred to by its shortened form of LED, is an electronic component that has the ability to emit light (hence the name) when subject to a current.

It comes from the Diode family, but with the added ability of being able to produce light. 

LEDs are an effective and efficient light source for many different applications. They can be found in mobile phones, traffic lights, camera flashes, street lamps, just to name a few.

They are also a great option for the hobbyist due to their low cost and availability. 

But, why do LEDs need resistors? LEDs need resistors to help limit the current that passes through them so they do not get damaged. Every LED has a current rating that should not be exceeded and resistors have the ability to limit the current to below the maximum allowable current allowed for the LED. 

This article shall dive deeper into the theory of why LEDs need resistors. Read on if you want to learn more!

Deeper look at LEDs

Before we take a look at why LEDs need resistors, we need to cover a few topics which will help give you a better understanding later of the question at hand (however, if you are well versed with the LED you can skip this section). 

Let’s first take a closer look at the LED. 

One of the great advancements in human history was the invention of the light bulb, thanks to one Thomas Edison

Before that, people had to illuminate their homes and businesses using candles which wasn’t the most effective or efficient way of lighting a space.

Light bulbs were revolutionary and made the task of illumination much easier. 

The next big breakthrough was the Light Emitting Diode (LED).

Just as the light bulb was more effective and efficient than the candle, the LED is more effective and efficient than the light bulb. 

Working principle of the LED

In the simplest definition, when current is passed through a LED, it emits energy in the form of light. But, let’s take a closer look at what happens inside the LED.

A LED is made of a semiconductor material that consists of two parts; P-type semiconductor and N-type semiconductor.

The p-type side of the LED contains a larger concentration of holes (positive charge), whereas the n-type side of the LED contains a large concentration of electrons (negative charge).

To make the LED produce light, a forward voltage  needs to be supplied. This is a crucial value as it is the voltage needed to unite the electrons and holes at the P-N junction. When they combine in the middle, light is emitted.

LEDs typically have a forward voltage of 1.8 – 3.3 volts. But, this number varies depending on the colour and size of the LED. The lower the colour on the light frequency, the lower its voltage drop. 

Also, LEDs come in many different shapes and sizes which will largely determine their forward voltage.

LED voltage and current ratings

Voltage and Current ratings are essential in the electrical and electronic world. It lets us know the range of values of voltage and currents needed to operate components and devices effectively. 

Most (if not all) of the time, the ratings given, indicate the maximum allowable values that they can handle. Exceeding these values can damage the component or device (sometimes permanently). 

The LED is no different. It has a set of ratings that let you know what is the ideal voltage and current to operate them at. 

This information can be found on the LED’s datasheet (every electrical/electronic component is accompanied with a datasheet).

While it comes with many ratings, two of them are of utmost importance ; the Forward voltage and the Forward current.

Aa we briefly saw above, the forward voltage tells us how much voltage is needed for the LED to emit light. The datasheet will usually indicate a further two values of forward voltage; the minimum forward voltage and the maximum forward voltage.

To emit light, a voltage that sits between these two values is needed. Supplying a voltage below the minimum will not turn on the LED, and supplying a voltage higher than the maximum will damage the LED. 

Forward current is a value that indicates how much current a LED can handle continuously. There is no minimum value. The value given is the maximum allowable current. 

However, the brightness of the LED is largely controlled by how much current is supplied to it. So, supplying the LED at the forward current rating indicated on the datasheet will make the LED shine at its brightest.

But, just like with the maximum forward voltage, exceeding this value will cause damage to the LED. 

Why do LEDs need resistors

So, why all the fuss with adding a resistor to a LED? 

Well, you might know the saying, “with great power, comes great responsibility”

Unfortunately for an LED, it is unaware of this great wisdom. 

When connected directly to a current source, it will draw as much power as it is allowed to do so (which is going to be its downfall).

As we just saw, LEDs have voltage and current ratings which should not be exceeded. This is especially true with the current. 

This is where a resistor lends a helping hand. 

A resistor is a passive electrical component which has the ability to limit the flow of current. The higher its resistance, less current is allowed to flow, and the lower its resistance, more current is allowed to flow. 

So, the main reason  LEDs use a resistor is to limit the amount of current to a value that is in the range of its forward current ratings, and not higher. 

Resistors will protect the LED from damage. 

Do LEDs require resistors all of the time?

There are some instances where a resistor might not be required when connecting a LED to a voltage source. Specifically when the voltage source (used to power the LED) is equal to the forward voltage drop of the LED.

In this scenario, no current limiting resistor is required.

For example, if you have a LED with a forward voltage drop of 3 volts, and the power is supplying 3 volts as well, you can connect them without the need of a resistor. 

How to choose the right resistor for a LED

LEDs come in a variety of shapes, sizes, colours, etc. This means that the forward current is going to be different for different types of LED.

So, there isn’t going to be one specific resistor you can use for all LED types. Also, there are many different power supplies of different voltage values which you could use to power a LED. 

So, how do you go about choosing the right resistor for the job?

Choosing the right resistor comes down to a little math, and a commonly used equation in electronics known as Ohm’s Law (seen below)

Where V is voltage, R is resistance and I is current

Below is a simple series circuit containing a voltage source, resistor and LED.

In order to calculate the resistance of the resistor required, we will need a few values which include;

  • Voltage value of power supply 
  • LED forward voltage (max)
  • LED forward current 

From the image above we can see that the power supply voltage is 5 volts, the LED’s forward voltage is 1.8 volts, and its forward current is 30mA.

Earlier we just saw the equation of ohm’s law. But, we need to find the resistance, so rearranging the formula gives us the following equation;

Where VS is the voltage of the power supply, VF is the LED forward voltage, and IF is the LED forward current

Next we plug the values into the rearranged equation. 

Note, the forward current of the LED is given in milliamps (mA). However, when used in an equation with volts (not millivolts), it needs to be converted to Amps (A). To do this just divide the 30mA by 1000 which gives us 0.03A. 

After the calculation, we get a value of 106 ohms. If there isn’t a resistor of 106 ohms, just move to the next highest available resistance value. 

Selecting a resistor when adding multiple LEDs in series and parallel

What if you are adding more LEDs to a circuit? How do you go about finding the right resistor?

Before answering that question, we need to first take a step back and take a look at two types of configurations of circuits; Series and Parallel

The earlier example of choosing a resistor for a LED was for a series circuit. But, if we added another LED, we could do so in a series or parallel configuration. 

Below shows both configurations of circuits if we were two add a second LED.

LEDs are polarised components, which means they have a positive side known as an Anode, and a negative side called the Cathode.

You can see in the series configuration, the LEDs are daisy chained together (the negative side of the first LED is connected to the positive side of the second LED). We would follow this pattern if we were to add more LEDs.

In the parallel configuration, LEDs are connected across each other. The positive side of the first LED is connected to the positive side of the second LED (and the same for the negative side). If we were to add more LEDs we would continue this pattern. 

Voltage and Current are the two variables that change in these two types of circuit configuration. 

In a series circuit, the current remains the same across all LEDs (or other components) regardless of how many you add. However, the voltage is shared.

In a parallel circuit, the voltage remains the same across all LEDs (or other components) regardless of how many are added. However, the current is shared. 

Let’s take a look at choosing a resistor for the different circuit configurations when adding more than one LED.

Choosing a resistor for a series circuit with multiple LEDs

Let’s take a look at a series circuit. 

When it comes to selecting a resistor for a series circuit with multiple LEDs, the process is not too different to what we saw earlier with just one LED.

We still use the same equation, however this time rather than having only one LED forward voltage, we are going to need the sum of all the forward voltages of LEDs being used. 

A quick example will clear up any confusion.

Say we are connecting 3 LEDs in series as seen in the image below. 

As you can see, all LEDs have different forward voltages. To calculate the resistance of the resistor needed, we just need to add all forward voltages which gives us a total voltage of 4 volts.

Now, we just plug this value into the same equation we used earlier for a single LED;

But, what about the current value? All three LEDs have different forward current values. So which one do we select?

You need to select the lowest forward current value of the three LEDs (in this instance 20mA). If you select a current value of 30mA, the LED with a 20mA maximum forward current rating is going to get damaged.

From the equation, we get a resistance value of 200 ohms.

One important note to make going forward is that if the sum of all your LED forward voltages is greater than the supply voltage, you will need to increase the supply voltage otherwise the LEDs will not light up.

So, if the three LEDs above each had a forward voltage of 2 volts, this means that the sum of their voltages equals 6 volts, which is greater than the supply voltage of 5V. 

You would need to bump up the power supply to 6V or greater. 

Choosing a resistor for a parallel circuit with multiple LEDs

Selecting a resistor for a parallel circuit is a bit tricky. Unlike a series circuit where only one resistor is needed, a parallel circuit requires a resistor for every LED.

This comes down to the fact that voltage is the same across all LEDs in a parallel configuration. When connecting multiple LEDs of different forward voltages, the voltage across the LEDs is restricted to the forward voltage of the LED with the lowest value. 

Even if the forward voltages are the same, and the LEDs come from the same batch, from the same manufacturer, there is going to be a slight deviation in their voltages (say 0.1 volts) which is going to cause problems.

If you don’t quite understand the theory, just realise that the best practice to avoid any problems is to use a separate resistor for every LED. 

Let’s take a look at how we go about selecting the right resistor for each LED.

Below is a parallel circuit using the three LEDs with the same values we just saw in the series circuit. 

As you can see, each LED has its own resistor. To calculate the resistance of each resistor, we just treat each LED and its resistor as its own loop with the power supply, which essentially becomes a series circuit, and we use the same equation we use for a series circuit to calculate each resistor for each loop. 

So, LED 1 has a forward voltage of 1 volt, and a forward current of 20mA. Therefore its resistor value will be 5 – 1/0.02, which equals 200 ohms (or the nearest available resistance).

LED 2 has a forward voltage of 1.2 volts, and a forward current of 30mA. Therefore its resistor value will be 5-1.2/0.03, which equals 126 ohms (or the nearest available resistance).

LED 3 has a forward voltage of 1.8 volts, and a forward current of 30mA. Therefore its resistor value will be 5 – 1.8/ 0.03, which equals 106 ohms (or the nearest available resistance).

If you add more LEDs, just follow the same process. 

Do you connect resistors to the positive or negative lead of the LED?

We briefly touched on the fact that a LED has polarity (or are polarised). Without going into too much detail, polarity defines a component having two poles; positive and negative

Components that have polarity include batteries, LEDs, polarised capacitors, motors, etc.

Polarised components need to be connected the right way in a circuit. Their poles need to match the terminals of the power supply (positive to positive, and negative to negative). 

Non-polarised components on the other hand, have no polarity and therefore can be connected in any orientation in a circuit. 

A resistor is a non-polarised component. 

So, does it matter if you connect a current limiting resistor to the positive or negative side of a LED? No, it does not matter which side of the LED you connect the resistor to. It will still perform the job of limiting current. 

Do all types of  LEDs require resistors

There are many different types of LED available to fill your next project full of light. They include;

  • Traditional through hole LEDs
  • SMD LEDs
  • RGB LEDs
  • Cycling LEDs
  • Infrared LEDs
  • High Power LEDs

There also exists a special type of LED that has an in-built resistor. This LED has a tiny SMD resistor that is encapsulated within the LED.

This type of LED does not require a resistor.