The variable resistor is a versatile and commonly used component in many electrical and electronic circuits. It has the capability of varying its resistance which provides handy in many different applications.

But, why is a variable resistor used in a circuit? A variable resistor is used in circuits to provide a range of resistances that can vary in values from its minimum resistance to its maximum. Common applications where this is helpful are volume control, voltage dividers, high/low pass filters etc. 

Other than just varying its resistance, some three pin variable resistors have the ability of varying the voltage at one of its pins. This allows it to be used for many different applications. 

What is a variable resistor?

You might be familiar with a standard resistor. It’s job is to provide resistance in a circuit to limit the amount of current flowing. 

So how does a variable resistor differ to a normal resistor? While they both perform the same function, which is to provide resistance to limit current, a variable resistance has the ability to vary its resistance.

Variable resistors are available with maximum and minimum resistances that range in values, so depending on the application or your needs, you will be able to find a variable resistor with the right max and min values.

There are a couple of ways variable resistors are constructed mechanically, which allow you to change the resistance. 

The most common way is using a knob which you can twist with your fingers. Some variable resistors (most commonly used when mounted on a  circuit board) have an indent that can be adjusted using a screwdriver.

Two ways to use a variable resistor in a circuit

The first way to use a variable resistor in a circuit is by using it to vary resistance. A variable resistor comes with 3 pins as seen below.

When using the variable resistor in a circuit for the purpose of only varying its resistance, only 2 of the 3 pins need to be connected. 

Now you know a variable resistor is a component that has the ability to change its resistance. However, the variable resistor has another way it can be used in a circuit.

It also has the capability to vary voltage as well. 

When used to vary voltage, it is known as a Potentiometer.

To use a variable resistor to vary voltage, 3 of the pins are connected as seen in the diagram below. The voltage will be varied from 0 volts to the voltage of the supply (5 volts in this case) that the variable resistor is connected to.

Main reason a variable resistor is used in a circuit

While the variable resistor operates the same way a resistor does, it has many applications when used in a circuit. 

As you have seen, the variable resistor is used as a means of varying the resistance, thereby allowing more current to flow or limiting it, or as a variable voltage source. 

But, why is it used in a circuit? 

Below are applications of variable resistors  when used in circuits to vary resistance or voltage

Applications where a variable resistor is used in a circuit to vary resistance

The most common reason a variable resistor is used in the applications below is to vary the resistance, which will ultimately have an effect on other parts of the circuit.

Light dimmer 

Lighting is essential in every household. Having a lighting in your house enables you to walk from one room to the next in the night, as well as read your favourite book etc.

Lighting also has an affect on our mood and how we feel.

Different rooms in your houses serve different purposes. But, sometimes one room can be used for many different functions.

For example, a living room can be used to host your close friends for a catch up session, or it could be used to watch a movie. 

The light levels you need for those two scenarios are completely different. A catch up session with friends requires higher light intensity, while watching a movie requires lower light levels. 

This is where the light dimmer comes to the rescue as it allows you to vary light levels with a twist of a knob. 

That knob is connected to a variable resistor. 

The variable resistor is used in the circuit to control the amount of current the lighting system receives thereby controlling its brightness.

Many stand alone lamps also use a variable resistor as a light dimmer. 

Volume Control 

While we all have different taste in music, it is the universal language we all share. It lifts our spirits, moves our bodies and helps us escape reality.

We can listen to it in our homes, our cars, and while we are out and about thanks to headphones and bluetooth speakers.

But, imagine if we could listen to our music at only one volume level and did not have the capability of increasing or decreasing it. 

That would suck!

This also applies to other forms of audio applications. 

Thanks to the variable resistor, we have the ability to vary the volume of the music we are listening to. 

Variable resistors were commonly used in analog circuits to control the volume of audio but, nowadays, digital circuits control the volume via software. 

Vary output voltage of voltage divider

A voltage divider is a commonly used circuit used in many different electronic applications. 

Its main purpose is to take a large voltage and scale it down to a smaller voltage. The circuit of the voltage can be seen below.

Below is the equation to calculate what resistor value is needed to achieve the required output voltage.

However, some of the time, the output voltage needs to be dynamic (able to be changed). This is where the variable resistor is useful in this circuit. 

Replacing resistor R2 in the circuit above with a variable resistor, gives us the ability to vary the output voltage.

Vary cut-off frequency in high and low pass filter

High and low pass filter circuits are found in many electronic applications.

A high pass filter will allow frequencies above the cut-off frequency to pass, while a low pass filter will allow frequencies lower than the cut-off frequency to pass. 

Their applications range from audio amplifiers and speaker systems, in order to reduce noise levels, eliminate distortion,  and amplify higher frequencies.

The variable resistor is used in these circuits to vary the cut-off frequency so the right frequencies can pass. 

Applications where a variable resistor is used in a circuit to vary voltage

When used with three pins, the variable resistor can vary the voltage of the supply it is connected to. Below is a common example where it is used in a circuit to vary voltage. 

Vary the output Pulse Width Modulation signal of a microcontroller

A microcontroller, is a device that has many features and capabilities. One of its many features includes Pulse Width Modulation (PWM) found at the output. 

PWM is the ability to reduce or increase the average power that is delivered to one of the outputs of the microcontroller.

This PWM signal can be sent to a motor to control its speed, or a light source such as an LED (light emitting diode) to control its brightness. This has many applications in and of itself.

The microcontroller also has another feature known as the Analog to Digital Converter (ADC) found at its input. It can take an analog voltage and convert into a digital signal.

This digital signal can be used in conjunction with PWM to set the average power at the microcontroller output. 

So, where does the variable resistor fit in?

The third pin of the variable resistor gets connected to the ADC input of the microcontroller. When the voltage is varied, the ADC converts this voltage into a digital value which then changes the PWM signal thereby controlling the output (a motor or LED).

Advantages of using a variable resistor in a circuit

The main advantage of using a variable resistor in a circuit is its ability to change its resistance.

Being able to change the resistance at any given time, thereby controlling the current, allows it to be used in many different applications covered earlier.

Difference between a resistor and variable resistor in a circuit

One of the main distinctions between a standard resistor and a variable resistor is that a standard resistor cannot vary its resistance.

When designing a circuit that a standard resistor will be used in, its value needs to be chosen ahead of time, whereas a variable resistors value can be changed whenever needed.