A traditional resistor has a fixed value, however, there is a version of the resistor that can vary its resistance, known as a variable resistor. 

But, what is the function of a variable resistor? Since the resistance of a variable resistor can be altered, variable resistors have a couple of functions which include; controlling current and voltage. 

This article will dive deeper into the variable resistor and its functions as well as applications.

What is a  variable resistor? 

Before we can dive into the different functions, let’s take a closer look at the variable resistor.

Resistance is one of the three major fundamental quantities in electricity (with voltage and current being the other two).

These three quantities form the crucial equation known as Ohm’s law.

As we saw above, a resistor is an electrical and electronic component that has a job of providing a known resistance in circuits. 

Resistors have the function of limiting the flow of current. 

Variable resistors are a type of resistor which have the added ability of being able to vary their resistance. 

While resistors are a two terminal device, variable resistors have three terminals. 

Variable resistors come in values similar to fixed resistors. But, in this instance, the value of the variable resistor denotes its maximum value. 

So a 10K variable resistor has a range of resistances from 0 – 10kohms. 

Construction and working principle of a variable resistor

There are many different types of variable resistor (which we shall see later on), however, the working principle and functionality remains the same.

Below is an image showing the different parts that make up a traditional mechanical variable resistor. 

So how does the variable resistor vary its resistance? 

The most common type of variable resistor will have a knob that you can twist. When you twist the knob, a wiper moves across a resistive track which sits between terminals 1 and 3. 

But, to vary the resistance, a connection will have to be made between terminals 1 & 2, or 2 &3. This is because the resistance between terminals 1 and 3 is fixed. 

What are the functions of a variable resistor

So if traditional resistors have the function of limiting current, what function does a variable resistor have? 

Variable resistors have two primary functions in a circuit, which are being able to adjust the current, and voltage.

Let’s take a look at how it goes about doing so

Variable resistor function #1: Adjusting current

The first main function of a variable resistor is its ability to adjust current.

We know that the basic function of a fixed resistor is to limit current, by providing a fixed resistance. Since variable resistors can vary their resistance, this allows them to be able to adjust current.

As we just saw above, to vary resistance, either terminals 1 & 2, or 2 & 3 need to be used. So, when a variable resistor is used to control current, only two of its terminals are utilised. 

One common application for a variable resistor when being used to adjust current is motor control. By adjusting the current through the motor, we can adjust its speed. 

Below is a circuit with a battery, variable resistor, and motor. 

As you can see, only two terminals are used. It doesn’t matter which combination, as long as it is not terminal 1 and 3. 

Variable resistor function #2: Adjusting voltage (configuration 1)

Adjusting voltage is the other main function of a variable resistor. 

To achieve voltage control, all three terminals of the variable resistor will have to be used. 

In the image below, you can see a simple circuit showing a battery, and a variable resistor wired to adjust voltage. 

In this setup, terminal 1 of the variable resistor is connected to the positive terminal of the battery, and terminal 3 of the variable resistor is connected to the negative terminal of the battery.

Note, since the variable resistor is not a polarised component, it does not matter if terminal 1 or 3 gets connected to the positive or negative terminals of a battery. 

The output voltage then appears at terminal 2. The range of voltage depends on the maximum input voltage (in this example, 5V), and how much you turn the knob. 

Variable resistor function #2: Adjusting voltage (configuration 2)

Another way to adjust voltage is two use a combination of a fixed resistor and variable resistor. This configuration is known as a voltage divider. 

In this setup, only two terminals of the variable resistor are wired (just like when it is used to adjust current). 

It is placed at the bottom in the R2 position. 

To calculate the voltage at the output, you will need to use the formula below.

So, by adjusting the resistance of the variable resistor, we can adjust the voltage at the output. 

For example, if the variable resistor is set to a resistance of 5k, this would give us a voltage of 1.67V (5 x 5k / (10k + 5k)).

If we now set the resistance to 10k, the voltage would now be 2.5V (5 x 10k / (10k + 5k)). 

Do a variable resistor, rheostat and potentiometer have the same function?

Now, you might have heard the terms potentiometer and rheostat mentioned in the same sentence when talking about variable resistors.

What are potentiometers and rheostats, and do they have the same function as a variable resistor? 

Let’s look at the term potentiometer first. 

We now know that a variable has two functions; to vary current, and voltage.

When a variable resistor is used to control voltage, it is often referred to as a potentiometer (or pot). It isn’t a separate device as such, just a different name. 

A rheostat, on the other hand, is a separate two terminal device that has only one one function which is to vary its resistance and therefore control current. 

When a variable resistor is used to control current, it has the same function as a rheostat. 

How are the functions of a variable resistor used in applications

There are many electrical and electronic applications that make use of the functions of the variable resistor either when used as a variable resistor (to control current), or a potentiometer (to control voltage).

Let’s take a look at some of the many applications of a variable resistor:

• Light dimmers
• Motor control 
• Volume Control (Television, Radios, Computer speakers)
• Control brightness, contrast, colour for older televisions
• Frequency attenuation (Radios)
• 555 timer circuits
• Used in measurement of distance and angles
• Tuning and calibration
• Control rate of charging of capacitors
• Control Pulse Width Modulation 

This isn’t an exhaustive list by any means. 

Different types of variable resistor and their functions

The variable resistor we have been discussing, is the traditional mechanical version. However, there are many different variations of the variable resistor that have different constructions and working principles, however the overall function remains the same (which is to vary resistance).

These variations do not have the ability to control voltage like a variable resistor, but they can be placed in the voltage divider configuration to achieve the same function of controlling voltage. 

Let’s take a look at the different types of variable resistors. 

Light dependent Resistor (LDR)

First on the list of the different types of variable resistor is the Light Dependent Resistor ( LDR). 

It is a component whose resistance varies depending on varying intensities of light, hence the name it has been given.

They are constructed using a  semiconducting material which has light sensitive properties. 

LDR’s can be used in a variety of applications which include;

• Light metres
• Fire and smoke alarms
• Burglar alarms
• Street lamps

Rheostat

We learnt a bit about the rheostat a bit earlier, when we discussed the difference between it and a variable resistor.

Their construction is very similar, however the main difference being that they only use two terminals rather than three.

This is why it is restricted to only being able to control current. 

They are often used for power control applications such as;

• Light dimmers
• Motor speed control
• Heaters
• Ovens

However, they aren’t as popular as they used to be due to their low efficiency. 

Force sensitive resistor

There are many applications where force or pressure needs to be measured. This is where the Force sensitive resistor (FSR) comes to the foreground. 

This type of variable resistor varies its resistance with varying amounts of force and is available in a number of different sizes, shapes, and force limits. 

With its capability there are a plethora of applications that it can be used for. Below are a few examples;

• Robot arm gripper
• Electronic drum kit
• Keyboards
• Electronic throttle and brake
• Gaming controllers

Thermistor

Next up is the Thermistor, which is a combination of the words “thermal” and “resistor” which is perfect since this is a device whose resistance changes depending on the temperature.

It can be further divided up into two categories;

• Negative temperature Coefficient (NTC)
• Positive temperature Coefficient  (PTC)

A NTC thermistor’s resistance has an inversely proportional relationship with temperature. This means as temperature increases, its resistance decreases. 

Whereas a PTC thermistor has a directly proportional relationship with temperature. So an increase in temperature sees an increase in resistance. 

Applications include;

• Fire alarms
• Ovens
• Refrigerators
• Automotive 
• Digital thermometers

Humistor

Last but not least is the Humistor and just like the thermistor, is a combination of two words “humidity” and “resistor”.

So you can guess, this type of variable resistor changes resistance depending on humidity levels. 

• Weather stations
• Agriculture
• Industrial process control
• Dehumidifier 

When would the function of a variable resistor replace a fixed resistor?

A fixed resistor is limited to one resistance, therefore the current or voltage is going to be a fixed value as well. 

A variable resistor would replace a fixed resistor in situations where current or voltage needs to be altered. So it really depends on the application.

We have seen the many applications where a variable resistor is utilized for its unique ability and using a fixed resistor in these applications would be redundant

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It comes in many different forms which include Mechanical, Electromechanical, and Piezoelectric.

A switch is an electronic component that has the function of allowing and preventing current flow when used in a circuit. 

But, can a buzzer function as a switch in a circuit? A buzzer cannot function as a switch in a circuit. It’s primary function is to produce an audio signal in the form of ‘buzzing’ or ‘beeping’ when a voltage is applied to it. It does not have the ability to open and close a circuit the same way a switch does.

The main function of a buzzer and switch in a circuit

There are plenty of electronic components used daily in circuits. Individually, they have a certain function in the circuit that contributes to the overall functionality of the circuit. 

A switch and buzzer are no different with each having a key function to play in a circuit. 

Understanding what each does will help you understand why a buzzer cannot function as a switch in a circuit. 

What is the main function of a switch in a circuit?

Let us first take a look at the switch. 

Below, is a simple circuit with a load connected to a battery.

In this circuit the battery is continuously supplying the load with power. 

However, at a certain point the battery is going to run out of energy and therefore will no longer be able to supply power to the load. 

Having this sort of setup in the real world is not very applicable. 

You are rarely going to need to have the battery constantly powering a load. 

This is where the switch plays a crucial role. 

An electrical/electronic switch is a component that connects or disconnects the conducting path in an electrical/electronic circuit, either allowing current to flow, or preventing it from flowing.

If we modify the diagram earlier and add a switch, we get the circuit below. The switch can be in two states ; Closed and Open. 

In diagram A, the switch is Open, causing a break in the conducting path, thereby preventing current from flowing through the LED and powering it. 

Diagram B has the switch closed which creates a conducting path and therefore allows current to flow from the battery to the LED. 

So, you can see, the main function of the switch is to either allow or prevent current from flowing in a circuit. 

What is the main function of a buzzer in a circuit?

Having the ability to produce sound in electronics has many different applications. 

The buzzer is an electronic component that has the ability to produce sound when a voltage is applied to it. 

This can be utilised in many applications. 

One of the biggest applications where a buzzer plays a crucial role is a smoke alarm. When you are sleeping your sense of smell and sight are impaired. 

If a fire starts in your home while you are sleeping, you will have no way of knowing. A buzzer provides a way of letting you know a fire has started through sound. 

Other applications include alarm clocks, doorbells, electronic musical instruments, greeting cards, security systems and many more applications. 

There are three different types of buzzers available ; Mechanical, Electromechanical, and Piezoelectric.

So, a buzzer’s main function in a circuit is to provide an audio signal. 

Why a buzzer cannot be used as a switch in a circuit

Now that you know the function of a switch and buzzer in circuit, we can discuss why a buzzer cannot function as a switch in a circuit.

The main function of a switch is to close and open a circuit, thereby allowing and preventing current to flow respectively.

While the buzzer’s main function is to produce sound when a voltage is applied to it. It is primarily an output device that functions to provide an auditory signal which can be utilised in many different applications. 

The buzzer does not have the ability to function as a switch, as it mechanically has no way of connecting and disconnecting the conducting path of a circuit the way a switch does.

What instance would a buzzer function a switch in a circuit?

There is an instance where a buzzer can disconnect the conducting path in a circuit.

This scenario could arise if the buzzer is damaged and causes an open circuit.

An open circuit  occurs when two points in a circuit are physically disconnected thus representing a switch that is ‘open’.

If a buzzer is damaged, there is a possibility that there is a break in circuit within the buzzer thereby causing an open circuit. 

However, this is far from a buzzer functioning as a switch in a circuit. You wouldn’t want to damage your buzzer just to open a circuit to make it ‘function’ like a switch.

Connecting a switch would be the more logical and sensible approach.

What other functions can a buzzer have in a circuit?

While a buzzer cannot function as a switch in a circuit, it does have another function other than just producing sound.

The piezoelectric type of buzzer has the ability to produce a voltage at its terminals when pressure is applied to the disc responsible for producing sound.

This opens a new set of possibilities for the piezoelectric buzzer. 

It can be used in applications where a sudden force or constant pressure needs to be sensed and/or measured. The piezoelectric buzzer can be connected to a microcontroller analog input which can interpret the voltages from the buzzer. 

Applications could include motion detection, door knock sensor, force measurement, pressure measurement, and many more.

What is the most basic circuit of a buzzer and how does it function

While there are many circuit configurations available for the buzzer, the most basic is one with a battery, switch, and buzzer (similar to our circuit at the beginning).

When the switch is closed, the battery energises the coil within the buzzer causing a rapid back and forth movement of a contact inside the buzzer causing a loud buzzing sound.

Opening the switch causes the coil to be de-energised which in turn will stop the buzzer from producing sound. 

The piezoelectric buzzer requires a bit more than just voltage. It requires some sort of oscillator that is capable of producing a sinusoidal type wave. Just connecting it to a DC voltage will cause a click and no buzzing sound. 

A microcontroller can be used to drive a piezoelectric type buzzer.

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