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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Components such as capacitors, inductors, diodes, thermistors, transformers, etc. 

However, one very crucial component that is missing from that list is the Resistor. I would be surprised if you weren’t able to find one in a circuit. 

The resistor can be further divided into two categories; Fixed Resistor and Variable resistor. 

But, what is the difference between a fixed resistor and variable resistor? The main difference between a fixed resistor and variable resistor is that a fixed resistor has a fixed resistance, whereas the variable resistor has variable resistance (it has a range of resistances which you can set using a knob or slider). 

There is more to just resistance when it comes to the differences between a fixed and variable resistor. Things like construction, application, etc. 

This article shall take a closer, more in-depth look. 

Deeper look at a fixed and variable resistor

Let’s take a closer look at the fixed and variable resistor. This will help you better understand the differences between the two later on. 

What is a resistor?  

Let’s take a closer look at the Resistor. 

As mentioned at the start, there are many different types of components and devices in the electrical and electronic world, each with their own unique abilities which help perform a certain function within a circuit. 

The Resistor is a fundamental electronic component that can be found in almost all circuits.

So what is the definition of a resistor? 

A resistor is a passive electrical and electronic component, whose main purpose is to ‘resist’ the flow of current in a circuit. Also, rather than having an ambiguous resistance value, resistors are created with a set resistance value. 

The bigger the resistance, the less current can flow, and the lower the resistance, the more current can flow.

It is known as a passive component due to the fact that it has no means of generating its own power, but rather dissipates power in the form of heat. 

The fixed resistor

As the name might suggest, fixed resistors have a resistance that is fixed regardless if there is a change in voltage.

It cannot physically change its resistance. 

For example, if you bought a 10 ohm fixed value resistor, this is the only resistance that the resistor is going to be able to provide.

In an ideal world, the resistor would provide a fixed resistance at all times. However this is not the case, as their resistance varies slightly with temperature (which we shall look at later). 

Fixed resistors are the most commonly used in circuits. Their value is chosen during the circuit design phase using calculations using Ohm’s Law. 

Different types of fixed resistor

Below are the different types of fixed resistor available;

• Wire Wound 
• Carbon composition 
• Carbon film
• Metal Film
• Metal-oxide film 
• Metal glaze
• Foil

Wire wound  fixed resistor

This type of resistor has an insulating ceramic rod that is wrapped in wire (with the wire being copper due to its high conductivity). 

It is the most widely used type of fixed resistor. 

Carbon composition fixed resistor

Have a cylindrical form with metal caps at either end of the cylinder. Inside the cylinder is a substance that is a mixture of carbon power and ceramic. 

While used primarily in the early 1960’s, this type of fixed value resistor isn’t used a lot anymore due to its high cost and low stability. 

Carbon film fixed resistor

Has a very similar construction to the carbon composition resistor. However, a carbon film is placed atop a ceramic substrate. 

This type of fixed value resistor produces less noise when compared to carbon composition resistors. 

Metal film fixed value resistor

The metal film resistor has the same construction as the carbon film. But, rather than using carbon as the material of the film, other metals are used.

Metals used in metal film resistors are usually Nickel Chromium, Tin or Antimony.

The resistance of metal film is less affected by temperature. 

Metal oxide fixed value resistor

Has the same construction as carbon film, and metal film resistors. Again, what sets them apart is the material used. 

The film in this resistor is a metal oxide such as Tin Oxide.

They cost less compared to carbon composition resistors, and can be used at higher temperatures. 

Metal glaze fixed value resistor

Use a composition of glass powder and metal particles to restrict the flow of current.

Its resistance is also less affected by temperature. 

Foil fixed value resistor

Last but not least are foil resistors. 

They are made using an alloy (a material composed of two or more metallic elements). The foil is created from an alloy of nickel and chromium. 

Of all the different types of construction for fixed value resistors, these are the most accurate and stable. 

They also produce far less noise.

What is a variable resistor?

Next, we have the Variable Resistor.

This type of resistor has the ability to vary its resistance between two set values (with the lower usually being 0 ohms).

Note, this does not mean it’s restricted to those values, it can have a range of values which sit within the range of its lower and upper resistance limits. Its incremental value depends on the resolution of the variable resistor. 

For example if you have a 10k variable resistor, you would be able to set its resistance at any value between 0 and 10k ohm. 

So, you could set it at 10, 100, 1k, 2.5k, 5.7k, 8k etc. 

Below are the most commonly used circuit symbols for variable resistors;

Construction of a variable resistor

The construction of a variable resistor is basically a fixed resistive element along with a wiper that sits on the resistive element. 

The slider can be adjusted (by means of a slider or knob which you control) to sit anywhere along the resistive element thereby adjusting the overall output resistance. 

As you can see there are three terminals and the resistive element is connected to terminals 1 and 3. To use it as a variable resistor, you have to make connections to terminal 1 and 3.

Other abilities of a variable resistor

The variable resistor is quite the versatile component. 

The great thing about it is that it has an added ability, which is to vary voltage. When used to vary voltage, it is known as a Potentiometer.

To use it as a potentiometer, you would connect terminals 1 and 3 to GND and VCC (it doesn’t matter which terminal gets connected to what). 

The varied voltage is then presented at terminal 2. 

The main difference between a fixed resistor and variable resistor

So, we have learnt what a fixed and variable resistor are individually. But, what is the main difference between the two?

While the overall function of both remains the same, which is to provide resistance, the main difference between a fixed and variable resistor is their Resistance values. 

Fixed resistors are created to have a specific known resistance value that is set and cannot be changed (hence why they are called fixed resistors).

Variable resistors on the other hand offer a range of resistances (between their minimum and maximum values) which you can alter by means of a slider or knob.

Also, they are used for specific applications in circuits (which we shall cover later in the article).

The main purpose of a fixed resistor and variable resistor

But, why do we need resistors in the first place? 

Every electrical and electronic component has something known as a Power Rating.

This is a value that indicates how much electrical power is needed by the component to work effectively. Also, the power rating value indicates the maximum allowable electrical power the device or component can handle.

Exceeding this value will cause damage to the component. The power rating can be broken further into; Voltage ratings, and Current ratings. 

This is because power is a product of voltage and current ( P = V x I).

Components will have values for maximum voltage and maximum current ratings that should not be exceeded.

Resistors are important as they allow us to protect components in circuits by limiting the current so that the current ratings of that component are not exceeded (and therefore power rating as well).

Sometimes you might not be able to change the power supply (which means the voltage is fixed), so you will need a resistor to limit the other variable in the power equation (in this instance the current).

However, while resisting current is their main task, they are used in electrical and electronic circuits for other reasons to achieve certain outcomes. 

Let’s take a look at different uses of fixed and variable resistors. 

Applications of a fixed resistor

If you were to pry open any electronic device and look at its circuits, I would be surprised if you didn’t find at least one resistor. 

Fixed resistors have many uses for different applications. Some of the uses include; 

• Reducing current flow 
• Adjusting signal levels
• Dividing voltages 
• Biasing active elements
• Terminating transmission lines 
• Pull-up / Pull-down 

Some of the many applications;

• High frequency instruments
• Power control circuitry 
• DC power supplies 
• Filter circuit networks
• Amplifiers
• Oscillators
• Telecommunications
• Electronic measuring instruments
• Wave generators
• Transmitter
• Modulators / Demodulators
• Instrumentation 
• Voltage regulators 
• Feedback amplifiers  

Applications of a variable resistor

While not as common as its conventional counterpart, variable resistors have more specific applications compared to fixed resistors.

Common uses of a variable resistor include;

• Varying resistance (when used as variable resistor)
• Varying voltage (when used as a potentiometer) 

Common applications of a variable resistor are;

• Televisions
• Audio control (Radios, Stereo systems, Cars, etc)
• Oscillators
• Motion Control
• Transducers 
• Home electrical appliances 
• Motor control (electric vehicles, scooters, golf carts, etc)

Different types of variable resistor and their applications

The variable resistor we just looked at is just one of the most common types which varies its resistance mechanically (twisting a knob, or moving a slider).

However, there are other forms of variable resistors each having its own unique way of varying resistance. 

Let’s take a quick look at each of them. 

Light dependent resistors

The  first on the list are Light Dependent Resistors (LDRs).

However, they have many other aliases that you might know it by, which include, Photoresistor, Photocell, or Photoconductor. 

The resistance of light dependent resistors changes depending on varying light levels. The amount of resistance that changes depends on the intensity of light, as well as the sensitivity of the LDR.

The less light that the LDR is subject to, the higher its resistance (in the order of megaohms) and the more light that shines upon the LDR, the lower its resistance (few hundred ohms).

They are created using a semiconducting material that gives them their light sensing abilities. 

Common applications of the light dependent resistors;

• Smoke alarms
• Photographic light meters
• Streetlights 
• Burglar alarms

Force sensitive resistor

Next up we have the Force Sensitive Resistor (FSR).

The resistance of the force sensitive resistor changes when it is subject to force, pressure or weight. 

How much the resistance changes is proportional to the amount of force being applied. 

When there is no force applied to them, it has an almost infinite resistance (which can be viewed as an open circuit).

When a light force is applied, the resistance ranges around 100kohm.

At maximum force, the resistance can be as low as 200 ohm. 

Note, these values will vary from one FSR to the next. 

FSRs can withstand forces up to 20lb (roughly 100 Newtons). 

Common applications of force sensitive resistors;

• Midi controllers (for music producing)
• Electronic drum kits
• Electronic throttle and brake (automotive)
• Gaming joysticks 
• Sports (target force and accuracy detection)

Thermistor

The Thermistor is a portmanteau of the words ‘thermal’ and ‘resistor’.

These words are chosen because of the fact that the thermistor is a device whose resistance varies depending on ambient temperature. 

There are two types of thermistor; Negative temperature coefficient (NTC) and Positive temperature coefficient (PTC).

The relationship between resistance and temperature in NTC thermistors is inversely proportional. This means as temperature rises, resistance decreases, and vice versa.

In PTC thermistors, the relationship between resistance and temperature is proportional. So, when temperature rises, so does the resistance, and vice versa.

Common applications of thermistors; 

• Fire alarms
• Refrigerators 
• Ovens
• Digital thermometer
• Automotive applications 

Humistor

Last but not least is the humble Humistor (no humsitor is not a combination of ‘humble’ and ‘resistor’).

Rather, the humistor is a combination of the words ‘humidity’ and ‘resistor’.

The resistance of this electronic device varies depending on humidity levels. 

Humidity is the amount of water vapour present in the air. The resistance of the humistor depends on the total amount of water vapour molecules it absorbs.

As humidity increases, the amount of water molecules absorbed by the humistor increases thus causing it to be more conductive which results in its resistance decreasing. 

On the other hand, the less water molecules absorbed, causes the humistor to be less conductive thereby increasing its resistance. 

Common applications of the Humistor;

• Agriculture
• Textile factories (where humidity can affect materials)
• Refrigerators
• Atmospheric environmental monitoring

Are fixed resistors and variable resistors interchangeable?

You could use a variable resistor in place of a fixed resistor as long as it’s able to provide the same resistance value. 

But, using a variable resistor in place of a fixed resistor is overkill as you are not using its full capabilities.

On the other hand, you cannot use a fixed resistor in place of a variable resistor as it cannot provide the same function (which is varying its resistance). 

Resistance values of fixed resistors

As you might know by now, resistors come in all shapes, sizes, materials, constructions, etc.

Resistance is an important variable when choosing a resistor. Every part of a circuit won’t necessarily require having a resistor of the same resistance value.

So, resistors come in a variety of resistance values. However, to avoid the confusion of allowing any resistance value under the sun, the resistances of resistors are organised into a set of preferred values or standard resistor values known as the E-series.

This enables you and me to choose resistors from a variety of different manufacturers while still having a consistent set of resistance values. 

The E-series defines a set of values within a certain decade (where the number next to the letter indicates the number of resistance values within that specific E-series)

For example, the E3 has a set of three resistance values; 1 ohm, 2.2 ohm and 4.7 ohm.

To get to the next decade, you would multiply a particular resistance value by 10. 

So, if we want to take the 1 ohm to the next decade’s value, we would multiply it by 10, which would give us a new resistance value of 10 ohms.

If however, you require a resistance value in the 1000s, you simply multiply the resistance value by a 1000 e.g; 1000 x 1 ohm  = 1000 ohms. 

You can do this for any of the resistances as long as it matches the value within the series. So, for the E3 series you are restricted to the three values of 1, 2.2 and 4.7.

Below are the other E-series available;

• E3
• E6
• E12
• E24
• E48
• E96
• E192

How to know the resistance of a fixed resistor?

Sooner or later, you are going to find a rogue resistor and unfortunately, resistors do not have their resistance values written on them. 

However, there is good news! 

While they might not have their resistance values written explicitly on them, resistors have bands of colours on them to help identify their resistance as well as their tolerances. 

This band of colours is known as the Resistor colour code.

Resistor colour codes can come in either a band of 4 or 5 colours.

Note, the last color band (tolerance value) is usually spaced apart further from the other colours so that there is no confusion when looking at the resistor from different points of view.

Below is a table of the colors associated with resistor color codes and their values.

The multiplier is the value you multiply the other numbers by to get the total resistance.

For example, let’s say we had a 4-band resistor with the colours, red, orange, yellow, and green.

• 1st digit = Red = 2
• 2nd digit = Orange = 3
• Multiplier = Yellow = 10⁴ (10000)
• Tolerance = Green = 0.5%

Therefore, the total resistance would be 23 x 10000 = 230000 ohms (or 230kohms), with a tolerance of 0.5%.

Another way of figuring out a resistor is to use a Multimeter, which has the ability to measure the resistance of materials (mainly conductors). 

Sometimes reading colour codes can be quite cumbersome, so using a multimeter might be the easier option.

Does a fixed resistor have constant resistance?

No, the resistance of a resistor is affected by Temperature.

The resistance increases as the temperature increases regardless of the material as well as the fixed length and area of the resistor (however, as we saw earlier, some fixed value resistors are more resistant to changes in temperature than others).

This happens because the atoms within a material get excited as the temperature increases. This causes the atoms to move about more hastily, making it harder for electrons to get through.

Superconductivity is a phenomenon whereby you subject conductors to extremely low temperatures, and almost eliminate all resistance of the conductor. 

The relationship between temperature and resistance can be summarized by the parameter known as Temperature Coefficient of Resistance (TCR). 

TCR shows the change in resistance as a function of the ambient temperature (this relationship is linear).

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