So, can a potentiometer be used as a fixed resistor? Yes, a potentiometer can be used as a fixed resistor. To do this you will need to set the potentiometer as a variable resistor and then set the desired value of resistance. Potentiometers come in a range of resistances, which you will need to choose accordingly. 

But, is using a potentiometer as a fixed resistor the more important question. This shall be discussed in detail in this article. 

What is the difference between a resistor and potentiometer

The main difference between a fixed resistor and a potentiometer is resistance. A fixed resistor is aptly named that because it has a ‘fixed’ value of resistance. Its resistance value cannot be altered. On the other hand a potentiometer is a component that can vary its resistance (when set up as a variable resistor) between a minimum value (usually 0), and a maximum value (which could be 1K, 2K, 5K, 10K, 22K, 47K, 50K, 100K, 220K, 470K, 500K, 1 M). 

The fixed resistor has two terminals, while potentiometers have three terminals. The potentiometer also has the ability to vary voltage (but we won’t worry too much about that in this article). 

Can a potentiometer be used as a fixed resistor?

Yes, a potentiometer can be used as a fixed resistor. The potentiometer has the capability to vary its resistance (between a minimum and maximum value). Its resistance can be any value between these two points. So you can set the resistance to your desired value. 

Note, the required resistance value will need to be within the range of the potentiometer. For example, if you require a resistance of 1K, and the potentiometer you have has a range of 0 -1K you are fine. You just set the pot to its maximum value. However, if you need larger resistance, say 2.2K, this pot will not be suitable. 

How to use a potentiometer as a fixed resistor?

A potentiometer can be used to vary voltage and resistance. But, if we require it to be used as a fixed resistor, we need to set it up to vary resistance. There are two ways to set up a potentiometer to vary resistance.

Option #1: 

The first option is to connect one of the outer terminals of the potentiometer to its middle terminal (as seen below). Then the pot becomes a two terminal device just like the fixed resistor. Then you only need to use the outer terminals. 

Option #2:

For the second option, we ignore one of the outer terminals and just use the middle and other outer terminals (as seen below). 

Is it effective to use a potentiometer as a fixed resistor?

While you can use a potentiometer as a fixed resistor, you gotta ask yourself is it really effective? And, unfortunately the answer is no. There are a couple of reasons to ponder and reconsider using the potentiometer as a fixed resistor.

The first reason is that the potentiometer is too big compared to a normal fixed resistor. If space isn’t an issue then you need not worry. However, if your circuit is confined to small boundaries, the potentiometer is going to take up a lot of space which isn’t ideal.

Next, the potentiometer does not have the same temperature stability as a fixed resistor. If it is going to be subjected to varying temperatures (high and low), the potentiometer is not going to provide a stable resistance.

Potentiometers have poor performance compared to a fixed resistance at higher frequencies. When potentiometers are subjected to high frequencies, their inductance and capacitance increase. This directly affects their overall performance.

Lastly, the potentiometer has less mechanical stability compared to a fixed resistor. To change the resistance of the pot you need to mechanically do so (using a knob, or slider). This means it has moving parts. Having moving parts means that there is a chance that the knob or slider could be disturbed thereby affecting the resistance as well.

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Resistors, Rheostats, and Potentiometers

Before we can take a look at the differences between these components, it will help to first learn a bit about them individually. So let’s take a quick look. 

What is a resistor?

A resistor is a crucial component used in almost every (if not all) electrical/electronic circuits. It is a two terminal passive component which has the main purpose of limiting the current flow. It does so by providing a known resistance. Higher resistances allow less current to flow, while lower resistances allow more current to flow. The resistor’s resistance comes down to the materials used as well as how it is constructed.  However, these materials need to be able to  allow current pass while still providing a resistance. 

While the resistor has the main function of limiting current, it has many other uses which include voltage division, heat generation, matching and loading circuits, gain control, as well as setting time constants. 

What is a rheostat?

Rheostats are components which have the ability to vary their resistance. By varying the resistance, rheostats can control the flow of current. Because of this unique ability rheostats are used in applications that include altering generator characteristics, light dimming, and motor speed control. 

Resistance of resistors, and rheostats come down to certain factors which are their length, cross-sectional area and material. The cross-sectional area, and material of a rheostat remain constant. So, to be able to vary its resistance, rheostats vary their physical length. 

A rheostat has the capability of varying its resistance because it has the ability to vary its length. While its cross-sectional area and what it’s made of remain constant, its length can be altered.

All rheostats come with three terminals. Two of the three terminals are fixed (in the above diagram its terminal 1 and 3). The middle terminal (terminal 2), is not fixed and can be moved backward and forward. By moving this terminal back and forth, we are changing its position on the resistive track (which is often a coil of wire as seen above) thus altering the rheostat’s length (and resistance in the process). 

While it has three terminals, only two terminals get connected to the circuit. This includes one of the fixed terminals, and the terminal that moves. 

What is a potentiometer?  

Finally, we have the Potentiometer. The potentiometer is very similar to the rheostat. It too has three terminals and the ability to vary resistance. However, the potentiometer has an additional capability which is to vary the voltage and act as a voltage divider.

The most commonly used potentiometer is the Rotary Potentiometer. This type of potentiometer has a knob which the user can rotate backwards and forwards to alter its characteristics. This knob is connected to a semi-circular resistive track via a contact. As the contact moves along the resistive track, the voltage is obtained between one fixed terminal and the middle sliding terminal.

Potentiometers also have three terminals as seen above. All three terminals are utilised to be able to use the potentiometer as a voltage divider. However, if you just want to vary resistance, only two of the end terminals are connected. 

What is the difference between a resistor and rheostat?

The main difference between a resistor and rheostat is that the resistance of a resistor is fixed, whereas the resistance of rheostat isn’t. The resistance of the rheostat can be varied by moving its middle terminal which changes the physical length of the rheostat, and thus changes its overall resistance. Resistors come with fixed values of resistance due to the fact that their length, cross-sectional area and materials used are all fixed and cannot be altered. 

What is the difference between a resistor and potentiometer?

The main difference between a resistor and potentiometer is that a resistor has a fixed resistance, while a potentiometer can vary its resistance. Just like the rheostat, the potentiometer has the ability to alter its physical length which results in its ability to alter its overall resistance as well. 

What is the difference between a rheostat and potentiometer?

The main difference between a rheostat and potentiometer is that the potentiometer can vary voltage and resistance, but a rheostat can only vary its resistance. A potentiometer has the ability to vary voltage because all three of its terminals are connected in a circuit. An input voltage is applied to the end terminals (the entire length of the potentiometer). This results in an output voltage between one of the fixed end terminals and the moving middle terminal. A rheostat is different because only two of its terminals are connected to a circuit so the input voltage cannot be divided. 

Also, a potentiometer can be used as a rheostat (and just vary resistance), but a rheostat cannot be used as a potentiometer and vary voltage. 

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The potentiometer can be used in two configurations where it can either vary voltage or vary resistance. 

Your next project might require speed control of a motor.

So, can you use a potentiometer to control motor speed? Yes, you can use a potentiometer to control motor speed. When used to vary voltage, a potentiometer can be connected to a motor and vary the voltage (and therefore power) delivered to a motor which will control its speed. 

I shall discuss the above circuit setup and a couple of other configurations you can use to control the speed of the motor using a potentiometer. 

How a potentiometer can be used to control motor speed

Before we dive into the circuit configurations of controlling the speed of the motor, let’s take a closer look at the Potentiometer. 

If you already know a potentiometer inside out, you can skip this section. 

The potentiometer

Below is the schematic symbol and of a Potentiometer. 

It is a three terminal device with a schematic symbol similar to that of a resistor. This is because it essentially is a resistor which has the ability to vary its resistance. 

The resistance at the both ends of the potentiometer is fixed, while the resistance of the middle pin can be changed by either rotating a knob or sliding a contact depending on its construction. 

However, the potentiometer has the added benefit of being able to vary voltage as well.

Varying resistance

To use a potentiometer as a variable resistor, only two of its terminals are connected (Terminals 1 and 2), while the third terminal (Terminal 3) can be left floating. 

Varying voltage

To use the potentiometer to vary voltage, all three of its pins are connected. 

Pin 1 is connected to the supply voltage, pin 3 is grounded, and Pin 2 is the output voltage. 

Note, the potentiometer is a passive device, therefore it can be used in any direction. Therefore, either of the end pins (1 or 3) can be connected to the supply voltage. 

The input voltage is applied across the entirety of the potentiometer, and the output voltage at Pin 2 is the voltage drop between one of the fixed ends and the sliding contacts.

A potentiometer’s role in speed control of a motor

So, how does the ability of a potentiometer to vary its resistance, and voltage help with controlling the speed of a motor? 

A Motor is a device that rotates when a voltage and current is applied to it.

It converts electrical energy to mechanical energy.

Motors come in a variety of sizes which require different magnitudes of voltages to operate. A smaller motor will require less power to rotate at its maximum speed compared to a larger motor.

The amount of voltage and current that a motor requires is given by its Voltage and Current Ratings. 

These values indicate the nominal maximum voltages and currents required to run at maximum speed. 

But, if you supply a motor with voltages less than its rated values, it is going to run at speeds lower than its maximum speed.

Say you have a motor that is rated for 12V and 1A. 

If we reduce the voltage by 50% (6V), the motor is now effectively going to run at 50% of its top speed. 

Since the potentiometer has the ability to vary voltage, that is how it can be used to control the speed of a motor. 

Circuit configurations to control motor speed with a potentiometer

So, now we have established that a potentiometer can be used to control the speed of a motor, let’s take a look at some circuit configurations.

There are two typical ways to use a potentiometer to control speed; Using the potentiometer by itself, and Using the potentiometer with an Arduino.

How to control motor speed using a potentiometer: Without an Arduino

The first circuit setup is the simplest and only requires a few parts;

• Motor
• Potentiometer
• Power Supply 

In this configuration, the potentiometer will have to be wired so that it can vary voltage (so all three pins will be used).

Below is the circuit schematic to control the speed of a motor using just a potentiometer.

The operation of this setup is simple; as you rotate (or the slide) the potentiometer, the voltage at its output pin (pin 2) which is connected to the motor, is going to vary between values from GND (0V) to the supply voltage (5V in this example).

This change in voltage is going to translate to different speeds of the motor. 

How to control motor speed using a potentiometer: With an Arduino

The second circuit setup will require an Arduino.

An Arduino is needed as it contains a microcontroller which has the capability to produce Pulse Width Modulation (PWM) at some of its designated pins.

Note, you are not restricted to using just an Arduino. As long as you have a microcontroller which can produce PWM you will be fine. 

What is PWM?

PWM is a technique used to vary the average power delivered to electrical and electronic devices. 

It is a digital waveform signal that varies between two values; High and Low.

The amount of time the pin of the Arduino is HIGH vs LOW is known as the Duty Cycle, which ultimately determines the average power.

Below are a couple of different PWM waveforms with different duty cycles. 

Each percentage value of duty cycle for each waveform determines how long the pin is on for, and is also how much percentage of the total power is delivered. 

So, if the duty cycle is 75% and the supply voltage is 5V, the total voltage at the output is now 3.75V (0.75 x 5).

This is great, as we can use PWM to control a motor speed. 

But, where does the potentiometer come in? 

The potentiometer is used to vary the duty cycle of the PWM via the Arduino’s Analog to Digital Converter (ADC) .

So, it directly controls the speed. 

Below is the circuit for using a potentiometer and arduino to control motor speed.

Why you would want to control motor speed using a potentiometer

There are a couple reasons for using a potentiometer to control the speed of a motor.

The main one being that it gives you a much more natural tactile feel when controlling the speed of the motor. 

Say you are building an electric scooter. Accelerating is much more natural by rotating the accelerator (via a potentiometer) than by just pushing buttons. 

How to choose the right potentiometer to control motor speed

As we saw earlier, motors have voltage and current ratings. 

These values determine the maximum voltage and currents they can operate under safely. Going over these values increases the chance of damaging the motor.

A potentiometer is no different. It too has maximum voltage and current rating values within which they operate efficiently. 

When using a potentiometer to control the speed of a motor, you will have to choose a potentiometer that matches the maximum ratings of the motor.

For example, if you have a motor that has ratings of 5V, and 1A, you will have to choose a potentiometer with ratings that are the same (5V, 1A) or higher.

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But, normal resistors have a fixed resistance which cannot be varied.

Potentiometers are three terminal devices which have the ability to vary their resistance. 

This is why a potentiometer can sometimes be referred to as a Variable Resistor. 

Other than just varying resistance, the potentiometer can also vary voltage. 

Light Dimmers are circuits which are connected to lighting fixtures that give you the ability to adjust the brightness of the light from 0% to 100% by altering the power that is delivered to the lights.

But, can you use a potentiometer as a light dimmer? Yes, a potentiometer can be used as a light dimmer. There are many different circuit configurations that a potentiometer can function as a light dimmer. These circuit configurations can include a microcontroller or be used without one. 

However, a note to be made is that a potentiometer by itself might not be able to replace existing light dimmer circuits already installed. It will need to replace the existing potentiometer in the light dimmer circuit. 

I shall cover these circuits in more detail in the rest of the article. 

Different configurations of potentiometers as a light dimmer

While a  potentiometer can be used as a light dimmer, there are different applications and circuit configurations available.

I will break them down into AC and DC applications. 

Potentiometer used as a light dimmer: AC application

Whether you are trying to create the right ambience, or you are sensitive to brighter lights, light dimmers have many advantages and are a great addition to a living or work space.

Light dimmers that are installed in your home or in your workplace are designed to deal with Alternating Current (AC). 

Alternating current differs  to Direct Current (DC) as it’s magnitude ‘alternates’ (changes) with time. 

AC applications deal with higher voltages which range from 120V – 240V.

Due to this, the components and circuits used in AC applications need to be able to handle these higher currents and voltages. 

Light dimmer circuits are designed specifically to handle high power. 

Also, the potentiometer is only one part of this light dimmer circuit. 

There are many other components that are used which contribute to the overall workings of the light dimmer. 

So, when answering the question whether a potentiometer can be used as a light dimmer, it can but not by itself.

You will need to use it in a circuit specifically designed to dim lights in AC applications.  

However, since there are high voltages and currents involved, it is not advisable to do so yourself. 

Hire an electrician who is trained to deal with AC circuitry and can replace the potentiometer for you.

Note, these types of light dimmers only work incandescent lamps and not fluorescent lights. 

Potentiometer used as a light dimmer:  With a Microcontroller

The next area where a potentiometer can be used as a light dimmer is Direct Current (DC) applications.

DC applications deal with much lower voltages and currents compared to its AC counterpart. 

Also the magnitude of the voltage is constant and does not vary with time. 

Microcontrollers are essentially small computers embedded on a single chip.

A microcontroller has a myriad of capabilities which include storing memory, pulse width modulation, serial communication, analog to digital conversion, timers, and much more.

The Analog to Digital Converter (ADC) and Pulse Width Modulation (PWM) are the two main functions we are concerned with for the light dimmer.

Analog to Digital Converter (ADC)

Microcontrollers are devices that deal with digital data.

Everything in the circuit outside of the microcontroller is analog (this means values vary in increments continuously from its minimum to maximum value).

For example voltages between 0 and 5V can vary in increments of 0.1 volts thereby giving you values such as 0.1, 0.2, 0.3 etc. 

This analog data needs to be converted to a digital value in order for the microcontroller to interpret it. 

It does this by using the ADC.

A voltage from a sensor, potentiometer can be fed into one of the designated ADC inputs which can then be converted to a digital value.  

Pulse width Modulation (PWM)

Pulse width modulation is a technique used to vary the power that is delivered to an output of a microcontroller.

Normally the output of a microcontroller can either be high (+5V), or low (0V). 

But, by varying the time that the pin is high vs low, we can vary the power at the output. 

This time that the pin is ‘on’ for a certain time period is known as the duty cycle. 

For example, if the pin is high for 50% of the time, the power output will be 50% of the maximum voltage (5V) thereby giving us a voltage of 2.5V (0.5 x 5).  

The output could be a speaker, motor, light etc.

The microcontroller has designated PWM output pins which outputs can be connected to. 

This is perfect for light dimmers as the power can be varied to the light thereby adjusting its brightness.

Micrcontroller and Potentiometer light dimmer circuit

So how do we get a light dimmer using a potentiometer and microcontroller? 

Below are the two configurations of a three terminal potentiometer. 

Diagram A is used to vary resistance. The middle terminal (pin 2) is not connected in this setup.

Diagram B is used to vary voltage. The middle terminal can be connected to the input of a transistor or microcontroller. 

When building the light dimmer using the potentiometer and microcontroller, we are going to need to set up the pot as seen in Diagram B (vary voltage).

Below is the final circuit setup which has a potentiometer, microcontroller and Light Emitting Diode.

Here’s how the circuit functions;

• Turning the potentiometer’s knob is going to vary the voltage (between 0V and 5V) at its middle terminal. 
• This voltage is connected to the micrcontroller’s ADC input
• The ADC then converts this voltage to a digital value
• This value is used to set the duty cycle of pulse width modulation
• Since the LED (or any other light source) is connected to one of the microcontroller’s PWM output pins, its brightness gets varied depending on how much you rotate the potentiometer
• Voila! You have a light dimmer

Potentiometer used as a light dimmer:  No Microcontroller

You might not have experience with microcontrollers or programming but still want to build a light dimmer using a potentiometer. 

Is it possible? Yes! You can still build a light dimmer using a potentiometer and not have to use a microcontroller.

The components you will need to build this light dimmer are;

• 2N700 N-channel MOSFET
• 1Megaohm resistor 
• 1Megaohm potentiometer
• Light as the output (LED, lamp etc) 

Below is the circuit for the light dimmer using the above components

Along with the potentiometer, the MOSFET is the other main component in this circuit.

As a voltage is applied to the Gate (G) of the MOSFET, current is allowed to flow through the Drain (D) and Source (S) of the MOSFET (which means current can flow through the light source thus powering it).

The amount of current that flows depends on how much voltage is present at the Gate of the MOSFET,

The more voltage the more current flows and vice versa and this is how the brightness of the light source is varied.

The amount of voltage present at the gate is determined by the potentiometer. 

When the potentiometer is at this maximum resistance (1 Megaohms) the voltage at the gate is at its maximum thereby allowing the maximum current to flow which causes the light to glow at its brightest.

On the flipside, when the potentiometer offers no resistance (0 ohms), the voltage at the gate is 0 (which means that no current flows) and the light is turned off.

Varying the potentiometer between values from 0 ohm to 1 Megaohm will give you a range of brightnesses. 

Different types of potentiometer that can be used for light dimmers

If you are designing a light dimmer that uses a potentiometer, you have different options available of how you physically vary its resistance.

However, while they are physically designed differently, they still all function the same.

The different Potentiometers available are;

• Rotary
• Multi turn
• Trimmer
• Digital
• Preset
• Linear 
• Slide

For a light dimmer I would recommend using the rotary style potentiometer as this will give you more control when adjusting the brightness of a light. 

Do potentiometers only work with LED light bulbs

LED light bulbs are becoming more popular and are replacing older incandescent bulbs.

They are more efficient and cost less as well. 

But, as you saw earlier, older light dimmer circuits that use potentiometers were used to dim conventional light bulbs.

So, if you are still using incandescent light bulbs you will still be able to dim them. The light dimming circuit however will be different compared to the ones used with LED light bulbs.

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