We’re going to discuss about something interesting and knowledgeable today, but before that I have a cool activity for you! Do you like solving riddles? Even if not, still give it a try because it’s quite easy.

 

“ I am an electronic component wearing coloured bands. They call me rebel.”

 

Think…

Think…

Very simple, Right? It’s a Resistor!

Resistors are the most basic electronic components in electrical circuits and are used to regulate the flow of electric current in a circuit.

Resistors can be classified as:

1. Fixed Resistors: Having fixed value of resistance(which is the property of a substance to oppose the flow of electric current).
2. Variable Resistors: Having value of resistance, over a defined range of values(of resistance).

Out of all the numerous resistors available, the most common and useful is “Potentiometer”. Potentiometer is a variable resistor. We are going to discuss about “Potentiometers” in this blog.

Let’s start the journey of varying resistance!

Introduction

The word Potentiometer(also known as “pot”) is coined from the combination of two words:

Potential Difference+Meter

Actually, in earlier days, these potentiometers were bulky instruments used to measure(meter) the voltage(potential) across any load and hence the name “Potentiometer”.

But now the story is different. With the passage of years and improvement in technology, we now have quite small, easy to use and light to carry potentiometers which are used in wide range of applications.

Let’s more clearly define potentiometer:

It is a three terminal variable resistor in which resistance is manually varied to control the flow of current.

This is how a potentiometer looks like:

 

Now that you have seen how a potentiometer looks, you might be be thinking how is this component manufactured? To understand this, we have to look into its structure and composition.

Inside a potentiometer

Structure

A potentiometer is made from the following:

• Resistive element(also known as track).
• 3 Terminals: 2 terminals(marked 1 and 3 in the picture below) are fixed and connected to the resistive element and the third terminal(marked 2) is connected to the sliding wiper(which slides over a resistive strip to make an electrical contact).

 

 

 

The resistance of the potentiometer is changed when the wiper is moved over the resistive path(or resistive element), which further controls the flow of electric current. The wiper is moved with the help of a shaft/rotator attached to it. You can see a yellow coloured rotator in the picture above.

The distance travelled by the wiper(through the resistive path) is from the starting point of the resistive element(which is connected to Vcc) to the current position of the wiper.

In the above figure, the distance travelled by the wiper(through the resistive path) is from point A to B.

If the wiper is adjusted in such a way that the resistive path is reduced, the resistance decreases and current increases(because the electric current has to travel only a small distance through the resistive path).

If the wiper is adjusted in such a way that the resistive path is increased, the resistance increases and current decreases(because the electric current has to travel a large distance through the resistive path).

Composition

Different materials are used to construct the resistive element(of a pot) which are discussed below:

Conductive Plastic:

• It’s really costly.
• It can be used in lower power applications only.

Carbon Composition:

• This is made from carbon granules.
• It has low cost

Wound Wires:

• These are nichrome wires wounded over an insulating substrate(wafer or thin slice).
• They are mostly used in high power applications.

Cermet:

• It is combination of ceramic and metals.

Potentiometer in a Circuit

To understand the use of a potentiometer in a circuit, let’s first see how to represent it in a circuit diagram.

So, we have two standard symbols to represent a potentiometer. One is the American standard and the other one is the International standard.

 

 

This zigzag lines with three terminals represent the American standard symbol of potentiometer and this rectangular box with three terminals represents the International symbol of potentiometer.

In an electrical circuit, a potentiometer is used as a Voltage Divider. It is clear from the name itself that a voltage divider divides the input voltage in an electrical circuit. To understand this, Let’s discuss the working of a potentiometer.

How Potentiometer Works

Look at the circuit diagram given below. You can see a potentiometer there. Its two terminals(numbered 1 and 2) are connected to the voltage source and 3rd terminal(numbered 3) is attached to the wiper.

 

The current position of the wiper is somewhere in the middle of resistive element/path R, dividing the resistive path into two portions R1 and R2.

So, we can say resistance R can be considered as two resistances(R1 and R2) connected in series.

                                                                     R=R1+R2

Voltage across the wiper(also known as wiper voltage) is the output voltage Vout, which in this case is voltage across R2.

Now, see R1 and R2 are in series, so the input voltage will divide(because when resistances are in series, current remains same but applied(or input) voltage is divided across the resistors). So, according to the Voltage Divider Rule:

                                                              Vout= {R2/(R1+R2)} x  Vs

Here Vs is the input/supplied voltage.

This is how a potentiometer works as voltage divider in an electrical circuit. You can notice here that by changing the value of resistive path R2(by sliding the wiper on resistive element R), we can change the output voltage Vout.
Note: When R1=0, the voltage across the wiper is same as the supply voltage. Also, when the wiper is at terminal 2, the effective resistive path for R2 is zero, hence the resistance R2  is zero.

Using Potentiometers

Now that we have understood theoretically how a potentiometer works, It’s time to do something more exciting. Are you ready? Let’s discuss the practical implementation of a pot.

1. The very first step is to check the terminals of the potentiometer. 
2. Refer to the terminals from left to right as 1, 2, and 3, keeping the pot in such a position that the shaft is facing up towards the ceiling and the 3 terminals are facing towards you.
3. Connect battery to outer terminals(1 and 3) of potentiometer i.e connect terminal 1 to ground and 3 to Vin(Input Voltage).
4. The middle terminal 2 is the output of pot which is to be connected with the device we want to control.
5. Now turn the knob (or dial/shaft) left and right and accordingly the output voltage of the pot will change.

 

 

Note: Potentiometers have a range of resistance. For example, a potentiometer of 10 kΩ can be adjusted from 0 Ω to its maximum of 10 kΩ(by turning it’s knob).

Now, I have an activity for you. You can use this potentiometer to control the brightness of an LED.

Here are the steps to follow:

1. Connect the battery to the outer terminals(1 and 3) of potentiometer i.e connect 1 to ground and 3 to Vin(Input Voltage).
2. The middle terminal 2 is the output of pot which is to be connected with the positive terminal of the LED( the longer leg), using a resistor of appropriate value(as shown in the diagram above).
3. Connect the negative terminal of the LED to the ground.
4. Now turn the knob (or dial/shaft/rotator) left and right.

It changes the brightness of the LED!

Note: Use resistance of appropriate value with LED otherwise it will burn out.

You too want to try this cool experiment? For that you first have to buy a potentiometer. Don’t know how to buy one? Don’t worry! I’m here to tell you 🙂 . First let’s see the different types of potentiometer and then we will discuss how to buy it.

Types of Potentiometers

There are different types of potentiometer available there in the market which are discussed below:

Based on the geometry of the moving element(wiper) and the shape of the resistive element, potentiometers are of two types: Rotatory type potentiometers and Linear type potentiometer.

 

Rotatory Type Potentiometers: In this, the wiper rotates over an arc shaped resistive material. You can see in the diagram below the resistive element is arc shaped over which the wiper moves. A rotator/shaft is there to rotate the wiper.

 

Here are some pictures of Rotatory Type Potentiometers:

 

 

 

Linear Type Potentiometers: In this type of potentiometers, resistive strip is straight instead of arc shaped and wiper moves on the straight resistive strip, with the help of a slider. They are also known by the names such as: slider, slide pot or fader.

This is how a Linear Type Potentiometer looks like.

How to Buy

When you buy a potentiometer, you have to look for the following things to choose the right potentiometer:

Range of the pot: As discussed earlier, a pot has a range of resistance. Like, if a we have a 10 kΩ pot, it’s resistance be adjusted from 0 Ω to its maximum of 10 kΩ(by turning it’s knob). So, depending on the requirement of the application, you have to select a pot of appropriate range of resistance. Range is listed on the pot itself. You can see that in the picture below.

 

 

Size and type of the pot: Potentiometers are available in different types, sizes and shapes in the market(You can see that in the picture below). Depending on the application, you have to choose the pot of appropriate size.

 

 

Power rating: Power rating of a potentiometer is the maximum power you can give to it. Power is nothing but energy transferred per unit time. Now, you might be thinking, where does this energy come from? Well, when current flows through the pot due to the presence of voltage across it, this electrical energy is lost by the resistive element of the pot in the form of heat(energy).

Mathematically power is the product of voltage(across the pot) and current(flowing through the pot). P=V*I(where P is the power, V is the voltage and I is the current).

So, you have to make sure that the potentiometer is rated for your circuit’s voltages and currents. Supplying more power to a potentiometer than its power rating, can damage it.

Tolerance: Like all resistors, potentiometer(variable resistance) also have tolerance. Tolerance is the shift(increase or decrease) in the value of resistance(of the pot) from its original expected value.

Tolerance may be listed on the device but not always. You can see the specifications of the pot using it’s datasheet(which is a document that provides the details of a component/ product) to know its tolerance value. Don’t know how to get the datasheet of any potentiometer? It’s very simple. Just Google it! You can easily find it there.

So, now you can easily buy a potentiometer and start experimenting with it. You’re gonna enjoy this!

Some Interesting Facts

Before ending this, it’s time to share some amazing facts about potentiometer.

 

 

1. The carbon potentiometer was invented by Thomas Edison(see in the picture) in 1872.
2. He called this device a “coiled resistance wire rheostat”.
3. Potentiometers are used in radios to control the volume, in televisions to control picture brightness, contrast, and colour response.