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Potentiometer: Construction, Working Principle, Types and its Applications

What is a potentiometer?


A potentiometer, also known as a variable resistor, is a three-terminal electronic device used to measure the electromotive force (EMF) of a cell and its internal resistance. It finds applications in various electronics equipment, allowing users to modify electrical circuits for desired outputs. In radios, potentiometers are commonly used for volume control and in other audio electronic equipment.



Potentiometer
Potentiometer

Potentiometers come in various forms, featuring three leads for easy prototyping on a breadboard. The component includes a knob that can be turned to display different values.



Symbol of Potentiometer
Symbol of Potentiometer
  • Potentiometer Pin 1 (fixed end) is connected to one end of the resistive path.

  • Potentiometer Pin 2 (variable end) is variable and connected to the wiper to provide the variable voltage.

  • Potentiometer Pin 3 (the fixed end) is connected to the other end of the resistive path.


Necessities for selecting the potentiometer


Potentiometers come in various types, shapes, and sizes, and selecting the potentiometer depends on the specific requirements below.


  • Requirements of the structure

  • Resistance to change the characteristics

  • Needs of application

  • Select the settings based on the requirements of the circuit.


Understanding variable resistors


Let us now explore the comparative analysis of potentiometers and rheostats in electrical applications.


Both the rheostat and potentiometer are variable resistors and are used in electrical circuits for controlling electric current flow. They both have some similarities, but they also have various differences when it comes to their design and applications.


A rheostat is a two-terminal variable resistor. It is used to handle high-current in the electrical circuits. Various forms of rheostats are available, including wire-wound and grid types, depending on the specific application and requirements of the current flow.


In practical terms, rheostats are often used in situations where you need to control the flow of electrical current through a circuit, and the current levels involved might be higher than what a typical potentiometer is designed to handle. This could be in applications like adjusting the brightness of a high-powered lamp or controlling the speed of an electric motor.


The capacity of a rheostat to handle a significant amount of current makes it suitable for tasks where power dissipation and heat generation are considerations. Potentiometers, on the other hand, are often used for lower-current applications, such as signal control in audio circuits.


Both potentiometers and rheostats are variable resistors, but potentiometers are used for voltage control and have three terminals, while rheostats are used for current control and have two terminals. The choice between them depends on the specific requirements of the circuit and the type of control needed.


Construction of a potentiometer


A potentiometer consists of a long wire with a uniform cross-section. The wire is composed of either manganin or constantan. In some cases, the long wire can be cut into certain pieces. Each piece will be associated with the end, focusing through a metallic strip. Usually, it would be a copper strip. Each piece of wire will be the length of one meter. For most of the parts, it consists of six bits of wire, and the whole length of the wire will be 6 meters. For the rest of the parts, the wire length shifts from 4m to 10m. The greater the wire length, the better the potentiometer precision.


The potentiometer is comprised of a driving circuit. It comprises a battery, key, rheostat, galvanometer, and rider. The potentiometer terminals are associated with the focuses where the potential contrast is to be calculated.


Working principle of the potentiometer


The basic working principle of a potentiometer is that the potential drop across any section of the wire is directly proportional to the length of the wire, given that the wire consists of a uniform cross-sectional area and a constant amount of current flows through the wire.



Schematic visualization of potentiometer operation
Schematic visualization of potentiometer operation


A potentiometer consists of a long resistive wire (L) and a battery that has an EMF voltage (V), whose voltage is termed the driver cell voltage. Let us assume a primary circuit that consists of a wire whose both ends are connected to the terminals of the battery. One end of the primary circuit is connected to the cell of EMF E to be measured, and the other end of the primary circuit is connected to the galvanometer G, and this circuit is considered to be the secondary circuit.


According to Ohm's Law,


V=IR where I is current, V is voltage, and R is the total resistance.


v=IρL/A (since R = ρL/A) ρ represents resistivity, while A corresponds to the cross-sectional area.

ρ and A are constants, and I is also a constant for a rheostat.


V=KL


E=LρX/A


E=KX


E=LρX/A=KX


X is the length of the potentiometer wire, E is the cell with the lower EMF, and K is constant.


The galvanometer G detects null when the potential difference equals zero, indicating no current flow.


X = length of null point


Unknown EMF can be estimated by finding X and K.


EMF has two cells: L1 is the null point length of the first cell with EMF E1, and L2 is the null point length of the second cell with EMF E2.


E1 = (l1/L)/v


E2 = (l2/L)/v


From E1 and E2, we get:


E1/E2 = L1/L2


Types of potentiometers


There are three types of potentiometers, namely,

  • Rotary potentiometer

  • Linear potentiometer

  • Digital potentiometer


Rotary potentiometer


We have already seen in the above topics that the knob is used to control the volume on a stereo player. When we turn the knob in either direction, it is possible to increase or decrease the volume of the stereo. This is the basic working principle of a rotary potentiometer.

The potentiometer assists in providing adjustable supply voltage to electronic and electrical circuits by converting its circular motion to variable resistance.



Rotary Potentiometer
Rotary Potentiometer


In this type of rotary potentiometer, there will be two contacts placed in a half circle with constant resistance throughout. The wiper is the third terminal, which is linked to the knob rotating. When the knob is moved, the wiper associated with it also moves accordingly across the resistance, and this varies the resistance of the potentiometer.


Types of rotary potentiometers


  • Single-turn potentiometer

  • Dual-gang potentiometer

  • Multi-turn potentiometer

  • Servo potentiometer 

  • Potentiometers using concentric shafts


Servo Potentiometer
Servo Potentiometer


Linear potentiometer


Linear potentiometers work exactly the same way as rotary potentiometers, except that linear potentiometers use linear motion to provide varying resistance instead of the internal resistance of the battery cell or for comparing the standard cell with the battery cell.



Linear Potentiometer
Linear Potentiometer


Types of linear potentiometers


  • Motarized fader potentiometer 

  • Dual-slide potentiometer

  • multi-turn slide potentiometer


Motorized Potentiometer
Motorized Potentiometer


Digital Potentiometer


Rotary and linear potentiometers are mechanical potentiometers. Hence, they have issues such as variations in the accuracy and other issues associated with the mechanical potentiometer, such as humidity, size, mechanical wear, wiper contamination, sensitivity to vibration, and resistance drift.


The digital potentiometer does not have any of the issues mentioned in the above types of potentiometers, such as accuracy of measurements or any other issues.



Digital Potentiometer with inbuilt Std Cell & Magnetic Stripper
Digital Potentiometer with inbuilt Std Cell & Magnetic Stripper


Types of digital potentiometers


Potentiometers can be classified into various types based on two important factors, namely,

  • materials used for the manufacturing

  • working principle


Based on the materials used for manufacturing, the digital potentiometers are classified as:

  • Carbon composition potentiometer

  • metal film potentiometer

  • conductive plastic potentiometer

  • cermet potentiometer


Disadvantages of a digital potentiometer


  • Temperature variations affect the internal resistance of the potentiometer.

  • It is not user-friendly like other types of potentiometers if there is no knob.

  • It cannot be used at high current magnitudes.


Advantages of a digital potentiometer


  • It can be used as a digital interface.

  • It has wide range of resolution.

  • In harsh conditions, this potentiometer performs well.

  • It is lightweight and portable.

  • Reliability is greater than the other mechanical potentiometer types.


Thermistor and Ohm Meter in Electronics


In electronic applications, alongside potentiometers, other components play crucial roles. Two such components are the thermistor and the Ohm meter.


Thermistor - A thermistor is a temperature-sensitive resistor, often used to measure or control temperature in electronic circuits. Its resistance changes with temperature variations, providing a valuable tool for temperature-dependent applications.


Ohm Meter - An Ohm meter is an essential tool for measuring resistance in electronic components. It helps ensure proper functioning and adherence to specified resistance values in circuits.


Potentiometer Values


The main feature of the potentiometer is its variable voltage and its capability to store or hold up multiple resistance values. To avoid or decrease the complexity of the circuit, manufacturers reduce the potentiometer resistance values to multiples of 10, 20, 22, 25, 47, and 50. 10K Ohms, 1K Ohms, 5K Ohms, and 100K Ohms are some of the famous potentiometer values.


Applications of Potentiometers


  • Enormous applications in electrical and electronic circuits and also in machinery

  • Allows for measuring the electromotive force or resistance of the cell.

  • Adjustable voltage divider in the circuit

  • It helps in audio applications in a radio receiver or television as a medium for volume control, absolute linearity control, tone control, picture brightness, color response, and contrast.

  • Power applications with machinery

  • Used in motion control to create a closed loop control

  • A potentiometer is used in position feedback devices called servomechanisms.

  • Since they deliver large outputs, they find applications in the design of displacement transducers.


Understanding the potentiometer's construction, working principle, types, and applications is crucial for its effective use in electronic systems. Whether it's for adjusting volume in audio equipment or controlling machinery, potentiometers play a vital role in diverse applications. Alongside potentiometers, integrating components like the thermistor and Ohm meter ensures comprehensive electronic functionality. Choose the right type based on your specific circuit requirements to achieve optimal performance.

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