Potentiometer Working Principle

Potentiometer

A potentiometer is a three-terminal electric device, which is widely used in various electric circuits and electronic devices. The term “potentiometer” is made up by combining two words, i.e., ‘Potential Difference’ and ‘Metering.’ As the name suggests, a potentiometer is used to meter or measure the potential difference across the circuit by varying the resistance in the circuit manually; hence, the potentiometer is referred to as a voltage metering device. The first potentiometer was invented and patented by inventor George Little in 1871. Potentiometers are passive electric devices, i.e., they do not produce energy and only store or dissipates it in the form of electric current. Potentiometer act as a versatile device; apart from measuring the potential difference, it is also used for other purposes such as in dividing the voltage, comparing the emf’s of cells and of course, and as a variable resistor. Potentiometers are also known as a short term called ‘pot.’ Let us learn about the construction, working, and much more about the potentiometers.

Key Components of Potentiometer

The following components are used in the electric circuit of the potentiometer.

1. Resistive Wire

A long wire of uniform and known resistance is used in the electric circuit of the potentiometer. The wire is usually made of alloys like manganin and constantan because they have a low-temperature coefficient and high specific resistance. Generally, long wires are preferred over short wires in the potentiometer because long wires cause a small potential gradient in the potentiometer.

Potentiometer Wire

2. Ammeter

An ammeter is an electronic instrument that is used to measure the amount of electric current flowing in the circuit. The ammeter is always connected in the series in the electric circuit to measure the current because they offer minimum internal resistance; hence it does not much affect the current to be measured.

Ammeter

3. Jockey

If the long wire is used in the circuit, then a pointed metal contact called a jockey is used to adjust the resistance value required at the terminals. It slides over the wire and provides the null point; a point where the galvanometer does not show any deflection is called the null point.

Potentiometer Jockey

4. Galvanometer

It is an electromechanical device that is used to detect and measure the electric current in the circuits. It consists of a needle attached to the moving coil that deflects in the presence of the current. The major difference between the galvanometer and the ammeter is that ammeter can only measure the magnitude of the electric current, while the galvanometer not only measures the magnitude of the current but also shows the direction of the current.

Galvanometer

5. Rheostat

A rheostat is a variable resistor, which is used to adjust the resistance in the electric circuit. It is used to lower the amount of current passing through the potentiometer and maintaining the constant electric current to increase the sensitivity of the potentiometer.

Rheostat

 

Construction of Potentiometer

Construction of Potentiometer

Figure-A: Here, the length AB represents the length of the Potentiometer wire, and the thick verticals lines represent the metal strips.

A potentiometer consists of a long piece of wire, which is of the same cross-section and uniform resistance throughout its length. As the wire used in the potentiometer is few metres long; hence, it is usually cut into few pieces, and then the wire pieces are attached with the help of thick metal strips at their ends as shown in figure-A above. The attachment of the wire in this way reduces the longitudinal area and also enhances the functioning of the potentiometer. The two ends of the wire are connected to a standard cell or battery of known voltage. The known voltage is called the driver cell voltage. The constant current I flows through the resistive wire, and the rheostat is attached in the circuit to vary the resistance R. The position of the jockey is varied over the potentiometer wire (AB), and the galvanometer is attached with the jockey as shown in the figure-A. Nowadays, small-sized potentiometers are constructed by manufacturers, which gives more accurate readings than the earlier used bulky potentiometers. Potentiometers may available in different shapes and sizes, but they always consist of three terminals. The two end terminals are attached to the resistive path, and the central terminal is attached to the movable wiper that is used to provide variable voltage. A knob, which is attached to the wiper is provided over the potentiometer to adjust the required value of resistance in the circuit.

Three Terminals of Potentiometer

Potentiometer Symbol

Two types of Potentiometer symbols are widely used to represent potentiometer in an electric circuit; one is the ‘American symbol’ and the other is an ‘International Standard Symbol.’ The American symbol consists of zigzag lines having three terminals, wherein the zigzag part is attached to the two straight lines. The International Standard Symbol is a small rectangular box consisting of three terminals, and this box is also attached to two straight lines.

Potentiometer Symbol

Working Principle of Potentiometer

The working principle of the potentiometer is that the voltage drop across any part of the uniform resistive wire is directly proportional to the length of the wire if a constant electric current is flowing through the resistive wire. Hence, no electric current will flow through the circuit if there is zero potential difference between any two parts of the wire. In the potentiometer, the input voltage is applied to the whole length of the resistive wire, and the output voltage is measured between the sliding contact and the fixed end of the circuit as shown in figure-1. The position of the jockey (sliding contact) is varied across the length of the uniform resistive wire to find the null position.

Output Voltage of Potentiometer

Figure-1

To understand the working of the potentiometer, Let’s consider a simple electric circuit consisting of two batteries, which are connected in parallel with each other through a galvanometer. As shown in the figure below (figure-2) the negative terminal of one battery is connected to the negative terminal of the other battery and similarly, the positive terminals of the battery are connected. If both the batteries are at the same potential, no amount of current will flow through the circuit, hence, the galvanometer does not show any deflection. The working of the potentiometer is based on this phenomenon.

Null Deflection by Galvanometer

Figure-2

Now, consider another electric circuit as shown in Figure-3. Here, a battery is connected to the rheostat and also a switch is fitted in the circuit. As the resistance per unit length of the wire is uniform throughout its lengths; hence, the voltage drop per unit length is also equal throughout its length. Suppose we get the voltage drop v per unit length of the resistive wire by varying the rheostat. Here, the positive terminal of the standard cell is attached to point A of the resistor, and its negative terminal is attached to the galvanometer. The other terminal of the galvanometer is connected with the sliding contact (jockey). The jockey slides over the resistor (potentiometer wire) and finds a point B at which no current flows through the galvanometer, i.e., the galvanometer does not show any deflection at point B. At point B, the electromotive force (emf) exactly equals the voltage across the resistor between points A and B. Let L be the distance between the points A and B of the resistor, hence we can say that the emf of the standard cell is equal to the product of length “L” and the voltage drop “v” across per unit length, i.e.,

E=Lv

Hence, in this way, the potentiometer can accurately measure the voltage between any two-point of the resistive element without causing much resistance to the circuit.

Working Principle of Potentiometer

Figure-3

Types of Potentiometer

There are various types of potentiometers constructed of different shapes and sizes, by the manufacturers, which are used for different purposes. No matter that what’s the type of potentiometer, they all work on the same principle. Let us discuss the types of Potentiometers.

1. Rotary Potentiometer

Rotary potentiometers are the most commonly used type of potentiometers, which are used to supply the suitable and smooth output voltage. It consists of two end terminals and a semicircular uniform resistance is placed in its middle, and the middle terminal is connected with the resistance along with the jockey (sliding contact), which is attached to the rotary knob. The jockey can be slide over the semi-circular resistance by rotating the knob of the rotary potentiometer, and the output voltage is obtained between the fixed end of the resistance and the jockey at the null point. Rotary transistors are widely used in radio transistors as a volume controller, wherein the rotating knob guides the amount of electric current to be supplied to the amplifier. Rotary potentiometers can have single or multiple resistive tracks/elements. The potentiometer consisting of multiple resistive elements are known as the ganged potentiometers, in which the sliding contact of the multiple resistances is connected to the same shaft. For example, to increase or decrease the volume in the stereo amplifiers or radio, the dual-gang potentiometer is used.

1. Single-turn Potentiometer

The wiper in the single-turn potentiometer can rotate about 270 degrees or covers the 3/4th of the complete turn. It is used in devices where a single turn is enough to give the desired resolution.

Single-turn Potentiometer

Single-turn Potentiometer

2. Multi-turn Potentiometer

These are used in devices, which requires extreme resolution and precision. The wiper present in the multi-turn potentiometer can undergo multiple rotations say 5, 10, or 20 rotations. They either consist of the wiper that follows the helical or spiral path or are constructed using the worm gear. The worm gear potentiometers are usually used as trim pots on the printed circuit boards.

Multi-turn Potentiometer

Multi-turn Potentiometer

3. Dual-gang Potentiometer

It consists of two resistive elements attached to the same shaft; hence, allows the parallel arrangement of the two channels. The potentiometers having more than two resistive elements connected to the same shaft are also possible, but they are not commonly used.

Dual-gang Potentiometer

Dual-gang Potentiometer

4. Concentric Potentiometers

It is a dual potentiometer, wherein both the potentiometers are individually adjusted with the help of concentric shafts. It enables dual changes using a single knob. The prime example of concentric potentiometers is the radios systems that were used in the old cars in which both the volume and the tone of the audios were changed by rotating the same knob.

Concentric Potentiometer

Concentric Potentiometer

5. Servo Potentiometer

Servo-potentiometers are motorized potentiometers, which can be controlled by servo motors. They are commonly used in remote controls to adjust the audios.

Servo Potentiometer

Servo Potentiometer

6. Presets and Trimmers

Presets and trimmers are small-sized potentiometers, which enables the users to easily adjust the resistance values in the circuit. They are generally available as closed square designs and open skeleton designs mounted on the printed circuit boards. Open skeleton preset’s are more likely to undergo mechanical wear and tear due to their open construction, hence they are not preferable for continuous use. They are enclosed in a plastic case to avoid the open skeletal structure from dirt and dust. Despite this drawback, they are widely used because of their low cost, mini size, and simple structure. In Presets, the minimum to its maximum values can be reached in just a single turn, this may not be suitable for the devices, which are sensitive to even the smallest changes in the potentiometers. Trim pots or Trimmer potentiometers are thus used as they are multi-turns devices. Like presets they can also be soldered over the PCBs.

Presets and Trimmers

Presets and Trimmers

2. Linear Potentiometer

The working of the linear potentiometer is the same as that of the rotary potentiometer with the only difference that in linear potentiometer the sliding contact moves linearly on resistor rather than the rotary movement. Linear potentiometers are also known as sliders, faders, or slide pots. The two terminals of the straight resistor are attached to the source voltage and a jockey can slide over the resistor through a path fixed along with the resistor. These potentiometers are commonly used in sound mixing equipment and music equalization.

1. Slide Potentiometer

They are used for adjusting the audios in various systems. Slider potentiometers are often built from conductive plastic. They are used for measuring distances and controlling single-channel; a system consisting of a single receiver and a single transmitter operating at the same frequency is known as a single channel.

2. Dual-Slide Potentiometer

It involves a single slider, which controls the dual potentiometers attached in parallel. It is widely used in devices that involve the control of dual parallel channels, i.e., professional stereos and audio control.

Dual-Slide Potentiometer

Dual-Slide Potentiometer

3. Multi-turn Slide Potentiometer

The multi-turn slide potentiometers are usually constructed with the use of a spindle, which controls the movement of the potentiometer wiper. These potentiometers are used in devices that demand high resolutions. Like worm-gear trimmers, multi-turn potentiometers are also used as the trim-potentiometers on the printed circuit boards (PCB).

Multi-turn Slide Potentiometer

Multi-turn Slide Potentiometer

4. Motorized Fader Potentiometer

These potentiometers are used in devices where both manual and automatic control is needed as motorized faders can easily be controlled using the servo motors.

Motorized Fader Potentiometer

Motorized Fader Potentiometer

3. Digital Potentiometer

Mechanical potentiometers are not suitable for operations where high precision is required. The main disadvantages of the mechanical potentiometers are mechanical wear and tear, resistance, vibrations sensitivity, and wiper contamination. Hence, to overcome these problems, digital potentiometers are used as a replacement for mechanical potentiometers. Unlike mechanical potentiometers, digital potentiometers can change the resistance value automatically according to the requirement. Digital potentiometers have wide applications that include tuning filters, controlling screen brightness and audio, and of course maintaining the output voltage. Digital potentiometers may overcome the disadvantages of mechanical potentiometers, but there exist certain drawbacks of digital potentiometers which are given below,

  • They are not preferred for high temperature and high power operations because their internal resistance get altered due to extreme temperature variations.
  • They can not be used for the high ranges of electric current.
  • They are not much easy to use due to the absence of the knob.
  • Their efficiency may get affected due to the parasitic (stray/unwanted) capacitance.
Digital Potentiometer

Digital Potentiometer

Characteristics of Potentiometer

1. Taper

Taper is one of the important characteristics of the Potentiometer. It is the relationship between the resistance and the mechanical position of the potentiometer. Commonly, there are two types of tapers, i.e., linear taper and logarithmic tapers.

1. Linear Taper Potentiometers

Characteristics of Linear Taper

Linear Taper Characteristics

If the relation between the position of the potentiometer and the resistance in the circuit is linear (as shown in the figure above), it is said to be a linear taper potentiometer. This implies that the voltage at the output terminal will be half of the voltage in the potentiometer if the potentiometer’s knob is kept at the medium position. Linear taper consists of the resistive element strip of the uniform density due to which the resistance between either end terminal and the middle terminal varies at a constant rate with the rotation of the control shaft. While constructing linear tapers, manufacturers marked the Linear tapers with the letter “B.”

Potentiometer with Linear Taper

Linear Taper Potentiometer

2. Logarithmic Taper Potentiometer

Logarithmic Taper Characteristics

Logarithmic Taper Characteristics

If the relation between the position of the potentiometer and the resistance gives a logarithmic curve (as shown in the figure above), it is said to be a logarithmic taper potentiometer. The logarithmic taper potentiometers are marked with the letter “A” while manufacturing. Linear tape does not suit all devices, for example, if linear tapers are used for controlling the gain/volume of the radio receivers, the volume that we will hear would change steadily at certain parts or ding again and again at other parts. Here, logarithmic taper potentiometers are used for this purpose. They are also known as the audio taper potentiometer or log-taper potentiometers. In these potentiometers, the resistance between the middle terminals and either end terminal of the potentiometer changes non-linearly with the position of the angular shaft. In a logarithmic potentiometer, the actual variations of the sound intensity are logarithmic, but it seems to increase in a linear manner on rotating the knob.

Logarithmic Taper Potentiometer

Logarithmic Taper Potentiometer

2. Marking Codes

Potentiometers are marked with some numbers of letters to indicate the value of resistance and the taper type. Usually, a three-digit coding format ( similar to SMD resistor code) is used to represent the values on the potentiometer. Here, the first digit of the number represents the value and the last digit represents the multiplier, for example, 1 kΩ is coded as 102, which means 10 Ω x {10}^{2} = 1 kΩ.

3. Resolution

The minimum possible variation in the resistance ratio is defined as the resolution of the potentiometer. The potentiometers made of conductive plastic gives the best resolution, while the wire-wound resistors have low resolutions. The resolution of the potentiometer is also affected by the arrangement of the wiper.

4. Hop-on and Hop-off resistance

The resistive element of the potentiometer is attached to the metallic part of low resistance, this metallic part connects the resistive elements to both the end terminals of the potentiometer. Hence, some changes in the resistance occur when the wiper passes through any of the end terminals, this resistance is called the hop-on and hop-off resistance.

5. Sensitivity of the Potentiometer

The ability of the Potentiometer to measure even the smallest differences in voltage is called the sensitivity of the potentiometer. The sensitivity of the potentiometer can be measured by the potential gradient, which is given by the decrease in the voltage per unit length of the wire, i.e., V/L, where V is the voltage difference between any two-point of the wire and L is the distance between these two points. From this expression it is clear that the potential gradient or sensitivity of the potentiometer and the length of the wire are inversely proportional, i.e., the shorter the wire, the larger will be the potential gradient and vice-versa. The larger the length of the wire, the smaller will be the potential gradient and hence larger will be the sensitivity of the potentiometer.

Applications of Potentiometer

1. Measurement of Voltage

The potential drop across the circuit can be measured with the help of a potentiometer using a simple electric circuit. A rheostat is adjusted in this circuit and the current flowing throw the resistor is adjusted in a way so that a specific voltage drop can be obtained per unit length of the resistor. Now we slide the jockey over the resistor and find the position where the galvanometer shows the zero deflection. We will note the null point on the resistor scale and depending on the null point the voltage in the circuit can be obtained.

Potentiometer Driver Cell: We can measure the voltage of any cell with the help of a potentiometer by comparing the voltage to be measure with the standard cell. Hence, to measure voltage a source voltage is needed to connect across the potentiometer circuit and this standard cell is known as the driver cell. The function of the driver cells is to deliver the current in the resistance of the potentiometer and a full-scale voltage can be thus obtained by multiplying the resistance and the current of the potentiometer. The sensitivity of the potentiometer can be changed by adjusting the voltage and the electric current flowing through the circuit can be controlled by adjusting the rheostat connected with the driver cell in series. It is to be noted that the potential of the driver cell should be greater than the potential that is to be measured.

Measuring the Voltage using the Potentiometer

This image represents the voltage measuring electric circuit using the potentiometer.

2. Comparing the Emf of a Battery Cell with a Standard Cell

Now, Let’s understand the method of comparing the emf of two cells through the derivation,

According to Ohm’s law,

V=IR   —- (equation-1)

Where R is the resistance of the wire

As we know that the resistance R is directly proportional to the length of the wire L and inversely proportional to its crossectional area A, i.e.,

R= ρ(L/A)  —– (equation-2)

Where ρ is the specific resistance.

Now, from equation-1 and equation-2

V= Iρ(L/A)

Where specific resistance ρ and cross-sectional area A remains constant, and the electric current through the circuit is also be kept constant by keeping ṭhe constant resistance with the help of the rheostat.

Therefore, I(ρ/A) = K (constant)

Thus, V=KL

Consider that a cell having lower emf E than the driver cell is attached in the circuit as shown below (e)

Therefore, E = L(ρx/A) =Kx

Where x is the length of some part of the potentiometer wire where the jockey shows the null deflection. At this point, the potential difference between the driver cell and the cell with emf E is zero so there will be no deflection in the galvanometer. The length (x) of the potentiometer from the one end of the wire till the point of the jockey where the galvanometer shows null deflection is known as the length of the null point.

If the values of the constant K and the Length x are known, the unknown EMF of the cell can be calculated and also the emf of the two cells can be compared with the help of the potentiometer.

Imagine that one cell has the EMF E1 and gives the null point at the length L1 and the second cell have the EMF E2 and gives the null point at the length L2.

Hence,

E1/E2=L1/L2

Comparing the Emf of a Battery Cell with a Standard Cell using Potentiometer

This image represents the electric circuit for comparing the emf of a battery cell with a standard cell by using the potentiometer.

3. Measuring the Internal Resistance of the Battery Cell

To measure the internal resistance of the battery, it is connected across the resistor and the galvanometer is attached to the circuit as shown in the figure below, the resistance of known value R is connected to the battery through a switch. When the switch is kept open, we adjust the position of the jockey and find the null point where the galvanometer shows the zero current. Once we find the null point we will measure the position of the jockey on the null point consider this length as L1.

Therefore,

E= KL1

Now, if we switch on the electric circuit, the electric current will start flowing through the resistance R and the battery and there will be a voltage drop in the battery due to the internal resistance of the battery. The voltage drop across the battery will be lesser than that of emf of the cell or the open-circuit voltage. Now, we again adjust the position of the jockey and find the point where the galvanometer indicates the zero current. Let this length be L2.

V=KL2

Thus

E/V=L1/L2

But,

E= I(R+r)

and, According to Ohm’s Law,

V=IR

Hence,

E/V= (r+R)/R

or

(r+R)/R=L1/L2

Hence, the internal resistance of the battery (r) is given by,

r=[(L1/L2)-1]R

Calculating the Internal Resistance of the Battery using the Potentiometer

This image represents the electric circuit for calculating the internal resistance of the battery using the potentiometer.

4. Other Applications

  • Potentiometers such as rotary and linear are used in the audio devices for varying the loudness or amplitude of the audio signals, and frequency attenuation.
  • They are also used as a voltage divider in electric circuits.
  • They are used in televisions for changing the brightness of the screen and controlling the contrast and the colours of the picture.
  • Potentiometers find their applications in the manufacturing of the displacement transducers as they give significant output signals.
  • Potentiometers along with other position feedback devices are used in the servomechanisms. This mechanism is used to measure the speed and angle of rotation of DC motors.
  • Highly precise potentiometers are used in analogue computers for computational purposes.
  • Potentiometers can be used as a tuner in the electric circuits to get the required output, and they are also used for calibration of the electrical components.

Rheostat vs Potentiometer

  • The rheostat and potentiometer both look similar but their working principles are completely different. The primary function of the potentiometer is to provide a variable voltage, while that of the rheostat is to provide the variable resistance.
  • A potentiometer consists of three terminals, and the rheostats consist of two terminals. Moreover,  a potentiometer can be easily be used as a rheostat if only two terminals of the potentiometer are connected in the circuit.
  • In the potentiometer, we obtain the ‘output voltage’ between the sliding contact and the fixed terminal, while in the rheostat ‘variable resistance’ is obtained in the sliding contact and the fixed terminal.
  • Generally, metallic ribbons and carbon discs are used to construct the rheostat, while potentiometers are made of graphite.
  • Rheostats consist of only a single turn, while potentiometers are available with single as well multi-turns.
  • The rheostat is connected in series with the source voltage, while the potentiometer is connected in parallel with the source voltage.
  • Rheostat has only a linear taper, while the potentiometers are available as both the linear and the logarithmic taper.
  • A rheostat is preferred for high power applications, whereas the potentiometer is preferred for low power applications.

Voltmeter vs Potentiometer

  • The voltmeter consumes some amount of electric current from the emf source, while the potentiometer does not consume electric current from the emf source.
  • The resistance offered by the voltmeter is limited, while the resistance offered by the potentiometer is endless.
  • The voltmeter has low sensitivity, while the potentiometer has a high sensitivity that can also be varied by changing certain parameters of the potentiometer.
  • The working of the voltmeter is based on the deflection technique, whereas the working of the potentiometer is based on the null deflection technique.

Advantages of Potentiometer

  • The prominent advantage of the potentiometer is that it can be easily used as the rheostat if we use only two terminals of the potentiometer rather than its three terminals.
  • Potentiometers are very affordable.
  • They are not complex to use, one can easily use them.
  • They can measure even the small emf’s as they have high sensitivity.
  • Their accuracy can be easily increased by increasing the length of the potentiometer wire.
  • They are available in small sizes that can be easily fitted into compact spaces.
  • Potentiometers have a robust fabrication, they can withstand high temperatures, moisture, dust, and chemical exposure.
  • Potentiometers have a long shelf-life, and can easily be used for several years without any failure.
  • They have low power dissipation.

Disadvantages of Potentiometer

  • The working of the potentiometer is very slow.
  • They lack high accuracy.
  • It has low bandwidth, hence the signal strength is not much significant.
  • A significant amount of force is required to move the slider of the linear potentiometer.
  • The chances of wear and tear are high due to the friction caused by sliding the wiper on the resistive track, which reduces the average shelf-life of the device.
  • To get the optimum results, the temperature of the resistive wire must remain uniform throughout the resistive element; however, it becomes difficult to maintain the uniform temperature throughout the length due to the flowing electric current.
  • Sometimes external factors like mechanical shocks or extreme temperature conditions hinder the accurate working of the potentiometer.

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