A potentiometer is one of the most commonly used variable resistors in electronic circuits. It is a three-terminal device that helps alter the resistance and control the flow of current in a circuit. Out the three terminals of the potentiometer, two terminals are fixed and one is variable. The variable terminal or the wiper can be a sliding contact or a rotating contact that helps form an adjustable voltage divider. A potentiometer is used to measure the EMF of a cell. EMF or electromotive force is the maximum terminal potential difference across two points when the circuit is open-circuited and no current is being drawn out of it. A potentiometer can also determine the internal resistance of a cell. A potentiometer relies on the comparative method to determine the unknown voltage; therefore, it is highly precise. When a potentiometer is perfectly balanced, no current flows through it, thereby allowing it to work free from source resistance. A potentiometer is a passive electronic component and is also known as a pot.
Construction of a Potentiometer
A potentiometer consists of a resistive element and a sliding contact. The sliding contact is known as the wiper and is used to vary the resistance offered by the potentiometer. The internal circuitry of a potentiometer comprises of a constantan (a copper-nickel alloy) or manganin (an alloy of copper, manganese, and nickel) wire and a metre rod. One end of the wire is connected to a switch called a tapping key, which is further connected to a rheostat. A driving voltage source is connected between the rheostat and an ammeter. The voltage source is responsible to maintain a constant flow of current in the circuit, while the ammeter functions to monitor this value of the current. The second terminal of the ammeter is connected to the other end of the constantan or manganin wire, thereby completing the circuit of a potentiometer.
Working Principle of a Potentiometer
A potentiometer basically works on the principle of varying the resistance of a fixed resistor by moving the sliding/rotary contact or the wiper. By displacing the position of the wiper, the length of the resistive constantan or manganin wire is altered. This change in the length of the wire is proportional to the change in the resistance offered to the circuit. Also, the resistance is inversely proportional to the cross-sectional area of the wire; however, it is difficult to vary the cross-section of a wire. Hence, the cross-sectional area of the wire and the current flowing in the circuit is maintained constant and the length of the resistive wire is made variable. In a potentiometer, input is applied across the fixed terminals, whereas the output is noted between the movable and a fixed terminal.
Types of Potentiometer
1. Rotary Potentiometers
In rotary potentiometers, the wiper tends to follow a circular path to move and vary the resistance of the circuit accordingly.
1. Concentric pot
A concentric potentiometer consists of two potentiometers that can be adjusted individually. The concentric shafts of the two potentiometers allow the user to have two controls on one single unit to vary two different resistance. These type of potentiometers are commonly used in car radios where the volume control knob and tuning knob are positioned adjacent to each other.
2. Single-turn pot
A Single Turn Pot supports rotation to only one position. Typically, the rotation is approximately 270 degrees, which is about three-fourth (¾) of the full turn. It is mostly used in applications where a single turn provides the desired control.
3. Servo pot
A servo pot contains a servo motor attached to the pot. The servo motor provides the necessary driving force and enables the wiper to be controlled automatically. It is best suited for applications where both manual and automatic control is desired, for example, remote-controlled audio equipment.
4. Presets and Trimmers
A preset or a trimmer is a special type of rotary potentiometer. It makes use of carbon-rich material or cement to form the resistive tracks. It is mostly used in applications where the need to vary the resistance arises very rarely. The rotary wiper of the preset can be adjusted with the help of a screwdriver. Presets are also known as trim pots. They are available in an open skeleton package and a closed square package.
5. Multi-Turn Pot
In Multi-turn pots, a worm gear is used to drive the wiper on the spiral or helical potentiometer structure. As the name suggests, multi-turn potentiometers have multiple rotations. The number of rotations is directly proportional to the precision of the pot, which means a multi-turn pot with 20 rotations is more accurate than a 5 rotation multi-turn pot. These pots are mostly used in applications where precision and resolution is the prime concern.
6. Dual-Gang Pot
A dual-gang pot has two potentiometers on a single shaft. A combination of single-turn potentiometers is mostly preferred for the construction of a dual-gang pot. As per the requirement, the number of potentiometers used in the dual-gang pot can be increased. These types of potentiometers are used in stereos where 2 channels are required to be tuned at the same time.
2. Linear Potentiometers
Linear potentiometers are those in which the wiper or the movable terminal of the pot moves linearly along the length.
1. Multi-turn slide
As the name suggests, a multi-turn slide is capable to undergo multiple rotations. Multi-turn slide-type potentiometers make use of a spindle that is responsible to move the wiper. These potentiometers have high accuracy and resolution. The precision of these potentiometers can be stretched by increasing the number of multiple rotations.
2. Slide pot
A slide pot consists of a single linear slider. It is also known as a fader. These types of potentiometers are best suited for applications where single-channel control or measurement is required such as audio devices. The material used for the construction of a slide pot or fader is usually conductive plastic.
3. Dual-slide pot
A dual slide pot makes use of a single slider to control the resistance of two potentiometers. The two potentiometers are connected parallel to each other. Since one slider is able to control the working of two potentiometers, they are used in applications where two control switches are required, for example, a stereo that needs volume and channel control.
4. Motorized Fader
A motorized fader is very much similar to the slide pot except that a motorized fader can be controlled with the help of a servo motor. The servo motor enables both manual and automatic control over the device. This is the reason why motorized faders are used in the applications such as audio mixers.
3. Digital Potentiometers
The working of a digital potentiometer is somewhat the same as a mechanical pot; however, the difference is that a digital pot makes use of a resistor ladder. At every step of the ladder, a switch is present. The closed switch determines the position of the wiper and hence, the resistance of the potentiometer. The force required to move the sliding contacts of a mechanical potentiometer is significantly high. Also, they are bulky in size, offer limited bandwidth and accuracy, are prone to mechanical wear and tear, and are sensitive to vibration. On the other hand, a digital pot or Digi-pot provides advantages like the reduced size of the circuit, high accuracy, improved reliability, low power dissipation, negligible resistance drift reduced capacitive effect, etc. Some of the popular digital potentiometer ICs are M62429 Digipot from Renesas and MCP41010 from microchip.
Symbol of Potentiometer
The symbol of a potentiometer consists of a standard resistor symbol with an arrow. A pot is a three-terminal device; therefore, the arrow is not overlayed on the resistor symbol as in the case of a variable resistor, but it is present as a third terminal.
Applications of Potentiometer
1. Comparing EMFs
To compare the e.m.f. of two cells, the cells are required to be connected in such a way that the positive terminals of both the cells are connected to the potentiometer, and the negative terminals are connected to a galvanometer via a two-way switch. The two-way switch enables the galvanometer to get connected with the cells one at a time. The second terminal of the galvanometer is connected to the slider. The slider is made to vary, and the deflection of the galvanometer needle is observed, thereby allowing us to compare the EMFs of the two cells connected.
2. Audio Controls
One of the major applications of a potentiometer can be found in stereos or radio receivers, where volume and tuning control is required. The potentiometers are able to vary the resistance by varying the position of the wiper, thereby allowing the user to regulate the volume of the device or tune the circuit to the desired frequency. Both rotary and linear potentiometers are useful in such applications.
A lot of features of television such as contrast control, brightness control, colour response adjustment, etc. require frequent customization and variation. These features can be controlled easily with the help of potentiometers. Earlier, mechanical potentiometers were used in the television sets to perform these tasks; however, with advancements in technology, the traditional rotary and linear pots have been replaced by digital pots.
The potentiometers used in amplifiers are responsible to control volume, bass, mid, tone, tempo, and many other audio-related features. The sliding-type potentiometers are mostly used in amplifier circuits to serve the purpose; however, the choice may vary from person to person.