Variable resistors are electronic components that allow for the adjustment of resistance in a circuit. They are used to control the flow of current in a circuit by changing the amount of resistance offered to the current. Variable resistors typically have a movable contact that can be adjusted to change the resistance value. This makes them more versatile than fixed resistors, as they can be used to adjust the resistance in a circuit to the desired level.

This guide covers the topics as below:

  • Types of variable resistors
    • Potentiometers
    • Rheostats
    • Trimmers
  • Applications of variable resistors
  • Selection and installation of variable resistors

Types of variable resistors

The three main types of variable resistors are potentiometers, rheostats, and trimmer potentiometers. Each type has specific characteristics and is suited for particular applications.

Potentiometers

Potentiometers are a type of variable resistor used to divide voltage in circuits. They have three terminals, a resistive track, and a movable contact that can be adjusted to change the resistance value. The resistive track is made of materials such as carbon, cermet, or conductive plastic, with a uniform resistance along its length. A sliding wiper moves along the track to change the resistance value between the wiper and the other two terminals. Potentiometers have many applications, such as volume control in audio systems, brightness control in displays, and control of speed or position in motorised systems. They come in various sizes and resistance values and can also act as variable voltage dividers.

Potentiometer
Figure 1: Potentiometer

Potentiometers offer precise control of resistance due to their smooth and continuous adjustment. However, they may wear down and produce noise or scratching sounds when adjusted. A potentiometer consists of three main parts: a resistive element, a wiper, and a mechanism for adjusting the wiper position. The resistive element is a thin strip of material mounted on a non-conductive base, marked with a resistance value. The wiper is a small contact that slides along the resistive element to determine the output voltage. The mechanism for adjusting the wiper position can take various forms, such as a knob, lever, screwdriver, or other tool.

Rheostats

A rheostat is a type of variable resistor that controls the flow of current in an electric circuit. It has three main components: a resistive element, a wiper arm, and electrical terminals. The resistive element can be made of different materials, such as coiled wire, carbon composition, or thin film, depending upon the type of rheostat and its application. The wiper arm varies the resistance of the rheostat by changing the length of the resistive element in the circuit. It's typically made of conductive material and moves smoothly along the resistive element without causing damage. The electrical terminals are used to connect the rheostat to a circuit and come in different types, such as screw, spade, or solder terminals.

Some rheostats may also have a shaft, knob, or housing for adjusting the wiper arm position and protecting internal components. Rheostats are used in applications where the current flowing in a circuit needs to be varied, like in dimmer switches or motor speed controllers. One disadvantage of rheostats is that they can dissipate a significant amount of power, especially for high currents. This is because the power dissipated in the rheostat is proportional to the voltage drop across it and the current flowing through it. To minimise power dissipation, rheostats are designed with a large surface area or cooling fins. Alternatively, other control methods like pulse width modulation can be used to control current flow in a circuit instead of using a rheostat.

Rheostat
Figure 2: Rheostat

Rheostats come in various types, each with unique characteristics based on their construction and intended use. Here are some commonly used types of rheostats:

  • Wire wound rheostats: These are made by coiling a resistive wire around a core made of ceramic or porcelain. They are usually employed in high power applications where significant heat is generated.
  • Carbon composition rheostats: In this type, carbon particles are mixed with a binder material and then molded into the desired shape. They are commonly used in low power applications.
  • Conductive plastic rheostats: Conductive particles are embedded in a plastic matrix to make these rheostats. They are often used when precise and repeatable adjustments are required.
  • Cermet rheostats: A thin layer of conductive material, such as metal oxide or cermet, is deposited onto a ceramic substrate to make cermet rheostats. They are generally used in high precision applications where stability and reliability are essential.
  • Film rheostats: A thin film of resistive material, such as carbon or metal alloy, is deposited onto a substrate to create film rheostats. They are typically used in low power applications where small size and low cost are crucial.

Trimmers

Trimmers, also known as trimmer potentiometers or preset potentiometers, are variable resistors that are specifically designed to be adjusted only once or occasionally, and then left in a fixed position. They are frequently used in electronic circuits to calibrate a sensor or instrument or adjust the gain, bias, or offset of an amplifier or other component.

The basic construction of a trimmer is similar to that of a regular potentiometer, with a resistive element, a wiper arm, and electrical terminals for connection to a circuit. However, trimmers are much smaller and more compact than potentiometers and may be mounted directly on a circuit board or other surface.

Trimmer potentiometers
Figure 3: Trimmer potentiometers

Trimmers are typically adjusted using a small screwdriver or other tool rather than by turning a knob or shaft like a regular potentiometer. This allows for precise adjustment of the resistance value without the risk of accidental or unintended changes.

Trimmers are available in various resistance values, power ratings, and types of resistive elements, such as carbon composition, cermet, or conductive plastic. Depending upon the required precision and range of adjustment, they may also be single-turn or multi-turn.

There are various types of trimmers available, each with its specific advantages and disadvantages, depending on the application. These types include:

  • Carbon composition trimmers: They are the most basic type of trimmer, made by mixing carbon particles with a binder material and molding the mixture into a desired shape. Carbon composition trimmers are inexpensive and have a wide range of resistance values. However, they can be less stable and have a limited lifespan.
  • Cermet trimmers: They are made by depositing a thin layer of conductive material, such as metal oxide or cermet, onto a ceramic substrate. Cermet trimmers are often used in high-precision applications where stability and reliability are crucial, as they have a high level of accuracy and a long lifespan.
  • Wire wound trimmers: They are made by winding a resistive wire around a ceramic or plastic core. Wire wound trimmers are often used in high-power applications where a lot of heat is generated. They are durable and have a wide range of resistance values. However, they can be bulky and expensive.
  • Conductive plastic trimmers: They are made by embedding conductive particles in a plastic matrix. Conductive plastic trimmers are often used in applications where precise and repeatable adjustments are required, as they have a high level of accuracy and are less expensive than other types of trimmers. However, they can be less durable than other types of trimmers and may have limited power handling capabilities.
  • Multi-turn trimmers: They are designed to provide a higher level of precision than single-turn trimmers, as they allow for multiple rotations of the adjustment screw. Multi-turn trimmers are often used in applications where very fine adjustments are needed, such as in calibration or tuning circuits.

Applications of variable resistors

Variable resistors are used in a wide range of applications in electronics and electrical circuits. Here are some of the most common applications of variable resistors:

  • Volume control: The volume control adjusts the level of the audio signal by changing the resistance of a potentiometer. The potentiometer is connected in series with the audio signal path in a typical volume control circuit. The input terminal of the potentiometer receives the audio signal, and the output terminal is connected to the next stage of the circuit, like an amplifier or speaker. The maximum volume level is determined by a fixed resistor called the load resistor, which is connected to the wiper or moving contact of the potentiometer. As the potentiometer is turned towards the opposite end from where it started, the resistance between the input and output terminals decreases, and the volume level increases. When the potentiometer is turned to its minimum resistance, the volume level is at its highest. Conversely, when the potentiometer is turned to its maximum resistance, the volume level is at its lowest.
  • Brightness control: To control the brightness of a light source like an LED or an incandescent bulb, you can use variable resistors, which are also known as potentiometers. The idea behind this is to regulate the current flowing through the light source by adjusting the resistance of the potentiometer. In a standard brightness control circuit, the potentiometer is connected in series with a fixed resistor and the light source. The voltage across the circuit is determined by the power supply, whilst the current flowing through the circuit depends upon the total resistance, which includes the resistance of the potentiometer and the fixed resistor. When the potentiometer is set to its maximum resistance, the total resistance of the circuit is at its highest, resulting in a minimum current flowing through the light source, causing the light to be dim. Conversely, as the potentiometer is turned towards its minimum resistance, the total resistance of the circuit decreases, leading to an increase in the current flowing through the light source, causing the light to become brighter.
  • Temperature control: Rheostats are often used for temperature control in applications that require precise management of electrical power delivered to a heating element. The rheostat acts as a variable resistor that regulates the amount of current flowing through the heating element. For instance, in electric stoves and ovens, a heating element is commonly connected in series with a rheostat and a temperature sensor. As the oven temperature increases, the temperature sensor's resistance decreases, and the rheostat is adjusted to reduce the current flowing through the heating element, thus maintaining the desired temperature. The rheostat can also regulate the speed of a fan or blower in temperature control applications. In a heating, ventilation, and air conditioning (HVAC) system, for example, the rheostat can be to adjust the fan's speed based on the room's temperature.
  • Motor control: Potentiometers and rheostats are commonly used for motor control, but they function differently. Potentiometers control the motor speed by adjusting the voltage applied to the motor. They are connected in series with a fixed resistor and the motor. When the potentiometer is turned, its resistance changes, which changes the voltage across the motor and therefore the speed. Potentiometers are suitable for low-power applications such as small DC motors or radio-controlled vehicles. On the other hand, rheostats control the current flowing through the motor, which affects the motor's torque or power output. They are connected in series with the motor, and the voltage across the circuit is kept constant. When the rheostat is turned, its resistance changes, which changes the amount of current flowing through the motor and, consequently, the motor's torque or power output. Rheostats are suitable for high-power motor control applications, such as industrial machinery or electric vehicles.
  • Calibration: Trim pots, also known as trimmer potentiometers, are used to fine-tune the values of resistive components in calibration applications. They are designed to be adjusted infrequently and are often used in manufacturing or testing to ensure that a circuit meets specific performance standards. Trim pots are used to adjust the output or behavior of a circuit by connecting them in parallel or series with other resistive components and adjusting their resistance to achieve the desired value or response. In audio amplifier circuits, trim pots may be used to adjust the gain of individual amplifier stages or to balance left and right channel outputs. In electronic filter circuits, trim pots may be used to adjust the cutoff frequency or resonance of the filter. Precision measurement circuits, such as digital multimeters or oscilloscopes, use trim pots to adjust calibration and ensure accurate readings.
  • Signal Amplification: Potentiometers are versatile components that can be used for signal amplification by adjusting the resistance in a voltage divider circuit. This circuit is a simple way to attenuate or amplify an input signal. In this circuit, the input signal is divided between two resistors, and the output voltage is taken from the junction of the resistors. By using a potentiometer as one of the resistors in the voltage divider, the resistance can be varied to adjust the output voltage level. For example, if the potentiometer is connected in series with the input signal, turning the potentiometer to a higher resistance value will attenuate the signal, while turning it to a lower resistance value will amplify the signal. Potentiometers can also be used in feedback circuits to adjust the gain of an amplifier. In this type of circuit, the potentiometer is used to adjust the amount of negative feedback, which can reduce the overall gain of the amplifier. By adjusting the potentiometer, the user can fine-tune the gain of the amplifier to match the desired output level.
  • Tuning: Potentiometers are widely used in electronic circuits for tuning applications that require precise and adjustable resistance to fine-tune the circuit's frequency response or other parameters. They are commonly used in radio or audio circuits as variable resistors to adjust the tuning frequency of a circuit. For instance, in a radio tuner, a potentiometer can be used to adjust the frequency of the local oscillator, which mixes with the incoming signal to create an intermediate frequency that can be amplified and demodulated. Potentiometers are also used in equalizer circuits to adjust the amplitude of different frequency bands, in tone control circuits to adjust the bass and treble levels, and in phase-locked loop circuits to adjust the frequency and phase of the output signal. They are often preferred over fixed resistors for tuning applications because of their flexibility and ease of use.

Selection and installation of variable resistors

Variable resistors, such as potentiometers and rheostats, are important components in electronic circuits, providing the ability to adjust resistance levels for a wide range of applications. Here are some tips for selecting and installing variable resistors:

Selection:
  • Determine the required resistance range and tolerance for your application. This will depend on the specific circuit and its desired performance characteristics.
  • Choose a variable resistor type that meets the requirements of your application. Potentiometers are typically used for voltage control and tuning applications, while rheostats are used for current control and power dissipation applications.
  • Consider the power rating of the variable resistor, which will depend on the amount of current and voltage that it will be subjected to. Make sure the power rating is sufficient for the application to avoid overheating or failure.
  • Choose a variable resistor with a suitable physical size and shape to fit into the circuit board or enclosure.
Installation:
  • Make sure to follow the manufacturer's recommendations for installation and use. Variable resistors can have specific requirements for mounting, electrical connections, and environmental conditions.
  • Pay attention to the orientation of the variable resistor, especially if it is a potentiometer. Potentiometers have three terminals, with the wiper terminal in the middle. Make sure to connect the input and output terminals correctly to ensure proper function.
  • Use appropriate tools and techniques for soldering or connecting the variable resistor to the circuit. Poor connections can lead to intermittent or erratic behavior.
  • Consider adding a protective cover or shield to the variable resistor to prevent accidental adjustments or damage.
  • Test the variable resistor after installation to ensure that it is functioning properly and within specifications.

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