What is a Variable Resistor?
A variable resistor is defined as a resistor whose value of electrical resistance can be changed on demand. It is a common component in electronic circuits that allows the adjustment of current or voltage according to Ohm’s Law.
The principle of operation of a variable resistor is based on changing the length of a resistive track by moving a wiper contact along it. The position of the wiper determines the effective resistance between the terminals of the variable resistor.
A variable resistor typically has three terminals: two fixed terminals at the ends of the resistive track and one movable terminal (the wiper). Depending on how the terminals are connected to a circuit, a variable resistor can function as a rheostat or a potentiometer.
A rheostat is a device that controls the current in a circuit by varying the resistance. It uses only two terminals: one fixed terminal and the wiper. The resistance between these two terminals increases or decreases as the wiper moves along the track.
A potentiometer is a device that controls the voltage across a load by dividing the input voltage. It uses all three terminals: both fixed terminals are connected to an input voltage source, and the output voltage is taken from one fixed terminal and the wiper. The output voltage is proportional to the ratio of the resistances on either side of the wiper.
Variable resistors can also be used as preset resistors, which are adjusted only once or rarely during circuit building or testing. Preset resistors have an adjustable screw that can be turned with a small screwdriver to set the desired resistance value.
Types and Characteristics of Variable Resistors
There are different types of variable resistors available in the market, depending on the material and shape of the resistive track, the method of adjustment, and the range and linearity of resistance.
The most common materials used for the resistive track are carbon, cermet (a mixture of ceramic and metal), and wire wound. Carbon and cermet tracks are suitable for high-resistance applications, while wire wound tracks are suitable for low-resistance applications.
The shape of the resistive track can be circular or linear. Circular tracks have a knob or dial that rotates to move the wiper along the arc-shaped track. Linear tracks have a slider or lever that moves horizontally to move the wiper along the straight track.
The method of adjustment can be mechanical or digital. Mechanical variable resistors use physical motion to change the position of the wiper, while digital variable resistors use digital signals to change the resistance in discrete steps. Digital variable resistors are also known as digital potentiometers.
The range and linearity of resistance are important characteristics of variable resistors. The range refers to the minimum and maximum values of resistance that can be obtained from a variable resistor. The linearity refers to how well the resistance changes proportionally to the position of the wiper.
Some variable resistors have a linear track, which means that their resistance changes linearly with respect to the position of the wiper. For example, if a linear variable resistor has a range of 0 Ω to 10 kΩ, then moving the wiper halfway along the track will result in 5 kΩ of resistance.
Other variable resistors have a logarithmic track, which means that their resistance changes logarithmically with respect to the position of the wiper. For example, if a logarithmic variable resistor has a range of 0 Ω to 10 kΩ, then moving the wiper halfway along the track will result in much less than 5 kΩ of resistance.
Logarithmic variable resistors are often used for volume control in sound systems because human ears perceive loudness on a logarithmic scale.
The type and value of resistance are usually marked on the body of the variable resistor. For example, a variable resistor labeled as 10K LIN has a maximum resistance of 10 kilo-ohms and a linear track.
Applications of Variable Resistors
Variable resistors have many practical applications in various fields of electronics and engineering. Some of the common uses of variable resistors are:
- Audio control: Variable resistors can be used to adjust the volume, tone, bass, and treble of sound systems, radios, headphones, speakers, etc. They are usually connected as potentiometers to vary the output voltage across the audio device. Logarithmic variable resistors are preferred for audio control because they match the human perception of loudness better than linear ones.
- Television: Variable resistors can be used to adjust the color, brightness, contrast, and position of the picture on a television screen. They are usually connected as potentiometers to vary the input voltage to the cathode ray tube (CRT) or the liquid crystal display (LCD) panel. Linear variable resistors are preferred for television because they provide a more uniform change in the picture quality than logarithmic ones.
- Motion control: Variable resistors can be used to control the speed, direction, and torque of motors, servos, fans, pumps, etc. They are usually connected as rheostats to vary the current through the load. Wire wound variable resistors are preferred for motion control because they can handle high currents and dissipate heat better than carbon or cermet ones.
- Transducers: Variable resistors can be used to convert physical quantities such as pressure, temperature, light, sound, etc., into electrical signals. They are usually connected as potentiometers to vary the output voltage across a sensor or a detector. For example, a thermistor is a variable resistor that changes its resistance with temperature, a photoresistor is a variable resistor that changes its resistance with light intensity, and a microphone is a variable resistor that changes its resistance with sound waves.
- Computation: Variable resistors can be used to perform arithmetic and logical operations in analog computers and calculators. They are usually connected as potentiometers to vary the input or output voltage across an operational amplifier or a comparator. For example, a summing amplifier can add two or more voltages by using variable resistors as weights for each input voltage, and a subtractor can subtract one voltage from another by using variable resistors to invert and scale one input voltage.
- Home Electrical Appliances: Variable resistors can be used to regulate the power consumption and performance of various home electrical appliances such as heaters, air conditioners, refrigerators, ovens, mixers, drills, etc. They are usually connected as rheostats or potentiometers to vary the current or voltage across the appliance. For example, a dimmer switch can control the brightness of a lamp by using a variable resistor to vary the current through the bulb, and a thermostat can control the temperature of a room by using a variable resistor to vary the voltage across a heating or cooling device.
These are some of the examples of how variable resistors can be used in different applications. However, there are many more possibilities and variations depending on the specific design and requirement of each circuit or device.
Variable resistors are versatile and useful components that can provide adjustable resistance in electronic circuits. They can function as rheostats or potentiometers depending on how they are connected to a circuit. They can also be classified into different types based on their material, shape, method of adjustment, range, and linearity of resistance.
Variable resistors have various applications in audio control, television, motion control, transducers, computation, home electrical appliances, and many more fields. They can control the current or voltage in a circuit by changing the length of a resistive track by moving a wiper contact along it.
Variable resistors are essential for tuning, calibrating, regulating, and modifying electronic circuits and devices according to the desired level or output.