6 Types of Variable Resistors: a Detailed Look at Their Uses

Potentiometer

Potentiometers, often referred to as "pots," enable fine control of resistance by using a dial or sliding element. The resistive element of a potentiometer can be adjusted using a wiper, controlled by turning a knob. Along with the “wiper” terminal,  potentiometers have two additional terminals, typically referred to as “input” and “output” terminals. Two common types of potentiometers are linear and rotary. Linear potentiometers find use in applications like volume control on audio devices, while rotary potentiometers are employed in settings requiring rotational adjustment, such as tuning radio frequencies. They come in various forms, including carbon film and metal oxide variants.

Rheostat

Rheostats are specialized variable resistors designed with only two terminals. While potentiometers are used to control voltage, the primary use of rheostats is to control current in a circuit, adjusting electrical resistance as needed. The two terminals are connected in series with a load (e.g., a light bulb or motor). Adjusting the position of a wiper along the resistance wire changes the resistance in series with the load, thus controlling the current. Common applications of rheostats include dimmer switches for lights and motor speed control.

Photoresistor

Photoresistors, also known as light-dependent resistors (LDRs), are two-terminal resistors that change in response to light levels. An LDR exhibits a decrease in resistance as light intensity increases, enabling it to sense and react to environmental light changes. This property makes an LDR ideal for applications like automatic lighting control and light-sensitive alarms. 

Wirewound Resistor

Wirewound resistors, constructed by winding a resistive wire around an insulating core, are known for their precision and ability to handle high power levels. Although having both fixed and variable variations, variable wirewound resistors allow for the length of the resistance wire included in the circuit to change, altering the resistance. Wirewound resistors also have two terminals. Wirewound resistors are used in applications that demand precise resistance values, such as in precision instruments and high-power electronic circuits.

Wirewound resistors come in both precision and power varieties. Precision wirewound resistors offer high accuracy and low tolerance, making them suitable for applications like voltage dividers and precision amplifiers. Power wirewound resistors are built to withstand high power levels, ensuring they can maintain their resistor value under challenging conditions, making them suitable for high-current circuits and power amplifiers.

Thermistor

Thermistors are temperature-sensitive resistors with two terminals that exhibit changes in electrical resistance with temperature fluctuations. They are classified into two primary types: negative temperature coefficient (NTC) and positive temperature coefficient (PTC). NTC thermistors decrease resistance with increasing temperature, while PTC thermistors exhibit the opposite behavior, making them essential in temperature control systems, such as thermostats, and are vital for temperature compensation in various electronic circuits.

Varistor

Varistors, also known as voltage-dependent resistors or VDRs, are specialized two-terminal variable resistors designed to protect electronic circuits from voltage spikes and surges. They exhibit a high electrical resistance under normal conditions but rapidly decrease their resistance when exposed to excessive voltage. This behavior allows varistors to shunt excessive voltage away from sensitive components by allowing high current to flow through the varistor instead. Varistors find use for surge protection in electronic systems.

Let's Sum It Up: A Comparison of Variable Resistor Types

To provide a quick reference, here's a table summarizing these six types of variable resistors:

Variable Resistance Usage for Voltage Dividers

Voltage dividers are circuits that divide an input voltage into smaller output voltages usually using resistors. Variable resistors, and especially potentiometers, are useful components in creating variable voltage divider circuits. By adjusting the resistance, you can finely control the output voltage.

Extended Applications of Variable Resistors

1. Digital Potentiometers: Digital potentiometers, or "digipots," are a modern variation of traditional variable resistors. They use digital signals to control resistance, offering precise and digitally adjustable resistance values. Digital potentiometers are commonly used in applications where remote control and digital interfaces are required, such as audio equipment and automated systems.
2. Trimpots: Trimpots, or trimmer potentiometers, are miniature variable resistors designed for fine-tuning and calibration in electronic circuits. They are often used in circuit design and manufacturing to set resistor values during production.
3. Multi-Turn Potentiometers: Multi-turn potentiometers provide increased precision by allowing multiple rotations to achieve fine resistance adjustments. These are used in applications such as in laboratory equipment and precision instruments.
4. Logarithmic Potentiometers: In addition to linear potentiometers, there are logarithmic or "log" potentiometers that provide a nonlinear resistance adjustment. These are commonly used in audio equipment to match the human perception of volume changes, as audio volume control is logarithmic in nature.
5. Conductive Plastic Potentiometers: Some potentiometers use conductive plastic tracks for resistance adjustment. These resistors offer high linearity and reliability, making them suitable for demanding applications such as in automotive sensors and industrial control systems.
6. MOSFET Variable Resistors: Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) technology allows for somewhat of a voltage-controlled resistance, where the voltage applied to the MOSFET gate terminal indirectly modulates its resistance. MOSFET variable resistors are used in electronic amplifiers, filters, and oscillators, offering voltage-controlled resistance for signal processing.
7. Cermet Variable Resistor: Cermet variable resistors, also known as cermet potentiometers or cermet trimmer resistors, are valued for their exceptional stability and durability. This potentiometer variation has a fixed, compact form factor designed to be mounted on printed circuit boards (PCBs) with the intended use of precision applications, such as audio equipment, instrumentation, and calibration circuits, where accuracy is particularly important.
8. Memristor: While not a conventional variable resistor, memristors are emerging as a new class of two-terminal non-volatile memory devices with variable resistance characteristics. Non-volatility, in this case, means that the memristors can retain their resistance state after the removal of a voltage or current signal. This value can be dynamically modified by applying voltage or current, giving the memristor the potential to revolutionize memory storage and neuromorphic computing.

Variable resistors are at the heart of electronic control and adaptability, and understanding their diverse types and applications is a vital step toward becoming proficient in electronics.