A resistor is a two-terminal component designed to impede the flow of electric current and dissipate power as it traverses through the components in a circuit. In an electrical circuit, the resistor serves as a passive component. Figure 1 shows the symbol of the resistor.
Figure 1: Symbol of Resistor
The ratio between voltage and current, defined as resistance, is quantified by Ohm’s Law. Ohm’s Law asserts that the current passing through a resistor within an electric circuit is directly proportional to the applied voltage, provided the temperature remains constant. Therefore, it is logical to assume that normal resistors have two terminals and they have constant resistance since their resistance cannot be changed.
V=IR
What is a Variable Resistor?
A variable resistor possesses an adjustable electrical resistance value, making it a crucial component in electronic circuits for adjusting current or voltage, according to Ohm’s Law. Typically, a variable resistor comprises three parts: two fixed ends on the resistive track and a movable part that facilitates changing the resistance.
Variable Resistor Symbol
The variable resistor symbol contains an arrow indicating the resistance variation.
Figure 2: Variable Resistor Symbol
Figure 3: Variable Resistor
How Variable Resistors Work
A variable resistor is characterized as a resistor with an adjustable electrical resistance value. This electronic component is frequently employed in electronic circuits to enable the adjustment of current or voltage by Ohm’s Law. A variable resistor usually has three parts: two fixed ends on the resistive track and a movable part that can be adjusted to change the resistance. The variable resistor symbol contains an arrow indicating the resistance variation.
Figure 4: Operation of Variable Resistor
RTotal=R1+R2
Output Voltage = (R1/R1+R2)x Input Voltage
Construction
When inspecting the internal structure of a variable resistor, as depicted in Figure 5, we observe a resistive track, known as static resistance, positioned between terminals 1 and 3. The knob links to Terminal 2, and the slider (wiper) maintains direct contact with the knob. Manipulating the knob positioned at the midpoint allows for the modification of resistance between terminals 1 and 2 or 2 and 3.
Figure 5: Variable Resistor
Types of Variable Resistors
Variable resistors permit adjustments in terminal configuration and resistance values based on diverse environmental parameters, which are crucial in various electronic applications.
Potentiometers
Potentiometers represent one of the most widely utilized categories of variable resistors. These devices manage voltage across a load by dividing the input voltage. All three terminals of a potentiometer are utilized; the two fixed terminals are connected to the input voltage source, and the output voltage is derived from one fixed terminal and the wiper. The output voltage is proportional to the resistance ratio on either side of the wiper. Potentiometers find preference in applications requiring precise voltage control. They are categorized into two main groups: mechanical and digital. Mechanical potentiometers, such as linear and rotary types, may encounter accuracy issues in vibrational conditions. Digital potentiometers are commonly favored due to the sensitivity challenges of their mechanical counterparts. A fundamental application of digital potentiometers is to address resistance drift problems arising in challenging environmental conditions.
Rheostats
A rheostat serves as a device that regulates the current within a circuit by adjusting the resistance. Operating with two terminals, namely one fixed terminal and the wiper, the resistance between these points fluctuates as the wiper traverses the track. As illustrated in Figure 6, the rheostat short-circuits the moving terminal with one of the side terminals. Situations requiring precise control over resistance or limiting current flow commonly use rheostats.
Figure 6: Symbol of Rheostat
Force Sensitive Resistor
A force-sensitive resistor undergoes a change in resistance when subjected to force, pressure, or mechanical stress. Alternatively termed force sensing resistors and occasionally abbreviated as FSR, these devices are made of material.
Thermistors
A thermistor functions as a resistance thermometer, representing a resistor whose resistance varies with temperature. The term is a fusion of ‘thermal’ and ‘resistor.’ Constructed from metallic oxides, it takes the form of a bead, disk, or cylinder encapsulated in an impermeable material like epoxy or glass. Thermistors might have a Negative Temperature Coefficient (NTC) or Positive Temperature Coefficient (PTC). In the case of an NTC thermistor, resistance decreases as temperature rises, and conversely, it increases as temperature drops. This type finds broader applications. On the other hand, a PTC thermistor operates differently. Its resistance increases with rising temperature and decreases as temperature decreases. PTC thermistors serve as fuses in many applications.
Humistors
A humistor is a variable resistor that undergoes changes in resistance corresponding to variations in the surrounding air’s humidity levels. Humidity refers to the quantity of gaseous water or water vapor present in the air.
Typically composed of materials such as organic polymers like polyamide resin, polyethylene, or metal oxides, humistors are alternatively known as humidity-sensitive resistors or resistive humidity sensors.
Magneto Resistor
A magneto resistor also referred to as a magnetic-dependent resistor (MDR), possesses the unique characteristic of altering its electrical resistance when subjected to an external magnetic field. This property makes magneto resistors valuable in various sensing applications, where they can measure the presence, strength, and direction of magnetic fields. Belonging to the broader category of magnetic field sensors or magnetometers, magneto resistors find widespread use in fields such as electronic compasses, magnetometry for determining magnetic field intensity and direction, and various types of position sensors, including angle, rotary, and linear position sensors. Magneto resistors play a crucial role in ferrous metal detection and vehicle/traffic detection systems. Leveraging the sensitivity of magneto resistors to changes in magnetic fields, these devices play an important role in enabling precise measurement and detection of magnetic phenomena.
Photoresistor
Photoresistors, commonly referred to as light-dependent resistors (LDR), are electronic components sensitive to light. Widely employed for detecting the presence or absence of light, as well as for measuring light intensity, LDRs exhibit high resistance in darkness, often reaching values as high as 1 MΩ. However, when subjected to light, their resistance decreases dramatically, sometimes plummeting to just a few ohms, contingent upon the intensity of illumination. LDRs demonstrate sensitivity that varies with the wavelength of the light, and they exhibit non-linear behavior. LDRs find applications across diverse fields, including light-sensing tasks.
Variable Resistor vs Potentiometer
A variable resistor is a two-terminal component that allows you to adjust its resistance by rotating or sliding a knob or wire along its axis. On the other hand, a potentiometer is a three-terminal component. It can function as a variable resistor but additionally allows you to obtain a variable voltage output by rotating its axis manually (in panel-mounted pots) or with a screwdriver (in trimmers, commonly used on PCBs for occasional adjustments).
Practical Applications of Variable Resistors
Motion Control
Variable resistors play a crucial role in regulating the speed, direction, and torque of various devices such as motors, servos, fans, and pumps. Employed as rheostats, they enable the adjustment of current flowing through the load. Motion control applications often prefer wire-wound variable resistors due to their ability to handle high currents and superior heat dissipation, surpassing the capabilities of carbon or cermet counterparts.
Audio Control Applications
Variable resistors are important for adjusting volume, tone, bass, and treble in devices like sound systems, radios, and headphones. They work as potentiometers, changing the output voltage. Audio control commonly uses logarithmic variable resistors because they better match how humans perceive loudness compared to linear ones.
Television
In adjusting color, brightness, contrast, and picture position on a TV screen, the variable resistors play their role. Connected as potentiometers, they modify the input voltage to the cathode ray tube (CRT) or liquid crystal display (LCD) panel. Manufacturers commonly choose linear variable resistors for television applications as they provide a more consistent picture quality alteration than logarithmic ones.
Home Electrical Appliances
Variable resistors find applications in controlling power consumption and performance of household devices like heaters, air conditioners, refrigerators, ovens, mixers, drills, etc. Connected as rheostats or potentiometers, they adjust the current or voltage flowing through the appliances. For instance, a dimmer switch uses a variable resistor to control the brightness of a lamp by varying the current through the bulb. Similarly, a thermostat uses a variable resistor to regulate the temperature of a room by adjusting the voltage across a heating or cooling device.
Light Dimming
Rheostats adjust the brightness of lights in various settings, including homes, theaters, and automotive interiors.
Conclusion
Variable resistors are vital in electronics, playing key roles in motion control, audio adjustment, TV settings, home appliances, and lighting. Understanding their construction, types, and applications highlights their versatile contributions to modern electronic systems. The adaptability of variable resistors positions them as crucial elements for achieving precision and control in various domains.
Fatima Razzaq is a freelance technical writer who served as an electrical engineering lecturer at Air University—a federally chartered public sector research university in Pakistan. Razzaq holds a Bachelor’s degree with distinction in electronic engineering from Ghulam Ishaq Khan Institute of Engineering Sciences and Technology (GIKI) and a Master’s degree in Sustainable Transportation and Electrical Power Systems from the University of Nottingham, Universidad de Oviedo, and La Sapienza University of Rome. Razzaq’s diverse work experiences in academia and industry continue to inform her prolific technical writing journey in the areas of electrical engineering, storage mechanisms, power electronics, electric vehicles, energy, and related topics.
