Understanding Variable Resistors: Functions, Applications, and Adjustments

Variable resistors serve an essential function in electronics. They provide the means to adjust current flow and resistance within circuits with precision. These components find their utility in diverse scenarios, where they cater to human needs and comforts, such as signal modulation, lighting control, and motor regulation. The choice of construction material—be it metal, carbon, or conductive liquids—is influenced by the application's specific electrical needs. Potentiometers, a special type of variable resistor, enhance their versatility by allowing the measurement and adjustment of voltage levels. This is achieved through their unique configuration of two fixed and one adjustable connector. Widely integrated into audio systems for the delicate task of tuning volume levels, their presence is felt both in the realms of home entertainment and intricate technological applications.

Catalog

Circuit Symbol of Variable Resistor

Variable Resistance Characteristics

Figure 1 illustrates the circuit symbol for a variable resistor, characterized by its simple yet meaningful design. It cleverly builds upon the ordinary resistor symbol by adding an arrow, suggesting the nuanced nature of its changing resistance. Within this symbol, one can distinguish two fixed pins alongside a moving pin, offering a glimpse into the dynamic nature of this component. This contemporary symbol is defined by the national standard, with the letters "RP" denoting the variable resistor.

Figure 1. Circuit Symbol of Variable Resistor

Traditional Symbols and Their Narratives

Many circuit diagrams, with their rich histories, present an older symbol depicted in Figure 2. This earlier version provides a clear explanation of how adjustments are made within a variable resistor and its actual integration into a circuit. In this design, the moving pin is connected to one of the fixed pins, short-circuiting a section of resistance within the body. Consequently, the resistance value reflects the measurement between the remaining fixed pin and the moving pin. Interestingly, the design in Figure 2 employs only two pins.

Figure 2. Old Circuit Symbol

Variable Resistors as Potentiometers

Moving to a different application, Figure 3 portrays the variable resistor when it functions as a potentiometer. Unlike Figure 2's configuration, this version features three independent pins. This separation highlights the versatility and adaptability inherent in using a variable resistor as a potentiometer.

Figure 3. Circuit Symbol of a Potentiometer

Exploring the Structure of a Moving-Component Resistor

Delving into the intricacies of the variable resistor unveils its operational mechanisms. Depicted in Figure 4 is the configuration of a small signal variable resistor, featuring a nuanced composition that includes: a rotating element, a carbon-based resistor body, and a trio of connecting terminals. The terminals are differentiated as two stationary connections and one that is dynamic, allowing movement. The rotating element's ability to move laterally is designed so that its contact points can traverse across the resistor surface when manipulated with a straight screwdriver inserted in the adjustment opening.

Figure 4. Anatomy of a Small-Signal Variable Resistor

Introducing a straight screwdriver into the adjustment port, gentle clockwise or counterclockwise turns of the screwdriver induce corresponding rotations in the moving element. A counterclockwise motion of the rotation element, moving the component upwards within the equivalent circuit, diminishes the span of the resistor body between stationary terminal 1 and the moving element, thereby reducing the resistance. Conversely, extending the span results in an increase in resistance.

At the extreme left position of the rotation element (uppermost vicinity), the resistance value between fixed terminal 1 and the dynamic terminal is null, while resistance between fixed terminal 2 and the dynamic terminal is maximized. This maximal resistance equates to the nominal resistance present between the two stationary terminals of the variable resistor. Sliding the rotation element to its rightmost position (lowest endpoint) nullifies the resistance between fixed terminal 2 and the moving terminal, while the resistance between the moving component and fixed terminal 1 reaches its maximum, aligning with the nominal value.

Classification of Variable Resistor

Variable resistors, marked by their adaptability, are categorized based on their material composition into film-type and wire-wound types.

Film-type Variable Resistors

Film-type variable resistors, often employed in delicate electronic symphonies like signal voltage circuits, utilize a rotary adjustment technique. Their essence lies in components such as:

- Resistor body: Typically crafted from synthetic carbon films.

- Movable contact: May be a flexible metal reed or a carbon element.

- Adjustment mechanism: Facilitates resistance modulation by adjusting with a small straight screwdriver or altering the contact positioning.

- Structural design: Includes configurations such as hermetic, semi-hermetic, and non-hermetic.

Film-type variable resistors possess a nuanced elegance, resonating with the orchestrated motion of human ingenuity.

Figure 5. Carbon Film Variable Resistor

Full-sealed Film Variable Resistor

These robust resistors, known as solid variable resistors, evoke a sense of security in their linked metals and plastics, expertly shielded against environmental perils. Crafted from a blend including carbon black, quartz powder, and organic binders, these resistors are enveloped by a protective metal casing. An access opening ensures that adjustments remain possible while maintaining exceptional dust resistance.

Figure 6. Quartz Powder

Semi-sealed Film Variable Resistor

Balancing ease and exposure, the semi-sealed film resistor echoes a dance with its surroundings. With a manufacturing lineage akin to its full-sealed counterpart, this variant’s metal reed and enclosing housing allow controlled adjustments while acknowledging the presence of elements dust may bring.

Non-sealed Film Variable Resistor

Reflecting vulnerability akin to raw emotional exposure, the non-sealed resistor, or chip adjustable resistor, wears its weaknesses openly. Constructed from a liquid suspension bound to glass fiberboard or bakelite, and lacking protective measures, it faces the risks of oxidation and degradation with a dignified resolve.

Figure 7. Glass Fiberboard

Figure 8. Bakelite

Wire Wound Variable Resistor

With attributes like low noise and thermal resilience, wire-wound variable resistors stand as bastions of power control within various circuits:

- High-Power Varistor Types: Subcategorized as axial porcelain tube and porcelain disc designs, these powerful entities are fondly called sliding wire resistors due to their capacity to slide seamlessly amidst electrical flows.

- Low-Power Varistor Types: Including round vertical, round horizontal, and square configurations, these resilient forms exist within compact, fully sealed structures.

Delving into further structural diversity, variable resistors can also be distinguished based upon orientation into vertical and horizontal categories, each serving unique scenarios in tech landscapes.

Figure 9. Wire Wound Variable Resistor

Characteristics and Structure of Variable Resistors

The physical attributes of a variable resistor distinguish it noticeably from typical resistors, offering various features that hint at its presence on a circuit board.

Size and Quantity

In terms of size, a variable resistor is larger than a general resistor. On a circuit board, variable resistors appear less frequently than their general counterparts, making them more conspicuous during inspection or repair.

Pin Configuration

A variable resistor is equipped with three distinct pins: a moving pin and two fixed pins. The fixed pins can usually be interchanged without affecting their function. However, the moving pin and the fixed ones are not interchangeable, emphasizing the designed roles within the resistor.

Figure 10. Pins in Variable Resistor

Adjustment Mechanism

An adjustment port is featured on the variable resistor, into which a straight screwdriver can be inserted. By rotating the screwdriver, one alters the position of the moving plate, thereby adjusting the resistance level, mirroring a subtle dance of precision and control.

Nominal Resistance Value

The nominal resistance value marked on a variable resistor indicates the resistance between the two fixed pins. This value also represents the maximum resistance between a fixed pin and the moving pin.

Vertical Variable Resistors

Primarily utilized in small-signal circuits, the vertical variable resistor boasts three pins oriented downward, with all components mounted vertically onto the circuit board. The resistance adjustment port is horizontally oriented.

Horizontal Variable Resistors

Similarly applied in small-signal circuits, horizontal variable resistors possess three pins aligned at 90 degrees to the resistor body, mounted vertically on the circuit board. Their resistance adjustment port points upward, allowing effortless access.

Compact Plastic-Case Variable Resistors

These smaller, circular variable resistors feature downward-facing pins, with the resistance adjustment port pointing upward, offering a compact solution for various applications.

Large Power Wire-Wound Variable Resistors

Designed for high-power usage, these variable resistors are notably large in size. The position of the moving plate can be adjusted left or right, showcasing a broad capability for resistance modification, suitable for robust operations.

Indispensable Functions of Variable Resistors

The essence of a variable resistor lies in its fundamental ability to change resistance within a specified range, unlike a fixed resistor which maintains a constant resistance level. In situations where alterations in resistance are needed without frequent adjustments, the variable resistor becomes the component of choice.

Composition and Application

A variable resistor consists of a resistor body and a sliding mechanism, allowing it to be adjusted. By modifying its resistance, it regulates the current flowing through a series circuit. This capability is particularly beneficial for safeguarding components sensitive to current fluctuations. It's often employed in circuits where steady resistance values are preferred, reducing the need for frequent recalibration. Meanwhile, they find their niche in small-signal circuits, providing nuanced control. In contrast, larger signal variable resistors find limited use, such as in tube amplifier designs.

Materials and Suitability

The material choice for a variable resistor significantly depends on its intended application. Options include metal wire, metal sheet, carbon film, or conductive liquid. For average current levels, metal-based resistors are prevalent, while carbon film types are apt for low-current scenarios. For handling higher currents, the electrolytic variant proves most effective. It's noteworthy that the intricate structure and operational dynamics of variable resistors contribute to a relatively higher failure rate compared to their ordinary counterparts.

Potentiometer—A Unique Variable Resistor

In the world of electronics, there exists a component that allows for manual adjustment, driving innovation and creativity in circuit design. This is the potentiometer. It operates by dividing the voltage across its two fixed terminals to yield the desired electrical signal strength, akin to how a valve manages water flow in a piping system. Through this function, it becomes integral to electronic circuits, influencing their performance with precision.

Potentiometers manifest in several forms, with the predominant types being wire-wound, non-wire-wound, and electronic potentiometers often utilized in audio circuits. Regardless of their construction, their core operational principle remains unchanged, illustrated by their symbols in Figure 11.

Figure 11. Potentiometer Symbol

Types of Commonly-used Potentiometers

As electronic technology evolves, potentiometers have diversified into a "big family" with a plethora of models and series. When categorized by materials, they include carbon film, wire-wound, and multi-turn potentiometers. Usage further divides them into rotary, solid core, fine-tuning, linear sliding, electric and stepper potentiometers, the latter two developed alongside Hi-Fi technology. You can observe visual representations in Figure 12.

Figure 12. Types of Potentiometer

The progression of science and technology has heightened expectations for electronic components. This extends to potentiometers, whose manufacturing precision has improved over time. With the growing enthusiasm for audio equipment, people's aspirations for superior potentiometers have intensified. To align dual-track potentiometers' resistance values, stepping potentiometers were innovated. Through series and parallel resistor connections, optimal dual-channel resistance synchronization is achieved.

Moreover, advancements in remote control technology have given rise to electric potentiometers, a distinctive category crafted specifically for this technology. By utilizing a motor's forward and reverse rotations to manipulate gears, these potentiometers preserve the inherent frequency characteristics and offer convenient control, making them a promising addition to electronic components.

Use of Potentiometers

When employing a potentiometer, recognition of its circuit symbol, depicted in Figure 13, is crucial. Additionally, understanding the connection between the schematic symbol and the actual device, notably the center tap's position, is essential. As potentiometers are a variant of resistors, attention should be paid to their resistance and power values within the circuit. Their application aligns with resistor principles, but circuit diagrams denote them as “RP” (or “w” in older diagrams).

Figure 13. Potentiometer in the Circuit

On the potentiometer, the resistance displayed is its total value. For instance, if the resistance between terminals A and B is 10 K, rotating the arm changes resistance between AC and BC from 0 to 10 K ohms. Beginners often struggle to locate the center tap initially. Resolving this involves comprehension of the relationship between AC, BC resistance, and the rotary axis position.

Referring to Figure 13, as point C moves toward A, AC resistance decreases while BC increases, and vice versa. To correctly identify terminals during usage, measure both ends of the potentiometer with a multimeter while rotating the shaft; the points with unchanging resistance are AB, leaving the remaining as the center tap.

Figure 14. Resistance Changing Curve

During operation, potentiometer resistance alters in three forms: exponential (Z), logarithmic (D), and linear (X), as illustrated in Figure 14. The variations in resistance influence their application; exponential types suit volume control in audio circuits, while linear types serve well in balanced potentiometer settings.