What is a Variable Resistor?

1. Circuit Symbols of Variable Resistors

Figure 1: Standard variable resistor (two-terminal) and potentiometer (three-terminal) symbols

Figure 2: Traditional variable resistor symbol (two-terminal configuration)

Figure 3: Potentiometer symbol (three-terminal configuration)

Variable resistors are represented in circuit diagrams using specific symbols:

• Standard symbol (Figure 1): A resistor with an arrow through it, indicating adjustability. The letter "R" denotes a resistor, and "W" indicates the wiper terminal.

• Traditional symbol (Figure 2): Often found in older circuit diagrams, showing a rheostat configuration where the wiper is connected to one fixed terminal.

• Potentiometer symbol (Figure 3): Shows all three terminals as independent connections, functioning as a voltage divider.

Note: Modern circuit diagrams typically use the standard symbol (Figure 1), while older diagrams may use the traditional representation (Figure 2).

2. Structure and Working Principle

Figure 4: Structure of a small-signal variable resistor

A typical small-signal variable resistor consists of:

• Resistive track: Made of carbon film, metal film, or other resistive material

• Wiper (moving contact): Slides across the resistive track

• Terminals: Two fixed terminals at track ends and one wiper terminal

• Adjustment mechanism: Usually a shaft or screw slot

Working Principle

When you rotate the adjustment shaft using a screwdriver:

1. The wiper (moving contact) slides along the resistive track

2. This changes the effective length of the resistive material between the wiper and each fixed terminal

3. As the wiper moves closer to one fixed terminal, resistance decreases between them while increasing between the wiper and the other terminal

4. The total resistance between the two fixed terminals remains constant

Example: When the wiper is at the extreme left position, resistance between the left terminal and wiper is zero, while resistance between the wiper and right terminal equals the component's full nominal resistance.

3. Classification of Variable Resistors

Variable resistors can be classified based on their construction materials and design:

3.1 Film-type Variable Resistors

Figure 5: Carbon film variable resistor

Film-type variable resistors use a synthetic carbon or metal film as the resistive element. They typically employ a rotary adjustment mechanism and are designed for small-signal applications.

Components:

• Resistor body (synthetic carbon film)

• Movable contact (metal reed or carbon contact)

• Adjustment mechanism

• Three terminals (two fixed, one moving)

Based on their enclosure design, film-type variable resistors are categorized into:

Full-sealed (Solid)

• Made from carbon black, quartz powder, and organic binders

• Sealed with metal casing

• Excellent dust protection

• Reliable contact performance

Semi-sealed

• Similar resistor body to fully-sealed type

• Uses metal reed for movable contact

• Sealed with plastic outer cover

• Easier to adjust but less dust-resistant

Non-sealed (Chip Adjustable)

• Suspension liquid coated on glass fiberboard or bakelite

• Limited dust protection

• Contacts prone to oxidation

• Higher failure rate

3.2 Wire-Wound Variable Resistors

Figure 9: Wire-wound variable resistor

Wire-wound variable resistors use resistance wire wrapped around an insulating core. They are designed for power applications and offer:

• Low noise characteristics

• High temperature resistance

• Ability to handle large currents

• Suitable for voltage/current adjustment in low-frequency circuits

Wire-wound variable resistors are further classified as:

High-power (Sliding Wire) Types

• Axial porcelain tube wire-wound

• Porcelain disc wire-wound

• Designed for high current applications

Low-power Types

• Round vertical wire-wound

• Round horizontal wire-wound

• Square wire-wound

• All feature fully sealed package structures

Application Guideline: For general-purpose applications with moderate current, metal variable resistors are recommended. For small currents, carbon film types are optimal. For high current applications, electrolytic variable resistors (with electrodes immersed in conductive liquid) offer the best performance.

4. Physical Characteristics

Figure 10: Terminal configuration of a variable resistor

Variable resistors have distinct physical characteristics that set them apart from fixed resistors:

• Size: Larger than standard resistors, making them easy to identify on circuit boards

• Terminal configuration: Three terminals (one moving and two fixed)

• Adjustment mechanism: Slot or shaft for screwdriver adjustment

• Resistance marking: Shows the nominal (maximum) resistance value

Vertical Variable Resistors

• Used in small-signal circuits

• Three pins vertically downward

• Mounted vertically on the circuit board

• Horizontal adjustment port

Horizontal Variable Resistors

• Used in small-signal circuits

• Pins at 90° angle to the resistor body

• Mounted vertically on the circuit board

• Upward-facing adjustment port

Important Note: Variable resistors typically have higher failure rates than fixed resistors due to their mechanical components and adjustment mechanisms. This should be considered when designing circuits that require long-term reliability.

5. Functions and Applications

Variable resistors serve several critical functions in electronic circuits:

Primary Functions

• Adjusting circuit current by varying resistance

• Dividing voltage (in potentiometer configuration)

• Protecting sensitive components by limiting current

• Calibrating electronic instruments

Common Applications

• Audio equipment (volume, tone controls)

• Light dimmers

• Motor speed controllers

• Signal generator characteristics adjustment

• Sensor calibration

Selection Guide

6. Potentiometers in Detail

Figure 11: Potentiometer symbols

A potentiometer is a special type of variable resistor that functions as a voltage divider. It divides the voltage applied to its fixed terminals according to the position of the wiper, providing adjustable output voltage.

The potentiometer concept can be compared to a water valve that controls flow, but for electrical signals instead of water. This makes it a crucial component in many electronic devices requiring manual adjustment.

6.1 Types of Potentiometers

Figure 12: Various types of potentiometers

Potentiometers can be categorized based on their construction materials and mechanical design:

By Material:

• Carbon film potentiometers

• Wire-wound potentiometers

• Conductive plastic potentiometers

• Cermet (ceramic-metal) potentiometers

By Design:

• Rotary potentiometers

• Linear sliding potentiometers

• Multi-turn potentiometers

• Digital/stepper potentiometers

• Motorized potentiometers

Advanced Types: With increasing demand for precision audio equipment, specialized potentiometers have been developed:

• Stepping potentiometers: Use series and parallel resistor networks to maintain synchronized resistance values in dual-track configurations, critical for stereo audio applications

• Electric potentiometers: Incorporate motors that drive the potentiometer through gears, enabling remote control while maintaining the frequency characteristics of traditional potentiometers

6.2 Using Potentiometers

Identifying Potentiometers in Circuits

When working with potentiometers, the first step is to recognize their symbol in circuit diagrams, as illustrated in Figure 13. The second critical step is understanding the relationship between the circuit symbol and the physical potentiometer, particularly the position of the center tap (wiper).

Figure 13: Potentiometer representation in circuit diagrams

Since potentiometers are variable resistors, it's essential to consider both their resistance value and power rating when incorporating them into circuits. While their application principles align with regular resistors, potentiometers are uniquely designated in circuit diagrams with the symbol "RP" (or "W" in older circuit diagrams).

Understanding Resistance Values

The resistance value marked on a potentiometer represents its total resistance. For example, in Figure 13, if the resistance between terminals A and B is 10kΩ, then the resistance values between terminals AC and BC will vary from 0 to 10kΩ as you rotate the wiper.

Pro Tip: Finding the Center Tap

New users often struggle to identify the center tap (wiper terminal) on a potentiometer. To solve this issue, use a multimeter to measure resistance between pairs of terminals while rotating the potentiometer shaft. The pair showing constant resistance are the outer terminals (A and B), while the remaining terminal is the center tap (C).

Referring to Figure 13, when the wiper (C) moves closer to terminal A, the resistance between A and C decreases while the resistance between B and C increases. Conversely, when the wiper moves toward B, the AC resistance increases while the BC resistance decreases.

Figure 14: Resistance changing curves for different potentiometer types

Types of Potentiometers by Taper

Potentiometers come in three primary taper types, each with different resistance variation patterns as shown in Figure 14:

• Exponential (Z): Resistance changes exponentially with rotation, making fine adjustments possible at one end of the range

• Logarithmic (D): Resistance follows a logarithmic curve, ideal for audio applications

• Linear (X): Resistance changes uniformly throughout the rotation range

These different taper types make potentiometers suitable for specific applications. For instance, logarithmic potentiometers are preferred for volume control in audio circuits because they match human hearing perception, while linear potentiometers are better suited for balance controls or applications requiring proportional adjustment.

Common Applications

• Volume controls (logarithmic)

• Light dimmers (linear)

• Sensor calibration

• Voltage dividers

• Motor speed controls

Selection Criteria

• Total resistance value

• Power rating

• Taper type (linear/log/exponential)

• Mechanical design (rotary/slider)

• Environmental considerations

Conclusion

This article has introduced the fundamental aspects of potentiometers, including their circuit symbols, structure, and functionality. We've explored the different taper types—exponential, logarithmic, and linear—and their specific applications. Understanding these characteristics enables you to select the appropriate potentiometer for your electronic projects, whether you're working with audio circuits, lighting controls, or precision measurement systems.

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