Variable Resistor Types: Potentiometer, Rheostat & Trimmer Complete Guide

Every circuit I’ve designed over the past two decades has included at least one variable resistor in some form. Whether it’s a tiny trimmer for calibrating a sensor offset, a panel-mount potentiometer for user volume control, or a wirewound rheostat handling motor current, these adjustable components remain indispensable in modern electronics. Understanding when to use each type separates functional prototypes from production-ready designs.

This guide breaks down the three main categories of variable resistors that PCB engineers encounter daily. I’ll cover their construction differences, application sweet spots, and the selection criteria that matter in real-world circuit design.

What Is a Variable Resistor?

A variable resistor is an electromechanical component that allows you to adjust its resistance value through physical movement of a wiper across a resistive element. Unlike fixed resistors with predetermined values, variable resistors provide continuous or stepped resistance adjustment, enabling dynamic control over current flow and voltage division within circuits.

The fundamental construction includes three key elements: a resistive track (made from carbon, cermet, wirewound, or conductive plastic), a wiper or sliding contact that moves along this track, and terminals for electrical connection. How you wire and use these terminals determines whether the component functions as a potentiometer (voltage divider) or rheostat (current controller).

Component	Terminals Used	Primary Function	Typical Power Rating
Potentiometer	3 terminals	Voltage division	0.1W – 2W
Rheostat	2 terminals	Current control	5W – 1000W
Trimmer	3 terminals	Calibration/adjustment	0.1W – 0.5W

Variable resistors appear in virtually every electronic domain, from consumer audio equipment to industrial motor controllers. The trick is matching the right type to your specific application requirements.

Understanding the Potentiometer

The potentiometer, commonly called a “pot,” represents the most widely used variable resistor type in electronics. It’s a three-terminal device designed primarily for voltage division, where two fixed terminals connect to the ends of the resistive track and the third terminal connects to a movable wiper.

How Potentiometers Work

When voltage is applied across the two end terminals, the wiper position determines the output voltage according to the voltage divider principle:

Vout = Vin × (R2 / Rtotal)

Where R2 represents the resistance between the wiper and the ground terminal. Moving the wiper changes this ratio, providing continuously variable output voltage from zero to the full input voltage.

The key characteristic of potentiometer operation is that minimal current flows through the wiper circuit. The wiper samples the voltage at a point along the resistive track rather than carrying the load current. This distinction is crucial because potentiometer wipers aren’t designed for significant current and will wear rapidly or overheat if misused.

Types of Potentiometers

Potentiometers come in several physical configurations optimized for different mounting and adjustment scenarios.

Rotary Potentiometers feature a rotating shaft that moves the wiper in an arc across the resistive element. These dominate panel-mount applications where users need frequent adjustment. Most provide approximately 270° of rotation from minimum to maximum resistance.

Slide Potentiometers use linear motion rather than rotation. Audio mixing consoles favor these because the slider position provides immediate visual feedback of the setting. The resistive element runs in a straight line with the wiper attached to the slider mechanism.

Multi-turn Potentiometers incorporate gear mechanisms that require multiple shaft rotations (typically 3, 5, or 10 turns) to traverse the full resistance range. This mechanical advantage provides much finer adjustment resolution, making them ideal for precision calibration where single-turn pots would be too coarse.

Multi-gang Potentiometers stack multiple resistive elements on a single shaft, allowing simultaneous adjustment of multiple circuit parameters. Stereo volume controls use dual-gang pots to adjust both channels equally.

Potentiometer Taper Types

The relationship between shaft rotation and resistance change isn’t always linear. Manufacturers offer different “taper” profiles:

Taper Type	Code	Resistance Curve	Common Application
Linear	B	Straight line	General purpose, sensors
Logarithmic	A	Logarithmic curve	Audio volume control
Anti-log	C	Reverse logarithmic	Balance controls

Linear taper pots change resistance proportionally with rotation—50% rotation equals 50% resistance. Audio applications require logarithmic taper because human hearing perceives loudness logarithmically. A linear pot would sound like most of the volume change happens in the first 20% of rotation.

The Rheostat Explained

A rheostat is a variable resistor specifically designed for current control in series with a load. While potentiometers divide voltage using three terminals, a rheostat uses only two terminals and must carry the full load current through its resistive element and wiper contact.

Rheostat Construction and Operation

The term “rheostat” comes from Greek words meaning “current controller,” which perfectly describes its function. When connected in series with a load, increasing the rheostat resistance reduces current flow according to Ohm’s Law:

I = V / (Rload + Rrheostat)

Traditional rheostats feature wirewound construction where nichrome or constantan resistance wire winds around a ceramic or porcelain core. This construction handles the significant currents and resulting heat that potentiometer-style carbon tracks cannot survive. The wiper slides over the wire windings, and the contact must be robust enough to carry the full circuit current.

Power ratings for wirewound rheostats range from 25W to over 1000W for industrial units, dwarfing the typical 0.25W rating of panel-mount potentiometers.

Types of Rheostats

Rotary Rheostats are the most common type, featuring a toroidal (donut-shaped) wirewound element with a rotating wiper arm. Most use open construction to facilitate heat dissipation, though enclosed versions exist for harsh environments.

Slide Rheostats (also called tubular rheostats) wind resistance wire around a cylindrical ceramic form with a sliding contact bar. These frequently appear in educational laboratories and testing setups where visibility and accessibility matter more than compactness.

Tapped Rheostats include fixed connection points at intervals along the resistance element, providing coarse stepped adjustment in addition to fine continuous control.

Rheostat Type	Construction	Power Range	Typical Use
Rotary Wirewound	Toroidal coil	25W – 500W	Motor control, heating
Slide/Tubular	Cylindrical coil	10W – 300W	Laboratory, education
Ceramic Disk	Flat disk element	50W – 1000W	Industrial power control

Modern Rheostat Applications

While solid-state devices like SCRs, triacs, and PWM controllers have replaced rheostats in many power control applications, wirewound rheostats remain relevant for:

• DC motor speed control in legacy equipment and hobby applications
• Light dimming for incandescent and resistive loads
• Laboratory power supplies where analog adjustment is preferred
• Load simulation for testing power supplies and batteries
• Heating element control in industrial ovens and furnaces
• Medical equipment including X-ray machines and respirators

The inefficiency of rheostat control (wasted power as heat) makes it unsuitable for battery-powered or energy-conscious designs. Modern practice uses potentiometers to set reference voltages for PWM controllers rather than placing variable resistors directly in the power path.

Trimmer Potentiometers (Trimpots)

Trimmer potentiometers, universally called “trimpots” or “presets,” are miniature variable resistors designed for occasional adjustment during manufacturing calibration or field service. Unlike panel-mount potentiometers intended for frequent user interaction, trimmers mount directly on PCBs and require a screwdriver for adjustment.

Trimmer Construction and Characteristics

Trimpots share the basic three-terminal potentiometer structure but optimize for different priorities. Their small size enables PCB mounting, while sealed or semi-sealed housings protect against contamination. The adjustment mechanism typically uses a small screwdriver slot rather than a knob.

Resistive Element Materials:

Material	Cost	Stability	Resolution	Life Cycles
Carbon Composition	Low	Moderate	Moderate	200-500
Cermet	Medium	Excellent	High	200-500
Wirewound	Higher	Excellent	Stepped	1000+
Conductive Plastic	Higher	Good	Excellent	1000+

Cermet (ceramic-metal) trimmers dominate professional applications due to their excellent temperature stability and resistance to humidity. Carbon composition types cost less but exhibit more drift over temperature and time.

Single-Turn vs Multi-Turn Trimmers

Single-turn trimpots provide the full resistance range within approximately 270° of rotation. They’re adequate for adjustments requiring only moderate precision, such as setting LED brightness or basic threshold levels. Their compact size and low cost make them the default choice for non-critical calibration.

Multi-turn trimpots use lead-screw or worm-gear mechanisms requiring 5, 10, 12, or 25 full rotations to traverse the complete resistance range. This mechanical reduction provides dramatically finer adjustment resolution—essential for precision applications like instrumentation amplifier offset trimming or reference voltage calibration.

Trimmer Type	Turns	Resolution	Best For
Single-turn	~1	Moderate	Coarse adjustment, non-critical
5-turn	5	Good	General precision
10-turn	10	High	Instrumentation
25-turn	25	Very High	Precision calibration

Trimmer Mounting Styles

Modern trimmers offer various mounting configurations:

• Through-hole (THT): Traditional leads for manual assembly
• Surface-mount (SMD): Gull-wing or J-lead packages for automated placement
• Top-adjust: Screwdriver access from component side
• Side-adjust: Access from board edge for stacked boards or enclosures
• Sealed: Protected against flux, solvents, and environmental contamination

Important Trimmer Limitations

Trimpots have finite adjustment life, typically rated for only 200-500 cycles. Using a trimmer where frequent adjustment is expected will cause premature wear, resulting in intermittent contact, noise, and eventual failure. If users need regular adjustment, specify a panel-mount potentiometer instead.

Additionally, trimmers can shift slightly after soldering due to thermal stress. For critical applications, allow the board to stabilize before final calibration, or specify high-temperature rated components.

Selecting the Right Variable Resistor

Choosing between potentiometers, rheostats, and trimmers requires analyzing your application’s specific requirements.

Key Selection Criteria

Function: Are you dividing voltage (potentiometer/trimmer) or controlling current in series with a load (rheostat)?

Power Dissipation: Calculate the maximum power the variable resistor must handle. Standard potentiometers rarely exceed 0.5W. If your application requires more, consider a wirewound rheostat or redesign using a potentiometer to control a power stage.

Adjustment Frequency: User-operated controls need panel-mount potentiometers. One-time factory calibration suits trimmers. Frequent high-current adjustment demands industrial rheostats.

Resolution Requirements: Coarse adjustment tolerates single-turn devices. Precision calibration requires multi-turn components.

Environmental Factors: Sealed trimmers and enclosed rheostats protect against contamination in harsh environments.

Variable Resistor Selection Quick Reference

Application	Recommended Type	Typical Value
Audio volume control	Logarithmic pot (A taper)	10kΩ – 100kΩ
Microcontroller ADC input	Linear pot (B taper)	10kΩ
Op-amp offset trim	Multi-turn cermet trimmer	10kΩ – 100kΩ
Motor speed (low power)	Wirewound rheostat	10Ω – 100Ω
Reference voltage set	10-turn trimmer	5kΩ – 50kΩ
LED brightness	Trimmer or pot to PWM driver	10kΩ
Sensor calibration	Cermet trimmer	Application dependent

Wiring Variable Resistors Correctly

Proper wiring ensures reliable operation and prevents component damage.

Potentiometer as Voltage Divider

Connect Terminal 1 to ground, Terminal 3 to the input voltage, and take the output from Terminal 2 (wiper). Rotation will vary output from 0V to full input voltage.

Potentiometer Wired as Rheostat

Connect Terminal 2 (wiper) to Terminal 1 or Terminal 3, then use the wiper and opposite end terminal as a two-terminal variable resistor. This configuration works for low-current applications but doesn’t provide the power handling of a true wirewound rheostat.

Important: When wiring a potentiometer as a rheostat, always connect the unused terminal to the wiper. This prevents open-circuit conditions if the wiper momentarily loses contact with the track.

Rheostat Connection

Connect one end terminal and the wiper terminal in series with your load. The unused end terminal can remain unconnected or tied to the wiper for safety.

Useful Resources for Variable Resistor Selection

Online Calculators and Tools

• DigiKey Potentiometer Parametric Search (digikey.com)
• Mouser Variable Resistor Selection Guide (mouser.com)
• Bourns Trimmer Selector Tool (bourns.com)
• Vishay Potentiometer Cross-Reference (vishay.com)

Manufacturer Technical Resources

• Bourns Application Notes for Trimmers and Potentiometers
• Vishay Spectrol Precision Potentiometer Guides
• Ohmite Rheostat Selection and Application Notes
• TT Electronics/BI Technologies Trimmer Datasheets

Component Distributors

• DigiKey Electronics (digikey.com)
• Mouser Electronics (mouser.com)
• Newark (newark.com)
• RS Components (rs-online.com)
• Arrow Electronics (arrow.com)

Reference Standards

• IEC 60393-1: Potentiometers for use in electronic equipment
• BS EN 141101: Lead-screw actuated and rotary preset potentiometers
• MIL-PRF-94: Variable resistors for military applications

Frequently Asked Questions About Variable Resistors

What’s the main difference between a potentiometer and a rheostat?

A potentiometer is a three-terminal device used for voltage division, where the wiper samples voltage along the resistive track without carrying significant current. A rheostat uses only two terminals and connects in series with the load, carrying full circuit current through its element and wiper. While you can wire a potentiometer as a rheostat for low-current applications, true rheostats feature robust wirewound construction designed for power handling that carbon or cermet tracks cannot survive.

Can I use a potentiometer for motor speed control?

For very small motors drawing under 100mA, a potentiometer wired as a variable resistor can work. However, this is inefficient and wastes power as heat. Modern practice uses the potentiometer as a control input to a PWM motor driver, which provides efficient speed control without burning energy in resistive losses. For larger motors, this approach is essential to avoid overheating the potentiometer.

Why does my trimmer drift after calibration?

Trimmer drift commonly results from several factors: mechanical shock or vibration loosening the wiper position, temperature cycling causing the element to expand and contract, or moisture absorption in non-sealed components. To minimize drift, use sealed cermet trimmers, avoid mounting near heat sources, and consider locking compound on the adjustment screw after calibration. For critical applications, specify components rated for your environmental conditions.

How do I choose between linear and logarithmic taper potentiometers?

Select linear (B) taper for applications where resistance should change proportionally with rotation, such as voltage references, sensor inputs, and general adjustment controls. Choose logarithmic (A) taper for audio volume controls because human hearing perceives loudness logarithmically, making log taper provide more natural-feeling volume progression. Anti-log (C) taper suits balance controls and specialized applications.

What causes scratchy or intermittent operation in variable resistors?

Scratchy operation typically indicates contamination (dust, dirt, oxidation) on the resistive track or wiper contact, or physical wear of the track surface. Sealed components resist contamination better than open types. For existing components, contact cleaner spray (DeoxIT or similar) can temporarily restore smooth operation. If the track is physically worn, replacement is the only permanent solution. Choosing higher-quality components with better wiper materials and sealed housings prevents these issues in new designs.

Final Thoughts on Variable Resistor Selection

After decades of specifying variable resistors, my core advice is this: match the component to the actual operating conditions, not just the schematic requirements. A trimmer that works perfectly on the bench may fail in the field if it’s exposed to temperature extremes or vibration the designer didn’t anticipate.

For new designs, I default to using potentiometers and trimmers as control inputs to active circuits rather than placing them directly in power paths. This approach leverages their precision while protecting them from the current and heat that cause premature failure. When true rheostats are necessary, spend the money on properly rated wirewound components rather than hoping small potentiometers will survive.

The variable resistor remains one of the most intuitive human-machine interface components available. A physical knob or slider provides immediate tactile feedback that touchscreens and digital interfaces struggle to match. Understanding these humble components and their proper application will serve you well throughout your engineering career.