Linear Potentiometer: The Engineering Guide to Working, Types & Applications

In the hierarchy of tactile interfaces and displacement sensors, the linear potentiometer—often colloquially called a sliding potentiometer or “fader”—occupies a unique space. While rotary pots dominate the volume knobs of the world, the linear variant is the go-to solution for precise displacement measurement and multi-channel control consoles.

From a hardware engineering perspective, a linear potentiometer isn’t just a slider; it is a precision-engineered electromechanical transducer. It converts mechanical travel into a proportional electrical signal. Whether you are designing a high-end mixing desk or an industrial actuator position sensor, understanding the parasitic capacitance, track linearity, and mechanical wear cycles of these components is vital.

What is a Linear Potentiometer?

A linear potentiometer is a variable resistor that operates via a sliding motion rather than a rotational one. Internally, the resistive element is laid out in a straight track. A wiper moves along this track, making electrical contact.

The Physics of the Slide

The fundamental equation governing this component is the voltage divider rule. If you apply a voltage $V_{in}$ across the two ends of the resistive track, the output voltage $V_{out}$ at the wiper is:

$$V_{out} = V_{in} \cdot \frac{R_{wiper}}{R_{total}}$$

In a high-quality sliding potentiometer, the change in resistance is directly proportional to the physical distance traveled. For an engineer, this linearity is the “North Star.” If the slider is at 50% of its physical travel, we expect exactly 50% of the input voltage, assuming a linear taper.

How a Linear Potentiometer Works

The “working” of a potentiometer involves three primary sub-assemblies: the resistive element, the wiper, and the housing/actuator.

1. The Resistive Element (The Track)

Modern industrial linear pots use either conductive plastic or cermet (ceramic-metal). Conductive plastic is favored in high-cycle applications (like audio faders) because it offers a smooth “feel” and can survive millions of strokes. Wire-wound tracks are rarer now but are still used for high-power applications where heat dissipation is a concern.

2. The Wiper Assembly

The wiper is usually a multi-fingered metal contact. Engineers prefer multi-fingered designs because they provide “redundancy” at the contact point. If a speck of dust gets on the track, the other “fingers” maintain the electrical connection, preventing signal dropout.

3. The Actuator (The Slider)

This is the mechanical interface. In a sliding potentiometer, the actuator must be designed to minimize “stiction”—the static friction that makes it hard to start the movement. High-end faders use dual-rail guides to ensure the movement is perfectly parallel to the track, preventing “canting” or jamming.

Types of Linear Potentiometers

Depending on your application—be it consumer electronics or aerospace—the construction varies wildly.

1. Slide Potentiometers (Faders)

Commonly found in audio equipment. These are designed for human interaction. They emphasize “feel” and travel length (typically 60mm or 100mm).

2. Softpot (Membrane Potentiometers)

A “softpot” is a thin, flexible linear potentiometer. It works by pressing a top layer onto a resistive bottom layer. These are excellent for medical devices or sleek consumer interfaces where a mechanical slider is too bulky.

3. Industrial Linear Displacement Transducers (LPDTs)

These look like hydraulic cylinders. The “slider” is a rod that extends or retracts. These are used to measure the position of heavy machinery or suspension travel in automotive testing.

4. Motorized Faders

A hybrid of a sliding potentiometer and a DC motor. These allow for “Total Recall” in digital mixing consoles. The computer can move the physical slider to a saved position.

Comparison of Linear Potentiometer Types

Type	Accuracy	Durability (Cycles)	Common Application
Slide Pot (Carbon)	Moderate	10k – 50k	Consumer Audio
Conductive Plastic	High	1M – 10M	Professional Audio / Industrial
Membrane (Softpot)	Low	100k+	Medical Interfaces
LPDT (Industrial)	Ultra-High	50M+	Robotics / CNC Machines

Critical Selection Criteria: An Engineer’s Checklist

When selecting a linear potentiometer for a PCB layout, you need to look beyond the resistance value.

Linearity (Independent vs. Absolute)

Standard pots have a linearity of $\pm 2\%$. For precision measurement, you may need $\pm 0.1\%$. If your software can calibrate the “zero” and “span” points, you can often save money by using a component with lower absolute accuracy but high repeatability.

Mechanical Travel vs. Electrical Travel

This is a trap for new engineers. The “Mechanical Travel” is how far the slider can move. The “Electrical Travel” is the portion of that movement where the resistance actually changes. There is usually a 1–2mm “dead zone” at each end of the track.

Resistance Taper

Linear Taper (B-type): The standard for displacement sensing.

Audio Taper (A-type): Logarithmic. Rarely used in linear sliders unless it’s a specific audio gain control.

Environmental Sealing

If your sliding potentiometer is going into a CNC machine, it needs an IP65 or IP67 rating. Conductive plastic tracks are sensitive to oil and metal shavings.

Applications of Linear Potentiometers

1. Audio and Broadcast Consoles

The most visible use of the sliding potentiometer. Engineers use them because they provide a visual representation of “levels” across multiple channels. You can look at a 48-channel desk and instantly see the “curve” of the mix.

2. Robotics and Actuator Feedback

In a closed-loop system, a linear pot is often attached to the arm of an actuator. The microcontroller reads the voltage to determine the exact position of the arm, allowing for sub-millimeter precision.

3. Automotive Suspension Sensing

Performance vehicles use LPDTs to monitor suspension travel in real-time. This data helps engineers tune dampening rates for better cornering.

4. Medical Syringe Pumps

Linear pots measure the travel of the syringe plunger to ensure that medication is being delivered at a precise, controlled rate.

Troubleshooting Sliding Potentiometers

As a PCB engineer, you’ll eventually have to debug a “noisy” slider. Here is the professional approach:

The “Crackle” (Contact Noise)

Usually caused by oxidation or debris on the track. In a lab setting, we use a specialized contact cleaner (like DeoxIT). Avoid using generic WD-40, as it leaves a residue that attracts more dust.

Linearity Drift

If a sensor was accurate a year ago but is now “off,” the resistive track has likely worn down, changing the local resistance ($R$ per mm). This is common in high-use areas of the track. The only solution is replacement.

Jitter in ADC Readings

If your microcontroller shows fluctuating values when the slider is still, it’s usually electrical noise.

The Fix: Add a $0.1\mu F$ decoupling capacitor as close to the wiper pin as possible. Ensure your analog ground ($AGND$) is isolated from your noisy digital ground ($DGND$) until a single “star point” connection.

Useful Resources & Databases

Penny & Giles Technical Library: The industry leaders in high-end faders and LPDTs.

Bourns Resistor Engineering App Notes: Great for understanding power derating.

Octopart / FindChips: To check stock and lead times for specific sliding potentiometer part numbers.

EIA-448 Standard: The standard for electromechanical component testing.

Frequently Asked Questions (FAQs)

1. Can I use a linear potentiometer as a rheostat?

Yes, by connecting only the wiper and one end terminal. However, ensure the current doesn’t exceed the power rating. Potentiometers are designed as voltage dividers (low current), not as high-load variable resistors.

2. What is the standard travel length?

In the audio world, 60mm, 88mm, and 100mm are the standards. In industrial sensors, travel can range from 10mm to over 2000mm.

3. Why are my fader values “backwards”?

This is a wiring issue. If 0V is at the “top” and 5V is at the “bottom,” your values will decrease as you slide up. Swap the VCC and GND wires on the end terminals to fix this.

4. Are motorized sliders difficult to program?

They require an H-bridge motor driver (like an L293D) and a PID control loop in your code to prevent the slider from “overshooting” its target position.

5. What is “Conductive Plastic” vs. “Carbon”?

Carbon is cheaper and found in consumer gear. Conductive plastic is an engineered polymer that offers significantly higher life cycles and lower contact resistance variation (CRV).

Final Design Considerations for PCB Layout

When integrating a linear potentiometer into your PCB, remember that these are mechanical components. They experience physical stress every time they are moved.

Mounting Holes: Always use the manufacturer-recommended mounting holes. Don’t rely solely on the solder joints to hold the fader in place.

Slot Orientation: If your slider moves vertically, ensure the PCB traces are routed in a way that doesn’t put stress on the pads during a “hard stop” at the end of the travel.

Shielding: Sliding pots have a large “opening” for the actuator. This can act as an EMI entry point. If you’re working with sensitive analog signals, consider a grounded metal shield over the body of the pot.