A potentiometer is a three-terminal variable resistor that acts as an adjustable voltage divider to control signal levels or reference voltages in a circuit. When you turn the shaft or slide the fader, you are not merely changing total resistance; you are altering the ratio of resistance between the wiper (the middle terminal) and the two outer terminals, which scales an input voltage down to a precise fractional output. Beginners frequently confuse potentiometers with rheostats (which are two-terminal devices wired specifically to vary current flow, not voltage) and rotary encoders (which are digital switches outputting quadrature pulses rather than continuous analog resistance).
The Math: Why Potentiometer Impedance Matters
To understand how a potentiometer behaves in a real circuit, we have to look past the ideal schematic and account for the load it drives. Let us run a worked numeric example using a standard 10kΩ linear potentiometer connected across a 5.00V DC supply.
If you rotate the shaft to exactly 30% of its mechanical travel, the resistance from the ground terminal to the wiper is 3kΩ, and from the wiper to the 5V terminal is 7kΩ. If you measure the wiper with a high-impedance digital multimeter (typically 10MΩ input impedance), it will read exactly 1.50V. This is the open-circuit ideal.
Supply: 5.00V | Pot: 10kΩ Linear | Wiper Position: 30%
Open-Circuit Wiper Voltage: 1.50V
Loaded Wiper Voltage (with 10kΩ load): 1.24V (17.3% sag)
Real circuits rarely have infinite impedance. If you connect that same wiper to a microcontroller ADC with a 10kΩ input impedance, or an external 10kΩ pull-down resistor, that 10kΩ load sits in parallel with the 3kΩ bottom segment of the pot. The equivalent resistance of that bottom segment drops to 2.307kΩ. The total circuit resistance becomes 9.307kΩ, and the wiper voltage sags to 1.24V. This 17% voltage drop is why you must always match your potentiometer impedance to your load, typically keeping the pot resistance at least 10 times lower than the load impedance to maintain accuracy.
Potentiometer Types: Tapers and Resistive Elements
Choosing the right component requires understanding two distinct classifications: the taper (how resistance changes relative to shaft rotation) and the resistive element (the physical material used for the track).
Understanding Tapers
- Linear (Taper B): Resistance changes at a constant rate with shaft rotation. A 50% turn yields exactly 50% of the total resistance. Used for sensor calibration, joystick axes, and voltage trimming.
- Audio / Logarithmic (Taper A): Resistance changes slowly at first, then rapidly. Because human hearing perceives loudness logarithmically, an audio taper pot provides a perceived linear change in volume. At 50% rotation, a 10kΩ audio pot will typically measure around 1kΩ to 1.5kΩ from the ground terminal, not 5kΩ.
- Reverse Logarithmic (Taper C): The inverse of the audio taper. Used in specialized audio circuits like tone controls or specific gain-staging amplifiers.
Resistive Element Materials
| Element Type | Resolution | Lifespan (Cycles) | Best Application | Typical Cost |
|---|---|---|---|---|
| Carbon Composition | Good | 15,000 - 30,000 | General prototyping, basic volume controls | $0.50 - $1.50 |
| Cermet (Ceramic/Metal) | Excellent | 200 - 1,000 (Trimpots) | PCB voltage trimming, precision calibration | $1.50 - $4.00 |
| Conductive Plastic | Superior (Smooth) | 1,000,000+ | High-end audio faders, studio mixing desks | $8.00 - $20.00 |
| Wirewound | Low (Discrete steps) | 5,000 - 10,000 | High-power current limiting, heavy-duty rheostats | $5.00 - $15.00 |
Where You Meet This in Practice
Different potentiometer types solve specific physical problems across electrical and electronic disciplines:
Audio Mixing and Amplifiers: You will almost exclusively find conductive plastic or high-grade carbon elements with an audio (logarithmic) taper here. A fader on a mixing console, like those using ALPS RK27 series pots, relies on the smooth travel and low wiper noise of conductive plastic to prevent audible scratching during live performances.
Microcontroller Sensor Calibration: When building a custom sensor array (e.g., tuning the threshold of a photodiode comparator), you need a linear taper. The microcontroller's ADC expects a direct, proportional voltage mapping to the physical knob position.
Power Supply Feedback Loops: Adjustable bench power supplies use multi-turn cermet trimpots (like the Bourns 3590 series) in the feedback network of the voltage regulator. Cermet is chosen here because its resistance remains highly stable across temperature variations, preventing your 12.0V output from drifting to 12.4V when the enclosure heats up.
Decision Tree: Picking the Right Potentiometer
Use this decision path to terminate your component search and select a specific part number for your bill of materials.
| If your application requires... | Then select this Element & Taper... | Concrete Part Number Pick |
|---|---|---|
| Trimming a DC voltage on a PCB where space is tight | Cermet, Linear, Multi-turn | Bourns 3296W-1-103LF (10kΩ, 25-turn) |
| A front-panel volume knob for a headphone amp | Conductive Plastic, Audio (Log) | ALPS RK27111200A9 (10kΩ Audio) |
| A joystick axis or DIY MIDI controller | Carbon or Plastic, Linear | Bourns PTV09A-4015F-B103 (10kΩ Linear) |
| High-current motor speed control (analog) | Wirewound, Linear | TT Electronics P260 (50Ω, 2W rating) |
Frequently Asked Questions
Why does my potentiometer output jump around when I read it with an Arduino?
This is usually caused by wiper noise or an impedance mismatch. Carbon track pots develop microscopic wear grooves over time, causing the wiper to momentarily lose contact, resulting in voltage spikes. Additionally, the Arduino Uno's ADC expects a source impedance of 10kΩ or less. If you are using a 100kΩ potentiometer, the internal sample-and-hold capacitor cannot charge fast enough during the ADC conversion clock cycles, leading to jittery readings. Stick to 10kΩ pots for 5V Arduinos, and 10kΩ to 50kΩ for 3.3V ESP32 boards.
Can I wire a 3-terminal potentiometer as a 2-terminal variable resistor?
Yes, this is called a rheostat configuration. You connect one outer terminal and the wiper to your circuit, leaving the second outer terminal floating. However, best practice dictates tying the unused outer terminal directly to the wiper. If the wiper experiences a momentary loss of contact due to dirt or vibration, the circuit will see the maximum resistance of the pot rather than an open circuit (infinite resistance), which can prevent downstream components from floating to undefined voltages.
What does the 'A' or 'B' stamped on the back of the pot mean?
These letters denote the taper, but they are not globally standardized. In most of the world (and for major manufacturers like Bourns and ALPS), 'B' means Linear and 'A' means Audio (Logarithmic). However, some older American-manufactured pots reverse this convention. Always verify the taper by measuring the resistance from the ground pin to the wiper at the 50% rotation mark with a multimeter before soldering it into an audio circuit.
If you are stocking a lab bench for general prototyping, microcontroller interfacing, and sensor calibration, do not overthink it. Buy a bulk kit of Bourns PTV09A-4015F-B103 (10kΩ linear, carbon element, 15mm slotted shaft). They cost roughly $0.80 each in quantities of ten, tolerate breadboard abuse, feature a standard 6mm shaft that fits standard knobs, and the 10kΩ value perfectly bridges 3.3V and 5V logic without drawing excessive current. Keep a separate drawer of 10kΩ cermet trimpots (Bourns 3296 series) for when you need to permanently calibrate a finished PCB.
