A potentiometer wired as a voltage divider outputs a variable DC voltage determined by the physical position of its wiper, governed by the formula Vout = Vin × (Rwiper / Rtotal). To properly test this circuit, you must verify the total end-to-end resistance out-of-circuit, check the wiper sweep for dead spots, and measure the loaded voltage output in-circuit. A healthy 10kΩ linear potentiometer on a 5.00V reference will output exactly 2.50V at mechanical 50% rotation, provided the downstream load impedance is at least 10× higher than the potentiometer's resistance.
Meter Setup and Safety Category for Low-Voltage DC Divider Testing
Before probing the circuit, configure your digital multimeter (DMM) correctly. A standard bench potentiometer operates at low voltage, but the safety category (CAT rating) depends entirely on the power supply topology, not just the nominal voltage.
Meter Setup Block:
- Dial Position: V DC (for in-circuit voltage tests) or Ω (for out-of-circuit resistance tests).
- Lead Jacks: Black lead in COM, Red lead in V/Ω (do not use the A/mA jack, which inserts a shunt resistor and will short your wiper to ground).
- Range: Auto-ranging, or manual 20V DC / 20kΩ range for maximum resolution on a standard 3.5-digit meter.
- Input Impedance: Ensure your DMM has ≥10 MΩ input impedance (standard for Fluke 117 or Klein MM400) to prevent the meter itself from loading the divider.
Probe Placement and Expected Readings (The Core Tests)
The following tests assume a standard 10kΩ linear taper (B10k) potentiometer powered by a 5.00V DC reference. Pin 1 is Vin (5V), Pin 3 is GND (0V), and Pin 2 is the Wiper (Vout).
| Test Phase | Probe Placement | Pot State | Expected Good Reading | Bad Reading (Fail State) |
|---|---|---|---|---|
| Total Resistance | Red on Pin 1, Black on Pin 3 | Power OFF, full sweep | 9.0kΩ to 11.0kΩ (±10% tolerance) | Open (OL) or < 8.5kΩ (internal short) |
| Wiper Sweep | Red on Pin 1, Black on Pin 2 | Power OFF, rotate slowly | Smooth transition from ~10Ω to 10kΩ | Jumping values, OL spikes, or stuck at 10kΩ |
| Vin Verification | Red on Pin 1, Black on Pin 3 | Power ON | 4.95V to 5.05V | < 4.8V (supply sag) or > 5.2V (regulator drift) |
| Vout at 50% | Red on Pin 2, Black on Pin 3 | Power ON, shaft at mechanical center | 2.45V to 2.55V | < 2.0V or > 3.0V (taper mismatch or loaded down) |
| Vout at 0% | Red on Pin 2, Black on Pin 3 | Power ON, fully counter-clockwise | 0.00V to 0.05V | > 0.2V (wiper not contacting ground track) |
Diagnosing Misleading Readings: Loading Effects and Taper Confusion
When a potentiometer voltage divider fails to output the mathematically expected voltage, the component is rarely broken. The error usually stems from circuit loading or taper misidentification.
The Loading Effect (Impedance Mismatch)
A voltage divider assumes no current is drawn from the wiper. If you connect a low-impedance load (like a 1kΩ resistor or an unbuffered ADC input) to the wiper of a 10kΩ pot, the load forms a parallel resistance with the bottom half of the potentiometer, severely sagging the output voltage.
Worked Numeric Example: You set a 10kΩ pot to exactly 50% (Rtop = 5kΩ, Rbottom = 5kΩ) with a 5V input. You connect a 1kΩ load from the wiper to ground. The new bottom resistance is 5kΩ || 1kΩ = 833Ω. The output voltage becomes Vout = 5V × [833 / (5000 + 833)] = 0.71V. You expect 2.5V, but measure 0.71V. The pot is fine; the load impedance is too low. The fix is to buffer the wiper with a unity-gain op-amp (like a TL072 or LM358) or use a lower-value potentiometer (e.g., 1kΩ).
Taper Misidentification (Linear vs. Audio)
Potentiometers come in different tapers, dictating how resistance changes relative to shaft rotation. According to All About Circuits, a linear taper (marked B10k) changes resistance proportionally. An audio/logarithmic taper (marked A10k) changes resistance logarithmically to match human hearing perception.
If you test an A10k (audio) pot and measure the wiper voltage at 50% mechanical rotation, you will read approximately 0.5V to 0.75V (10-15% of Vin), not 2.5V. This is not a defect; it is the designed taper curve. Always check the component marking: 'A' denotes audio, 'B' denotes linear.
Decision Tree: Selecting the Right Replacement Potentiometer
If your resistance sweep test shows dead spots, infinite resistance jumps, or physical shaft play, the carbon track or cermet element is degraded. Use this decision path to select the exact replacement part.
| Application Scenario | Required Specifications | Concrete Part Pick |
|---|---|---|
| PCB-mounted calibration or trimming (rarely adjusted) | Cermet element, multi-turn (25 turns), 10kΩ, top-adjust | Bourns 3296W-1-103LF |
| Panel-mount audio volume control | Audio taper (log), 10kΩ or 50kΩ, 16mm, knurled split shaft | Alpha (Taiwan Alpha) RD164-10K-A or Bourns PTD902-2015K-A103 |
| Panel-mount linear sensor scaling / motor speed | Linear taper, 10kΩ, conductive plastic or carbon, 20mm bushing | Bourns 53C10K (Conductive Plastic, high cycle life) |
| High-power rheostat / heavy current divider (>0.5W) | Wirewound element, 5W+ rating, panel mount | Vishay 534B1103JC (Wirewound, 2W to 5W range) |
Troubleshooting FAQ: Wiper Noise and Terminal Failures
Why does my voltage reading jump erratically when I barely touch the shaft?
This is caused by poor wiper contact pressure or oxidation on the resistive track, measured as Equivalent Noise Resistance (ENR). According to Bourns technical specifications, a healthy carbon pot should exhibit an ENR of less than 3% of its total resistance. For a 10kΩ pot, the dynamic contact resistance should not fluctuate by more than 300Ω during slow rotation. If your DMM shows voltage jumps exceeding 100mV on a 5V rail while moving the shaft slowly, the wiper fingers are worn. Cleaning with DeoxIT D5S-6 can temporarily restore contact, but replacement is the only permanent fix for mechanical wear.
I measure 5V at Pin 1 and 0V at Pin 3, but the wiper (Pin 2) reads 0V regardless of rotation. What is broken?
This specific failure mode indicates an open circuit between the wiper and the resistive track, or a broken wiper pigtail inside the housing. Verify by switching your DMM to the Ohms range (power off). Measure between Pin 1 and Pin 2. If the meter reads OL (Open Line) at all shaft positions, the wiper connection has failed internally. Measure between Pin 1 and Pin 3; if that reads 10kΩ, the track is intact, but the wiper assembly is physically detached. The part must be replaced.
Can I use a linear pot in an audio volume circuit to save money?
You can, but the volume control will feel entirely non-functional. Because human hearing perceives loudness logarithmically, a linear pot will cause 80% of the perceived volume change to happen in the last 15% of the knob's rotation. You will spend all your time making micro-adjustments at the top end. Always use an audio (logarithmic) taper for human-interface volume controls, and reserve linear tapers for voltage references, sensor scaling, and op-amp gain feedback networks.
