A standard potentiometer features three terminals: two fixed end pins (Pin 1 and Pin 3) that connect to the resistive track, and one movable wiper pin (Pin 2) that slides across the track. To verify a potentiometer is functional, you must measure the total resistance across the two outer pins, then measure the resistance between the wiper and one outer pin while sweeping the shaft through its full rotation. A good linear 10kΩ potentiometer will read exactly 10kΩ (±20%) across the outer pins, and sweep smoothly from 0Ω to 10kΩ on the wiper pin without any infinite spikes or dropouts.
Meter Setup and Safety Categories
Testing a potentiometer is strictly a low-voltage, de-energized bench procedure. However, using the correct multimeter category and settings prevents damaged equipment and false readings.
Meter Setup Block
- Dial Position: Set to Resistance (Ω). If your meter is not auto-ranging, select a range one step higher than the potentiometer's rated value (e.g., use the 20kΩ range for a 10kΩ pot, or the 2MΩ range for a 1MΩ pot).
- Lead Jacks: Black lead in COM, Red lead in V/Ω.
- Zero Check: Touch the probe tips together. The display should read between 0.1Ω and 0.5Ω (this is your lead resistance). Note this value; you will subtract it if you are testing very low-value pots (under 100Ω).
Identifying and Probing the Potentiometer Pins
Before probing, you must correctly identify the wiper. On a standard through-hole pot (like the Bourns 3296W trimpot or a standard 16mm Alpha panel mount), the pins are usually arranged in a straight line. Pin 1 is the counter-clockwise (CCW) end, Pin 3 is the clockwise (CW) end, and Pin 2 (the middle pin) is the wiper. If the pins are arranged in a triangle, look at the phenolic board: the wiper pin is physically connected to the metal bracket or the distinct center trace.
- Test Total Resistance (Outer Pins): Place your red probe on Pin 1 and your black probe on Pin 3 (or vice versa; polarity does not matter for resistance). Rotate the shaft. The reading should remain completely static.
- Test Wiper Continuity (Pin 1 to Pin 2): Move the black probe to the middle pin (Pin 2). Leave the red probe on Pin 1. Turn the shaft fully counter-clockwise. The reading should drop near 0Ω.
- Sweep the Track: Slowly rotate the shaft clockwise while watching the multimeter display. The resistance should climb steadily and smoothly until it reaches the total resistance value measured in Step 1.
- Test Wiper Continuity (Pin 3 to Pin 2): Move the red probe to Pin 3, keeping the black probe on Pin 2. Sweep the shaft back. The resistance should smoothly decrease from maximum to near 0Ω.
Expected Readings: Good vs. Bad Values
The table below outlines exact expected readings for a standard 10kΩ B-taper (linear) potentiometer. According to All About Circuits, standard carbon-track potentiometers carry a tolerance of ±20%, meaning a 10kΩ pot can legally read anywhere from 8kΩ to 12kΩ out of the box.
| Test Point | Shaft Position | Good Reading (10kΩ Linear Pot) | Bad Reading (Indicates Failure) |
|---|---|---|---|
| Pin 1 to Pin 3 | Any position | 8.0kΩ – 12.0kΩ (Static) | OL (Open), 0Ω (Shorted), or fluctuating values |
| Pin 1 to Pin 2 (Wiper) | Fully CCW | 0Ω – 50Ω | > 100Ω (Dirty wiper contact) |
| Pin 1 to Pin 2 (Wiper) | 50% Mechanical Rotation | 4.5kΩ – 5.5kΩ | Sudden jumps to OL or dropping to 0Ω |
| Pin 1 to Pin 2 (Wiper) | Fully CW | 9.5kΩ – 10.5kΩ | Reads significantly lower than Pin 1-3 total |
Common Mistakes That Give Misleading Readings
When testing potentiometer pins, bench technique is just as critical as the multimeter itself. Here are the most frequent errors that lead to false diagnostics.
1. The Finger Resistance Parallel Path
Human skin has a dry resistance ranging from 100kΩ to over 1MΩ. If you are testing a high-impedance potentiometer (e.g., a 1MΩ volume pot) and you hold the metal probe tips and the outer pins simultaneously with your bare fingers, your body acts as a parallel resistor. Using the parallel resistance formula ($R_{total} = \frac{R_1 \times R_2}{R_1 + R_2}$), a 1MΩ pot measured in parallel with your 1MΩ body resistance will falsely read as 500kΩ. The Fix: Use alligator clip test leads, or hold only the insulated plastic probe barrels.
2. Measuring In-Circuit
If you attempt to measure the potentiometer pins while the component is still soldered to a PCB, the multimeter's test current will flow through parallel paths (like surrounding resistors, op-amp inputs, or capacitors). This will almost always result in a total resistance reading much lower than the pot's rated value. The Fix: Desolder at least two of the three pins (preferably all three) to isolate the component before testing.
3. Confusing Audio Taper with Linear Taper
Many hobbyists assume that at exactly 50% physical shaft rotation, the wiper should read 50% of the total resistance. This is only true for B-taper (linear) pots. If you are testing an A-taper (logarithmic/audio) potentiometer, the 50% mechanical mark will typically yield only 10% to 15% of the total resistance. This is not a defect; it is an intentional design meant to match the logarithmic hearing response of the human ear. Consult the manufacturer's datasheet (e.g., Alps or Bourns) to verify the taper curve before condemning the part.
Frequently Asked Questions
Which pin is the wiper on a standard potentiometer?
On a standard inline 3-pin potentiometer, the wiper is the middle pin (Pin 2). On triangular pinout trimpots (like the Bourns 3386 series), the wiper is typically the pin that sits alone on one side of the component, or the pin physically connected to the metal adjustment screw bracket. If you are unsure, measure the resistance between all pairs while turning the screw; the pin that yields changing resistance against both of the other two pins is the wiper.
Why does my multimeter show infinite resistance on the middle potentiometer pin?
If measuring from an outer pin to the middle wiper pin yields an "OL" (Over Limit / Infinite) reading, the wiper has lost physical contact with the resistive carbon or cermet track. This is usually caused by heavy oxidation, dirt ingress, or a bent wiper arm. You can sometimes rescue a dirty track by injecting a small amount of DeoxIT D5 contact cleaner into the housing and sweeping the shaft back and forth 50 times. If the track is physically worn away (common in cheap carbon pots after years of use), the component must be replaced.
Can I use the outer potentiometer pins as a fixed resistor?
Yes, the two outer potentiometer pins (Pin 1 and Pin 3) simply represent the ends of a fixed resistive track. You can use them as a standard fixed resistor in a low-power circuit. However, standard potentiometers are typically rated for only 0.1W to 0.5W of power dissipation and have a wide tolerance (±20%). They are not suitable as precision or high-wattage fixed resistors. For better stability, tie the unused wiper pin (Pin 2) to one of the outer pins to prevent open-circuit noise if the wiper gets bumped.
How do I test an audio taper (logarithmic) potentiometer with a digital multimeter?
Test the outer pins first to establish the baseline total resistance. Then, connect your probes to Pin 1 and the wiper (Pin 2). Rotate the shaft to the 12 o'clock (50%) position. For an audio taper pot, expect the multimeter to read roughly 10% to 15% of the total resistance. Rotate to the 3 o'clock (75%) position; the reading should jump to roughly 50% of the total resistance. If the sweep is smooth and matches this non-linear progression without dropping to zero or spiking to infinity, the audio taper track is in good condition. For deeper verification of taper curves, reference the Bourns 3386 series datasheet which provides exact graphical resistance-vs-rotation curves.
