To build an accurate SPICE potentiometer subcircuit model, you must measure the physical component's end-to-end resistance, wiper contact resistance, and taper linearity using a high-resolution digital multimeter (DMM) or 4-wire Kelvin setup. For a standard 10kΩ linear pot, a good end-to-end reading is 10,000Ω ±5% (9,500Ω–10,500Ω), wiper contact resistance is under 100mΩ, and midpoint voltage division deviates less than 2% from ideal. Default SPICE resistor models fail to capture wiper bounce and parasitic track resistance, making these physical bench measurements critical for high-fidelity analog simulation.
Meter Setup and Safety Categories
Before probing, configure your DMM to eliminate lead-resistance errors, which can easily mask the low-ohm wiper contact resistance you need for your SPICE model.
Meter Setup Block
- Dial Position: Resistance (Ω). If your meter has a dedicated 4-wire Kelvin function (often labeled with a 4-wire symbol or accessible via a secondary button), select it. Otherwise, use standard 2-wire Ohms.
- Lead Jacks: For 2-wire, insert black into COM and red into V/Ω. For 4-wire Kelvin, use the dedicated Sense and Source terminals (often requiring a specialized banana-to-clip cable).
- Range: Manual range. Set to the 20kΩ or 200kΩ range for a 10kΩ pot to prevent auto-ranging delays from masking intermittent wiper dropouts during mechanical sweeping.
- Null/Relative (REL): Short the probe tips together and press the REL/NULL button to zero out the test lead resistance (typically 0.2Ω to 0.5Ω for standard silicone leads).
Expected Readings and Failure Diagnostics
The table below defines the numerical pass/fail criteria for a standard 10kΩ linear (B10K) carbon track or cermet potentiometer. These values form the foundational parameters for your SPICE subcircuit.
| Test Point | Ideal Value | Good Reading (Pass) | Bad Reading (Fail / Misleading) |
|---|---|---|---|
| End-to-End (Pin 1 to 3) | 10,000 Ω | 9,500 Ω – 10,500 Ω | < 9kΩ (shorted track) or OL (open track) |
| Wiper to CW End (Pin 2 to 3) | 5,000 Ω (at 50%) | 4,800 Ω – 5,200 Ω | Erratic jumping (dirty wiper / carbon dust) |
| Wiper to CCW End (Pin 2 to 1) | 5,000 Ω (at 50%) | 4,800 Ω – 5,200 Ω | Sum of Pin 2-1 and 2-3 ≠ Pin 1-3 total |
| Wiper Contact Resistance | 0 Ω | 10 mΩ – 100 mΩ | > 500 mΩ (oxidized wiper, requires cleaning) |
| Track Parasitic Capacitance | 0 pF | < 5 pF | > 20 pF (causes high-freq rolloff in AC sim) |
Misleading Reading Traps
When extracting parameters for a SPICE simulator like LTspice, two common bench mistakes will corrupt your model:
- Measuring In-Circuit: If you measure a potentiometer while it is still soldered to a PCB, parallel resistive paths (like pull-up/pull-down networks or op-amp feedback loops) will artificially lower your end-to-end reading. Always desolder at least two legs of the pot before measuring.
- Ignoring Lead Resistance on Wiper Tests: If your test leads have 0.4Ω of resistance and you are trying to measure a 50mΩ wiper contact, a 2-wire meter will read 0.45Ω. You will mistakenly model a massive 450mΩ wiper resistance in SPICE, causing severe DC offset errors in low-impedance audio or precision DC circuits. You must use the REL function or a 4-wire Kelvin measurement for the wiper.
Probe Placement and Test Point Execution
Follow this exact probing sequence to extract the physical data required for your SPICE netlist.
- End-to-End Resistance (R_total): Place the red probe on Pin 3 (Clockwise) and the black probe on Pin 1 (Counter-Clockwise). Rotate the shaft fully back and forth. The reading should remain rock-solid. If it fluctuates by more than 1%, the carbon track is worn or the terminations are cracked.
- Wiper Continuity and Taper (R1 + R2): Move the red probe to Pin 2 (Wiper). Keep black on Pin 1. Slowly rotate the shaft from CCW to CW. The resistance should climb smoothly from near-zero to R_total. Repeat with red on Pin 2 and black on Pin 3; the value should drop smoothly from R_total to near-zero.
- Wiper Contact Resistance (R_wiper): This requires measuring the resistance between the wiper pin and the track at the extreme ends. Set the meter to its lowest Ohms range (or 4-wire mode). Place probes on Pin 2 and Pin 1. Turn the shaft fully CCW until it hits the mechanical stop. The reading is your R_wiper. (Refer to Fluke's guide on 4-wire measurements for high-precision setups).
- Taper Linearity (Voltage Divider Method): Resistance measurements alone don't perfectly capture logarithmic (audio) tapers. Apply a precise 5.000V DC reference across Pin 1 (GND) and Pin 3 (5V). Measure the DC voltage at Pin 2 at 25%, 50%, and 75% mechanical travel. For a linear pot, expect 1.25V, 2.50V, and 3.75V. For an audio taper, expect highly non-linear steps (e.g., 0.4V, 2.2V, 4.1V) depending on the manufacturer's specific curve.
| Feature | 2-Wire Standard DMM | 4-Wire Kelvin Setup |
|---|---|---|
| Best Used For | End-to-end track resistance (>100Ω) | Wiper contact resistance (<1Ω) |
| Lead Resistance Error | Adds 0.2Ω - 0.5Ω to reading | Eliminated via separate sense lines |
| SPICE Model Impact | Overestimates R_wiper, ruins low-Z sims | Provides exact R_wiper for high-fidelity models |
| Equipment Needed | Standard multimeter | Bench DMM (e.g., Keysight 34461A) or micro-ohmmeter |
Translating Bench Data into a SPICE Subcircuit
Standard SPICE only recognizes fixed resistors. To simulate a potentiometer, you must build a subcircuit using the bench data you just collected. A basic 3-terminal SPICE potentiometer model consists of two resistors (R1 and R2) and a series wiper resistor (R_wiper).
Using our 10kΩ B10K bench data, the SPICE netlist parameters are calculated as follows:
- R_total: 10,000Ω (from end-to-end test).
- R_wiper: 0.05Ω (from 4-wire Kelvin test at mechanical stop).
- Position Variable (x): A parameter from 0 to 1 representing shaft rotation.
- R1 (CCW to Wiper):
(1 - x) * (R_total - R_wiper) - R2 (Wiper to CW):
x * (R_total - R_wiper)
If your voltage divider taper test revealed a logarithmic (audio) curve, you cannot use simple linear equations for R1 and R2. Instead, you must map your bench voltage readings to a piecewise linear (PWL) lookup table or use a SPICE behavioral resistor (B-source) with a mathematical approximation of the log curve. For high-frequency AC simulations, add a 2pF to 5pF capacitor in parallel with R1 and R2 to model the parasitic track capacitance measured on your LCR meter, preventing unrealistic high-frequency gain in your simulated op-amp feedback loops.
