The schematic symbol for a potentiometer is fundamentally a standard resistor symbol intersected by an arrow representing the movable wiper. However, whether that resistor is drawn as a zig-zag line or a solid rectangle depends entirely on the drafting standard used in your region. Misidentifying the wiper pin or misinterpreting the taper code on the physical component will result in reversed controls, dead spots, or shorted traces. Below is the definitive reference for identifying, wiring, and testing potentiometers on the bench.
Schematic Symbols: IEC 60617 vs IEEE 315 Standards
Before you wire a board, you must know which drafting standard the schematic follows. The International Electrotechnical Commission (IEC 60617) standard dominates in Europe, Asia, and modern global CAD tools, using a rectangular box for resistive elements. The IEEE 315 / ANSI Y32.2 standard remains common in legacy US documentation and older American textbooks, using a zig-zag line.
| Component Type | IEC 60617 Symbol (Global/EU) | IEEE 315 / ANSI Symbol (US Legacy) | Practical Application |
|---|---|---|---|
| Potentiometer (3-Terminal) | Rectangle with an arrow pointing to the center line | Zig-zag line with an arrow pointing to the center | Volume controls, voltage dividers, user-adjustable setpoints. |
| Rheostat (2-Terminal) | Rectangle with an arrow pointing to one end, other end unconnected | Zig-zag line with an arrow pointing to one end | Current limiting, motor speed control (high power). |
| Trimmer / Preset Pot | Rectangle with an arrow crossing the line (often with a 'T' or diagonal slash) | Zig-zag line with an arrow crossing it, or a non-pointed line | One-time calibration, bias adjustment (e.g., Bourns 3296W). |
| Ganged Potentiometer | Two pot symbols linked by a dashed mechanical line | Two pot symbols linked by a dashed mechanical line | Stereo audio volume (dual-channel tracking). |
Never route your main load current through the wiper terminal (Pin 2). The wiper contact is typically rated for significantly less current (often 50mA to 100mA) than the end terminals. If you need a high-current variable resistor, wire it as a rheostat but place a fixed power resistor in series to prevent a dead short if the wiper reaches zero ohms.
Pinout Mapping and Taper Code Reference
Physical potentiometers rely on standardized pinouts and alphanumeric stamping to denote resistance and taper (the rate of resistance change relative to shaft rotation). While major manufacturers like Bourns and Alpha adhere to global conventions, regional legacy markings still surface in repair work.
| Marking / Pin | Designation | Electrical Function | Standard Convention & Notes |
|---|---|---|---|
| Pin 1 | CCW (Counter-Clockwise) | Ground / Reference Low | Connects to 0V/GND in standard voltage divider configurations. Yields minimum output when shaft is fully CCW. |
| Pin 2 | Wiper | Variable Output | The moving contact. Connects to the ADC input, op-amp, or load. Always the middle pin on standard single-turn PCB pots. |
| Pin 3 | CW (Clockwise) | VCC / Reference High | Connects to the positive supply voltage. Yields maximum output when shaft is fully CW. |
| Taper 'B' | Linear | Constant resistance change per degree | Marked as B10K or 103B. 50% rotation = 50% resistance. Used for position sensing and balanced voltage dividers. |
| Taper 'A' | Audio / Logarithmic | Exponential resistance change | Marked as A10K or 103A. 50% rotation ≈ 10-15% resistance. Matches human hearing perception for volume controls. |
| Taper 'C' | Reverse Logarithmic | Inverse exponential change | Marked as C10K. Rare; used for specific tone controls or reverse-acting feedback loops. |
Rows People Get Wrong: Faded Markings and Regional Traps
The most common bench mistake involves the taper codes on older or region-specific components. In modern global manufacturing, A = Logarithmic and B = Linear. However, on vintage US-manufactured equipment (pre-1980s), you will frequently find the reverse: A = Linear and B = Logarithmic. If you are repairing a 1970s American amplifier and replace an 'A' taper pot with a modern Asian 'A' (Log) pot, your volume knob will act erratically.
Another frequent error is misidentifying the wiper on 3-pin trimmer potentiometers (like the blue Bourns 3386 series). Unlike panel-mount pots where the wiper is strictly the physical center pin, some right-angle PCB trimmers place the wiper on the outer edge of the pin row. Always verify the wiper pin with a multimeter before soldering; assuming Pin 2 is the wiper on an unfamiliar footprint will short your reference voltage to ground.
Safe Interpretation and Bench Verification
When you pull a salvaged potentiometer from a scrapped chassis, the silkscreen markings are often faded, scratched, or obscured by conformal coating. Do not guess the taper or the pinout based on physical appearance. Use this three-step multimeter verification protocol to safely map the component.
- Identify the Wiper and Total Resistance: Set your DMM to the Ohms (Ω) range. Probe all three pins in pairs. The pair that shows the highest, stable resistance (e.g., 9.8kΩ on a 10kΩ pot) are the two end terminals (Pins 1 and 3). The remaining pin is your Wiper (Pin 2).
- Map the Rotation Direction: Leave one DMM probe on the Wiper and place the other on one of the end terminals. Turn the shaft fully counter-clockwise (CCW). If the resistance drops to near zero (usually < 2Ω), the terminal you are probing is Pin 1 (CCW/GND). If it reads maximum resistance, you are probing Pin 3 (CW/VCC).
- Determine the Taper: Connect the DMM between the Wiper and Pin 1 (CCW). Rotate the shaft to the exact mechanical midpoint (50% rotation).
- If the DMM reads approximately 50% of the total resistance, it is a Linear (B) taper.
- If the DMM reads roughly 10% to 15% of the total resistance, it is an Audio/Log (A) taper.
- If the DMM reads 85% to 90% of the total resistance, it is a Reverse Log (C) taper.
By relying on electrical measurement rather than faded ink, you eliminate the risk of wiring a logarithmic taper into a linear feedback circuit, which would cause severe non-linearity in your control loop or ADC readings. Always document your findings with a marker on the component housing before dropping it into your parts bin.
