The variable resistor symbol in a circuit schematic depends entirely on the drafting standard used: IEC 60617 uses a rectangle with a diagonal arrow, while IEEE/ANSI uses a zigzag line with an arrow. In practice, the wiper (Pin 2) is always the moving contact, and the outer pins (1 and 3) provide the fixed total resistance. If you are reading a modern schematic, assume IEC rectangular symbols unless the drawing explicitly references ANSI Y32.2.
The Complete Variable Resistor Symbol & Pinout Reference
Use this table to decode the specific component type and terminal assignments from your schematic. The physical pinout applies to standard carbon-film and cermet components like the Bourns 3296W trimpot or ALPS RK09 panel potentiometer.
| Component Type | IEC 60617 Symbol | IEEE 315 / ANSI Symbol | Standard Pinout (1-2-3) | Primary Circuit Function |
|---|---|---|---|---|
| Potentiometer (3-Terminal) | Rectangle with diagonal arrow crossing outside | Zigzag with diagonal arrow crossing outside | 1: CCW, 2: Wiper, 3: CW | Voltage divider, signal attenuation |
| Rheostat (2-Terminal) | Rectangle with arrow pointing into one end | Zigzag with arrow pointing into one end | 1: Fixed, 2: Wiper (3: Floating or tied to 2) | Variable current limiting, dimming |
| Trimmer / Preset | Rectangle with T-shaped arrow (no diagonal cross) | Zigzag with T-shaped arrow | 1: CCW, 2: Wiper, 3: CW | One-time calibration, bias setting |
| Tapped Resistor | Rectangle with multiple fixed arrows pointing in | Zigzag with multiple fixed arrows | Multiple fixed terminals (no wiper) | Fixed voltage drops, motor starting |
Regional Standards: IEC vs. IEEE/ANSI vs. Legacy
Schematic symbols are not universal. The visual representation of a variable resistor changes based on the governing standards body of the region where the schematic was drafted.
- IEC 60617 (Global / Europe / Modern International): Uses a solid rectangular box to represent the resistive element. The arrow represents the wiper. This is the dominant standard in modern CAD software (Altium, KiCad, Eagle) default libraries.
- IEEE Std 315 / ANSI Y32.2 (North America / Legacy US): Uses a jagged zigzag line to represent the resistor. While largely superseded by IEC in new professional designs, you will still see this in older US military specs, vintage audio schematics, and older educational textbooks.
- Legacy European (DIN): Occasionally used an open, unfilled rectangle or a specific cross-hatch pattern. If you encounter a box with a diagonal line crossing entirely through it and an arrow, it is an older DIN variant of a potentiometer.
Rows People Get Wrong: Faded Markings & Pinout Traps
Schematics are clean; physical components are not. Here is where bench technicians and hobbyists make critical errors when translating the variable resistor symbol circuit to a physical breadboard or PCB.
1. Faded Silkscreen on Trimpots
On cermet trimpots like the ubiquitous Bourns 3296W, the '1, 2, 3' pin designations are often printed in tiny, easily rubbed-off ink. If the markings are missing, do not guess based on physical orientation, as pinouts flip between top-adjust and side-adjust variants.
The Multimeter Test: Set your DMM to resistance (Ohms). Probe two pins while turning the adjustment screw through its full travel. If the resistance remains perfectly fixed (e.g., exactly 10.0kΩ on a 10k pot), you are probing Pins 1 and 3. The remaining unprobed pin is Pin 2 (the wiper).
2. The Taper Code Confusion (A vs. B)
The schematic symbol rarely indicates the taper (linear vs. logarithmic), but the physical component does. Misinterpreting this ruins audio circuits and sensor scaling.
- 'B' Taper (e.g., B10K): Linear. The resistance changes at a constant rate relative to shaft rotation. Used for voltage dividers, sensor scaling, and oscilloscope offsets.
- 'A' Taper (e.g., A10K): Logarithmic (Audio). The resistance changes slowly at one end and rapidly at the other, mimicking human hearing perception. Used strictly for audio volume controls.
Trap: Some legacy European manufacturers reversed these codes (A for linear, B for log). Always verify the taper by measuring the resistance at the exact 50% physical rotation mark. On a 10k linear pot, 50% rotation yields ~5kΩ. On a 10k log pot, 50% rotation typically yields ~1.5kΩ to 2kΩ.
3. Rheostat Wiring (Floating Pin 3)
When a schematic calls for a rheostat (2-terminal variable resistor), you only use Pins 1 and 2. However, leaving Pin 3 electrically floating can cause it to act as an antenna, injecting noise into high-impedance op-amp circuits. The correct physical wiring practice is to jumper Pin 3 directly to the wiper (Pin 2). This ensures that if the wiper momentarily loses contact with the carbon track due to vibration or dirt, the circuit opens gracefully rather than snapping to maximum resistance.
Variable Resistor Symbol Circuit FAQs
How do I read a 3-pin variable resistor symbol circuit for a voltage divider?
In a voltage divider configuration, the schematic will show Pin 1 connected to your reference voltage (VCC), Pin 3 connected to Ground (GND), and Pin 2 (the wiper) routed to your load or ADC input. As the wiper moves toward Pin 1, the output voltage approaches VCC; as it moves toward Pin 3, it approaches 0V. Ensure your ADC input impedance is at least 10x higher than the potentiometer's total resistance to prevent loading errors.
What is the difference between a potentiometer and rheostat symbol?
The difference lies in the arrow placement. A potentiometer symbol (3-terminal) features an arrow that crosses completely over or outside the resistor body, indicating a third, independent wiper terminal used for voltage division. A rheostat symbol (2-terminal) features an arrow that points directly into one end of the resistor body or loops back to the wiper, indicating that the third terminal is either omitted or internally tied to the wiper to function as a variable current limiter.
Why does my variable resistor circuit symbol have a box with a diagonal line through it?
If you see a rectangular box with a diagonal line crossing entirely through it, accompanied by an external arrow, you are looking at a legacy DIN or early IEC variant of a potentiometer. The diagonal line was historically used to distinguish a variable/adjustable component from a fixed resistor (which was just an empty rectangle). Modern IEC 60617 standards have largely dropped the internal diagonal line in favor of just the external arrow, but it remains common in schematics drafted before the late 1990s.
How to wire a variable resistor when schematic pin numbers don't match the datasheet?
Schematic pin numbers (1, 2, 3) represent logical circuit functions (CCW, Wiper, CW), not necessarily the physical left-to-right layout of the component's metal legs. If the datasheet for your specific part (e.g., an ALPS RK09 or a Bourns PTV09A) shows a different physical pin order, always trust the physical datasheet for PCB footprint routing. For breadboarding, ignore the stamped numbers and use the multimeter wiper-identification test outlined above to map the logical schematic pins to the physical metal terminals.
