The variable resistance symbol represents any component designed to alter its ohmic opposition to current flow, either manually or via environmental stimulus. On US-based schematics (IEEE/ANSI), it is drawn as a rectangle intersected by a diagonal arrow. On European and international schematics (IEC), it appears as a zigzag line with a diagonal arrow. Below is the complete reference for identifying these symbols and selecting the exact physical component for your workbench.
The Complete Variable Resistance Symbol Reference Table
Read this table by matching the visual geometry on your schematic (rectangle vs. zigzag) to the component type, then verify the terminal count before ordering parts. The core difference between standards is purely cosmetic: IEEE 315 uses rectangles for all resistors, while IEC 60617 uses zigzags.
| Component Type | IEEE/ANSI (US) Symbol | IEC 60617 (EU) Symbol | Terminals | Primary Function & Schematic Notation |
|---|---|---|---|---|
| Potentiometer | Rectangle with diagonal arrow pointing to center tap | Zigzag with diagonal arrow pointing to center tap | 3 | Voltage division. Arrow touches the middle of the resistor body. |
| Rheostat | Rectangle with diagonal arrow, no center tap line | Zigzag with diagonal arrow, no center tap line | 2 | Variable current limiting. Arrow crosses the body but connects to only one end. |
| Trimmer (Trimpot) | Potentiometer symbol with a 'T' or perpendicular line at the arrow tail | Potentiometer symbol with a 'T' or perpendicular line at the arrow tail | 3 | Calibration. The 'T' indicates it requires a tool (screwdriver) to adjust. |
| Photoresistor (LDR) | Rectangle in a circle with two inward-pointing arrows | Zigzag in a circle (or just zigzag) with two inward-pointing arrows | 2 | Light-dependent resistance. Inward arrows indicate light entering the device. |
| Thermistor (NTC/PTC) | Rectangle with a diagonal line through it ending in a 't°' or '-t°' | Zigzag with a diagonal line through it ending in a 't°' or '-t°' | 2 | Temperature-dependent. '-t°' denotes NTC (resistance drops as temp rises). |
| Varistor (MOV) | Rectangle with a diagonal line crossing it, labeled 'U' or 'V' (non-linear) | Zigzag with a diagonal line crossing it, labeled 'U' or 'V' | 2 | Voltage-dependent. Clamps transient spikes. Often drawn with a bent 'knee' line. |
Regional Standards: IEEE 315 vs. IEC 60617
The divergence in resistor symbology stems from two primary drafting standards. In North America, the IEEE 315 / ANSI Y32.2 standard dictates that all resistors—including variable types—are drawn as rectangular blocks. This was adopted to make automated schematic tracing and early CAD rendering easier, as drawing perfect zigzags at varying zoom levels caused visual clutter.
Conversely, the international IEC 60617 standard retains the traditional zigzag line for resistors. If you are reading legacy European equipment manuals (like older Philips or Siemens audio gear), you will exclusively see zigzags. Modern EDA tools like KiCad and Altium Designer allow you to toggle between IEEE and IEC symbol libraries globally, but when reverse-engineering paper schematics or PDFs from manufacturers, you must recognize both geometries as functionally identical.
Rows People Get Wrong (and Costly Miswiring Risks)
Misinterpreting a variable resistance symbol usually results in a non-functional circuit or, in power applications, a destroyed component. Here are the most common schematic misreads:
1. Confusing the Potentiometer (3-pin) with the Rheostat (2-pin)
A potentiometer symbol explicitly shows three connection points: two outer stators and one wiper. A rheostat symbol shows only two. The Mistake: Designers often use a 3-pin potentiometer in a circuit that calls for a 2-pin rheostat, leaving the third pin floating. If the wiper loses physical contact with the carbon track due to vibration or dirt, the circuit becomes an open loop. The Fix: When substituting a pot for a rheostat symbol, always jumper the wiper pin to the unused outer lug. This ensures the maximum resistance is maintained rather than an open circuit if the wiper lifts.
2. Misidentifying the Varistor (MOV) as a Potentiometer
Because both symbols feature a resistor body with a diagonal line, beginners sometimes confuse a Metal Oxide Varistor (MOV) with a variable resistor. The Mistake: Soldering a 10kΩ trimpot into an AC mains snubber circuit meant for an MOV. The trimpot will instantly vaporize under a 120V/240V transient spike. The Fix: Look for the 'U' or 'V' notation, or the bent 'knee' line near the symbol, which explicitly denotes non-linear voltage clamping.
3. Ignoring the Trimmer 'T' Notation
If the arrow on the potentiometer symbol has a small perpendicular bar or a 'T' at its tail, it indicates a factory-calibration trimmer, not a user-facing knob. Using a standard panel-mount Bourns potentiometer here will ruin the physical enclosure design, as trimmers are meant to be buried inside the PCB housing.
Component Decision Tree: From Schematic to Workbench
Use this decision path to translate the schematic symbol into a concrete, orderable part number. Do not guess based on resistance value alone; the physical construction (carbon vs. wirewound vs. cermet) dictates the component's survival in your specific circuit.
| If Your Schematic Shows... | And The Application Is... | Select This Exact Component Type & Example Part |
|---|---|---|
| 3-Terminal Potentiometer | User-facing audio volume or analog sensor scaling (low power, <0.5W) | Carbon Track Panel Pot: Bourns PTV09A-4025F-B103 (10kΩ, Audio Taper). Use audio taper (logarithmic) for human hearing applications. |
| 3-Terminal Trimmer ('T') | One-time PCB calibration, op-amp offset nulling, or voltage reference trimming | Cermet Multi-Turn Trimmer: Bourns 3296W-1-103LF (10kΩ, 25-turn). The 25-turn gearing allows micro-adjustments without overshooting the target voltage. |
| 2-Terminal Rheostat | Manual motor speed control, high-current bench power supply limiting (>1W dissipation) | Wirewound Power Rheostat: Vishay 534B1 Series (e.g., 534B1103JC, 10kΩ 2W). Carbon track pots will burn open under high current; you must use wirewound. |
| LDR (Inward Arrows) | Automatic night-light switching or ambient light lux measurement | Cadmium-Sulfide Photoresistor: Advanced Photonix VT90N2 (Dark: 1MΩ, Light: 2kΩ). Note: RoHS restrictions are phasing out CdS; consider ALS-PT19 ambient light sensors for new mass production. |
| NTC Thermistor (-t°) | Inrush current limiting for SMPS or 3D printer hotend temperature reading | Glass Bead NTC: EPCOS B57560G104F (100kΩ, 1% tolerance). Glass encapsulation prevents moisture drift at high temperatures. |
Safe Interpretation When Schematics Are Faded or Unmarked
When repairing vintage gear or working with poorly reproduced PDFs, the diagonal arrow on a variable resistance symbol might be faded, making it impossible to tell if the schematic calls for a 2-pin rheostat or a 3-pin potentiometer. Never guess and apply power.
The DMM Wiper Test: To determine the physical pinout of an unmarked 3-terminal variable resistor on your bench, set your digital multimeter to the Ohms (Ω) range. Place one probe on the suspected wiper (usually the middle physical pin) and the other probe on an outer pin. Rotate the shaft. If the resistance changes smoothly from near-zero to the stated maximum, you have found the wiper. To confirm, keep the first probe on the wiper and move the second probe to the other outer pin. Rotate the shaft again. The resistance must change inversely (as one goes up, the other goes down). If you measure a fixed resistance between two pins regardless of shaft position, those are the two outer stator lugs, and the remaining pin is the wiper.
