The standard schematic symbol rheostat represents a two-terminal adjustable resistor. Visually, it is a fixed resistor symbol (a zig-zag line in US/ANSI standards, or an empty rectangle in international standards) intersected by a diagonal arrow pointing to the center. That arrow represents the mechanical wiper. Unlike a potentiometer, which uses three terminals to divide voltage, a rheostat uses only two terminals—one end of the resistive track and the wiper—to vary current, act as a variable load, or control motor speed.
While the concept is simple, misinterpreting the symbol on a schematic or miswiring the physical component on the bench leads to open-circuit spikes, runaway loads, and blown driver transistors. Below is the definitive reference for reading, drawing, and wiring rheostat symbols across global standards.
Rheostat Symbol Reference Chart (IEEE vs. IEC)
Schematic symbols are governed by two primary standards: IEEE 315 (ANSI Y32.2), which dominates North American industrial and academic schematics, and IEC 60617, which is the legal standard in the EU, UK, and most international manufacturing. The table below maps every variant of the symbol rheostat you will encounter on a spec sheet or wiring diagram.
| Standard / Region | Symbol Shape | Arrow Style | Terminal Count | Primary Use Case |
|---|---|---|---|---|
| IEEE 315 (US/ANSI) | Zig-Zag Line | Diagonal arrow touching center | 2 (Wiper + 1 End) | Motor speed control, high-current variable loads, legacy US schematics |
| IEC 60617 (EU/Global) | Empty Rectangle | Diagonal arrow touching center | 2 (Wiper + 1 End) | Standard European schematics, IEC-compliant control panels |
| IEEE Trimmer Variant | Zig-Zag Line | Arrow with T-bar or circle tail | 2 (Wiper + 1 End) | PCB-mounted calibration, one-time factory set, bias adjustment |
| IEC Trimmer Variant | Empty Rectangle | Arrow with T-bar or circle tail | 2 (Wiper + 1 End) | SMD/THT trimmer resistors in modern EU consumer electronics |
| Potentiometer (Reference) | Zig-Zag / Rectangle | Arrow pointing completely through | 3 (Wiper + 2 Ends) | Voltage division, audio taper, sensor biasing (NOT a rheostat) |
Source references: IEEE 315 Graphic Symbols Standard and the IEC Symbols Portal.
The Rows People Get Wrong (and How to Fix Them)
Even experienced hobbyists and junior technicians make three specific errors when translating the symbol rheostat from paper to the workbench. Here is how to avoid them.
1. Wiring All Three Terminals
The most common mistake is treating a rheostat symbol like a potentiometer. A physical wirewound rheostat (like an Ohmite 25W panel-mount) often has three physical solder lugs: Pin 1 (End A), Pin 2 (Wiper), and Pin 3 (End B). The schematic symbol explicitly shows only two connections. If you wire all three, you are building a potentiometer (voltage divider), not a rheostat (variable resistor). For a true rheostat configuration, you must connect your circuit only to the Wiper and one of the End pins.
2. The 'Floating' Unused Terminal Trap
When you wire a 3-pin physical part as a 2-pin rheostat, what do you do with the third, unused pin? Leaving it floating is a critical reliability hazard. Carbon tracks and wirewound elements degrade over time. If the wiper momentarily loses physical contact with the resistive track (wiper bounce), a floating configuration results in an instantaneous open circuit. In a motor drive or LED dimmer, an open circuit means infinite resistance, which can cause voltage spikes that destroy your switching MOSFETs.
3. Confusing the Arrow for Current Direction
In schematic reading, arrows on diodes or transistors indicate conventional current flow. The arrow on a symbol rheostat does not indicate current direction. It is purely a mechanical indicator showing that the resistance value is adjustable via a physical wiper. Current can flow bidirectionally through a standard rheostat.
Physical Identification When Schematic Markings Fade
When repairing legacy industrial equipment—such as old Clarostat motor controllers or vintage audio amplifiers—you will frequently encounter physical rheostats where the silkscreen pinout has burned off, or the original schematic is missing entirely. Here is the exact multimeter procedure to safely map a 3-pin physical component to a 2-pin rheostat symbol.
- Isolate the Component: De-energize the circuit and discharge any parallel capacitors. Desolder at least two of the three pins to remove parallel circuit paths that will skew your ohmmeter readings.
- Find the Ends: Set your multimeter to the highest resistance range (e.g., 20kΩ or 2MΩ). Probe Pin 1 to Pin 3. This reading should match the nominal stamped value of the rheostat (e.g., 500Ω). This reading will not change when you turn the shaft.
- Find the Wiper: Move your black probe to Pin 2 (the middle pin). Keep the red probe on Pin 1. Turn the shaft fully counter-clockwise, then fully clockwise. The resistance should sweep from near 0Ω up to the nominal value. Pin 2 is your wiper.
- Map to the Symbol: Connect your circuit's 'hot' or source line to Pin 2 (Wiper). Connect the load line to either Pin 1 or Pin 3. Jumper the remaining unused pin directly to Pin 2 to implement the wiper-bounce protection mentioned above.
For deeper study on component identification and schematic reading, the All About Circuits Reference Textbook provides excellent visual breakdowns of passive component mappings.
Regional Standards and Code Compliance
Knowing which standard applies to your region dictates how you should draw your own schematics and how you interpret imported machinery manuals.
- North America (US/Canada): Governed by IEEE 315 and ANSI Y32.2. You will see the zig-zag line. If you are designing control panels for US industrial clients, use the zig-zag to avoid confusion with IEC terminal blocks or relays.
- Europe, UK, and Asia: Governed by IEC 60617. The rectangle is mandatory. Note that the old UK standard (BS 3939) used a rectangle but sometimes included specific terminal dot conventions that differ from modern IEC. If you are maintaining pre-1990s British machinery, verify if the schematic predates the IEC harmonization.
- Automotive and Aerospace: Often use proprietary internal standards (like Boeing BACW10 or specific OEM wiring manuals) which may depict the rheostat as a simple rectangle with a slash through it, omitting the arrow entirely. Always check the legend box in the bottom right corner of the schematic first.
Ultimately, the symbol rheostat is a functional instruction, not just a drawing. It tells you to limit current, not divide voltage. Respect the two-terminal rule, tie off your floating wipers, and match your drawing standard to your regional AHJ (Authority Having Jurisdiction) requirements to ensure your designs are both safe and universally readable.
