The schematic symbol of a resistor depends entirely on your regional standard: North America predominantly uses the zig-zag line (IEEE 315/ANSI Y32.2), while the rest of the world uses an empty rectangle (IEC 60617). Both represent the exact same component and behave identically in circuit simulation and physical builds.
The Complete Resistor Schematic Symbol Reference Table
Use this reference table to identify fixed, variable, and specialized resistors on any schematic. The table maps the physical component to both major global drafting standards.
| Component Type | IEEE/ANSI Symbol (North America) | IEC Symbol (Global/EU/UK) | Standard Reference | Practical Application |
|---|---|---|---|---|
| Fixed Resistor | Zig-zag line | Empty rectangle | IEEE 315 / IEC 60617-4 | Current limiting, voltage division, pull-up/pull-down networks. |
| Variable Resistor (Potentiometer) | Zig-zag with 3rd terminal arrow | Rectangle with 3rd terminal arrow | IEEE 315 / IEC 60617-4 | User-adjustable voltage dividers (volume knobs, joystick inputs). |
| Variable Resistor (Rheostat) | Zig-zag with 2 terminals, arrow across | Rectangle with 2 terminals, arrow across | IEEE 315 / IEC 60617-4 | Adjustable current limiting (motor speed controls, dimmer circuits). |
| Trimpot (Preset) | Zig-zag with T-shaped adjustment arrow | Rectangle with T-shaped adjustment arrow | IEEE 315 / IEC 60617-4 | Factory calibration, one-time bias adjustments on PCBs. |
| Thermistor (NTC/PTC) | Zig-zag with diagonal line and 't°' or '-t°' | Rectangle with diagonal line and 't°' | IEC 60617-4 | Temperature sensing, inrush current limiting (NTC), overcurrent protection (PTC). |
| Photoresistor (LDR) | Zig-zag with inward-pointing arrows | Rectangle with inward-pointing arrows | IEEE 315 / IEC 60617-4 | Light-activated switches, ambient light sensing for displays. |
| Varistor (MOV) | Zig-zag with diagonal line and 'V' or 'U' | Rectangle with diagonal line and 'V' | IEC 60617-4 | Transient voltage suppression, surge protection on AC mains inputs. |
| Tapped Resistor | Zig-zag with multiple fixed tap lines | Rectangle with multiple fixed tap lines | IEEE 315 / IEC 60617-4 | High-power voltage division, vintage audio output transformers. |
Regional Standard Variants: IEEE vs. IEC vs. Legacy UK
While the NEC (National Electrical Code) governs physical wiring colors and installation rules in the US, it does not define schematic symbols. Schematic symbols fall under IEEE/ANSI in North America and IEC globally. However, old UK wiring colors and legacy British standards often cause confusion when reading older schematics that mixed conventions.
| Standard | Region | Primary Symbol Style | When You Will Encounter It |
|---|---|---|---|
| IEEE 315 / ANSI Y32.2 | USA, Canada | Zig-zag line | Modern US schematics, legacy American military/aerospace prints, older textbooks. |
| IEC 60617 | EU, UK, Asia, Global | Empty rectangle | Modern global datasheets, European automotive prints, modern UK consumer electronics. |
| BS 3939 (Legacy) | Old UK / Commonwealth | Rectangle (often with internal markings) | Pre-2000 British telecommunications and industrial control schematics. |
| DIN 40700 (Legacy) | Old Germany / EU | Rectangle with specific internal hatching | Vintage European audio equipment, old Siemens/Bosch industrial manuals. |
Which standard applies to you? If you are designing a board in Altium or KiCad in 2026, the default libraries usually provide both. Always use the IEC rectangle for international commercial products, as it scales better on dense, multi-layer PCB schematics and is the globally recognized default. Use the IEEE zig-zag only if your company's internal style guide mandates it or if you are maintaining legacy American documentation.
The 'Rows People Get Wrong' Notes
Even experienced bench technicians misread specific symbol variants. Here are the most common schematic interpretation errors and how to avoid them:
- Potentiometer vs. Rheostat: A potentiometer symbol always shows three connection points (two ends of the resistive track plus the wiper). A rheostat symbol shows only two connection points (one end of the track and the wiper, often with the unused end drawn unconnected or tied to the wiper). Wiring a 3-terminal pot as a 2-terminal rheostat in a high-current path will burn out the wiper track.
- Photoresistor vs. LED Arrows: The arrows on a photoresistor (LDR) point inward toward the symbol, indicating light entering the device. On an LED or photodiode, the arrows point outward, indicating light emitting from the device. Reversing these in your mental model leads to incorrect sensor vs. emitter placement.
- NTC vs. PTC Thermistors: The basic thermistor symbol just shows a temperature-dependent resistor. To distinguish them, look for the negative sign on the diagonal line (
-t°for NTC, resistance drops as heat rises) versus a standardt°or positive slope indicator for PTC (resistance rises with heat). NTCs are for sensing; PTCs are usually for self-resetting fuses. - Varistor (MOV) vs. Thermistor: Both use a diagonal line through the resistor symbol. The varistor has a
VorU(voltage-dependent), while the thermistor has at°(temperature-dependent). Swapping an MOV for an NTC on an AC mains input will result in a catastrophic short circuit and exploded component upon power-up.
Safe Interpretation When Markings Are Faded or Missing
When physical color bands are burnt off, or a schematic is damaged and the symbol is obscured, follow this bench procedure to safely identify and verify the component:
- Visual Context Clues: Look at the physical package. A blue or orange epoxy body usually indicates a metal oxide varistor (MOV). A black epoxy disc is almost always an NTC thermistor. A standard beige/tan cylinder with painted bands is a carbon or metal film fixed resistor.
- Desolder One Leg: To get an accurate reading, use a soldering iron and desoldering wick to lift one leg of the through-hole resistor out of the PCB pad. For SMD resistors, remove the component entirely using hot air.
- Measure and Calculate: Measure the isolated component with a digital multimeter. If the reading is open (OL), the resistor has failed open (common in carbon composition types subjected to voltage spikes). If it reads within 1% to 5% of a standard E24/E96 series value, you have identified it.
- Trace the Circuit: If the component is completely destroyed (charred), trace the copper on the PCB. If it connects directly from a high-voltage rail to a sensitive IC input pin, it is likely a current-limiting or pull-up resistor. Check the IC datasheet for recommended external component values.
For deeper standard references on component drafting, consult the IEC standard symbols database or review the foundational component theory outlined by All About Circuits.
Frequently Asked Questions
What does the schematic symbol of a resistor with an arrow through it mean?
An arrow drawn diagonally through the center of a standard resistor symbol (zig-zag or rectangle) indicates a variable resistor. If the arrow connects to a third terminal, it is a potentiometer (voltage divider). If the arrow simply crosses the body and connects to one of the main terminals, it is a rheostat (variable current limiter). If the arrow has a bar across its end (a T-shape), it represents a factory-adjustable trimpot.
How do I tell the difference between a capacitor and a resistor symbol?
In both IEEE and IEC standards, these are distinctly different. A resistor is a continuous path (zig-zag or rectangle) indicating opposition to current flow. A capacitor symbol always features a break in the line—represented by two parallel plates (IEEE) or one flat plate and one curved plate (IEC, indicating polarity). Current cannot flow continuously through a capacitor symbol, whereas it flows through a resistor symbol.
Why do some European schematics use the American zig-zag resistor symbol?
This usually happens for three reasons: the engineer was trained using older American textbooks, the CAD software (like older versions of Eagle or OrCAD) defaulted to the US library and the user didn't switch it, or the schematic was drawn for a US-based client who mandated IEEE 315 compliance. While IEC 60617 is the official standard in Europe, the zig-zag is universally understood by engineers globally, so it rarely causes functional confusion.
What is the symbol for a surface mount (SMD) resistor?
There is no unique schematic symbol for an SMD resistor. Schematics represent the logical electrical function, not the physical packaging. An SMD 0402 resistor and a massive 50W wirewound chassis-mount resistor will both use the exact same standard fixed resistor symbol (zig-zag or rectangle). The physical footprint (e.g., 'R0402' or 'R2512') is only specified in the PCB layout document and the Bill of Materials (BOM), not on the schematic itself.
