If you are asking what is the symbol of a resistor, the direct answer depends on your regional drafting standard. In the United States, the standard symbol for a fixed resistor is a zigzag line (IEEE/ANSI standard). In Europe and most of the rest of the world, the standard symbol is a simple rectangular box (IEC 60617 standard). Both symbols represent the exact same passive, two-terminal component that restricts current flow and drops voltage according to Ohm's Law.
The Complete Resistor Symbol Reference Table
Before designing a printed circuit board (PCB) or troubleshooting a schematic, you must identify the specific type of resistor being used. The table below maps the component type to both major drafting standards, alongside the standard reference designator (RefDes) and practical application.
| Component Type | IEEE/ANSI Symbol (US) | IEC 60617 Symbol (Global) | RefDes | Practical Use Case |
|---|---|---|---|---|
| Fixed Resistor | Zigzag line | Hollow rectangle | R | Current limiting, pull-up/pull-down, voltage division |
| Variable (Potentiometer) | Zigzag with 3rd terminal arrow | Rectangle with 3rd terminal arrow | VR / POT | User-adjustable voltage dividers (volume knobs, dimmers) |
| Variable (Rheostat) | Zigzag with 2 terminals, arrow across | Rectangle with 2 terminals, arrow across | RH | Variable current limiting (motor speed control) |
| Preset / Trimmer | Zigzag with T-bar adjustment arrow | Rectangle with T-bar adjustment arrow | TR / RV | Factory calibration, one-time circuit tuning |
| NTC Thermistor | Zigzag with diagonal line (flat bottom) | Rectangle with diagonal line (flat bottom) | TH / RT | Inrush current limiting, temperature sensing |
| PTC Thermistor | Zigzag with diagonal line (pointed up) | Rectangle with diagonal line (pointed up) | TH / RT | Resettable fuses, overcurrent protection |
| Photoresistor (LDR) | Zigzag in a circle with inward arrows | Rectangle in a circle with inward arrows | LDR / PR | Light-activated switches, streetlight controllers |
Regional Standards: IEEE (US) vs IEC (Global)
The divergence in resistor symbols stems from two distinct governing bodies. In North America, schematic symbols are governed by IEEE Std 315 (formerly ANSI Y32.2). This standard popularized the zigzag line, which visually mimics the physical wire-wound resistors used in the early 20th century. According to All About Circuits, the zigzag remains the default in almost all US-based university engineering programs and legacy military schematics.
Conversely, the International Electrotechnical Commission (IEC) maintains the IEC 60617 standard. The IEC rejected the zigzag in favor of a simple rectangle to standardize passive component drawing across international borders and to make automated schematic parsing easier for early CAD software. The IEC symbolic database enforces this rectangular geometry for all basic passive components, differentiating them only by internal markings or external terminals.
Modern ECAD tools like KiCad 8 and Altium Designer 24 allow you to toggle between IEEE and IEC symbol libraries. Never mix them on the same schematic sheet. Mixing a zigzag fixed resistor with an IEC rectangular capacitor will cause confusion during design reviews and may lead to netlist errors if the fab house uses automated optical inspection (AOI) software calibrated to a single standard.
Which Standard Should You Choose?
- Choose IEEE (Zigzag): If you are designing for US-based manufacturing, submitting to US military/aerospace contractors, or collaborating with North American engineers who learned on legacy OrCAD libraries.
- Choose IEC (Rectangle): If you are designing for European or Asian manufacturing, releasing open-source hardware to a global audience, or working in an environment where IEC 60617 is mandated by corporate policy.
Rows People Get Wrong: Common Schematic Mistakes
Even experienced drafters make specific errors when drawing advanced resistor symbols. Here are the most common mistakes and how to correct them.
1. Potentiometer vs. Rheostat Terminal Count
A potentiometer is a three-terminal device used as a voltage divider. The schematic symbol must show the two end terminals and a third 'wiper' terminal with an arrow pointing at the resistive element. A rheostat is a two-terminal device used for variable resistance. Drafters frequently draw a rheostat symbol but accidentally leave the third terminal connected to the wiper, effectively shorting one half of the resistive track and altering the circuit's taper.
2. NTC vs. PTC Thermistor Directionality
The thermistor symbol features a diagonal line crossing through the resistor body. The direction of this line dictates the temperature coefficient. If the diagonal line has a small horizontal 'flat' at the bottom (pointing down-left), it is an NTC (Negative Temperature Coefficient) thermistor—resistance drops as heat rises. If the line points sharply upward without a flat, it is a PTC (Positive Temperature Coefficient) thermistor. Drawing the arrow backward will cause a PCB designer to source the wrong protective component, potentially leading to a thermal runaway event.
3. The Tapped Resistor Confusion
A tapped resistor has a fixed, factory-set connection point along its resistive element, unlike a variable resistor where the wiper moves. The IEEE symbol for a tapped resistor shows a fixed line connecting to the zigzag, without an arrowhead. Drafters often mistakenly add an arrowhead, incorrectly implying the tap is adjustable by the end-user.
Safe Interpretation When Physical Markings Are Faded or Missing
Schematics are only half the battle. When troubleshooting a legacy board, you will frequently encounter physical resistors where the color bands are burned off, or SMD (Surface Mount Device) resistors where the silkscreen has rubbed away. Here is how to safely bridge the gap between a faded physical component and the schematic symbol.
Decoding Unreadable SMD Resistors
If the schematic calls for a specific value but the physical SMD resistor is blank or damaged, you must verify the footprint and surrounding circuit topology. SMD resistors typically use three coding systems:
- 3-Digit Code: First two digits are significant figures, third is the multiplier (e.g.,
103= 10 × 10³ = 10,000Ω or 10kΩ). - 4-Digit Code: First three digits are significant figures, fourth is the multiplier (e.g.,
1002= 100 × 10² = 10,000Ω or 10kΩ). - EIA-96 Code: Two numbers and a letter used for 1% tolerance parts (e.g.,
01C= 10kΩ). You must reference an EIA-96 lookup chart for these.
The 'Lift and Measure' Protocol
If the physical markings are entirely gone and you need to verify if the installed part matches the schematic's R14 designator, never measure resistance in-circuit while the board is powered. Furthermore, measuring in-circuit while powered down will yield false low readings because the multimeter's test current will flow through parallel IC pins and capacitors.
Follow this bench procedure:
- De-energize the board and disconnect all power sources, including backup CMOS batteries.
- Discharge large electrolytic capacitors using a high-wattage bleed resistor (e.g., 1kΩ 5W) to prevent meter damage or shock.
- Use a soldering iron to desolder and 'lift' one leg of the through-hole resistor, or completely remove the SMD resistor from the pads.
- Set your multimeter to the appropriate resistance range. For values under 10Ω, use Kelvin (4-wire) clips if your meter supports them to eliminate test lead resistance.
- Compare the measured value against the schematic. If the measured value is >5% outside the schematic's stated tolerance (e.g., measuring 12kΩ on a schematic calling for a 10kΩ 5% resistor), the component has suffered thermal drift and must be replaced.
Understanding what is the symbol of a resistor on paper is only the first step. True proficiency requires mapping those IEEE and IEC symbols to the physical realities of the workbench, accounting for regional standards, and knowing exactly how to verify the component when the silkscreen fails.
