Complete Inductor Schematic Symbol Reference Table
This table covers the most common inductor variants you will encounter in both modern CAD software (like Altium or KiCad) and legacy paper schematics.
| Inductor Type | IEEE/ANSI Symbol (US) | IEC 60617 Symbol (Intl) | Core Material | Primary Application |
|---|---|---|---|---|
| Air Core | 4 adjacent semi-circles, no underlying line | Single loop or rectangle with no core modifier | Air / Non-magnetic ceramic | High-frequency RF, tuning circuits |
| Iron Core | 4 semi-circles with a solid straight line underneath | Rectangle with a solid straight line on one side | Laminated iron / Steel | Audio crossovers, 50/60Hz mains filtering |
| Ferrite Core | 4 semi-circles with a dashed (broken) line underneath | Rectangle with a dashed line on one side | Ferrite (iron oxide ceramic) | Switch-mode power supplies (SMPS), EMI chokes |
| Tapped Inductor | Semi-circles with a vertical line extending from the middle loop | Rectangle with a tap line extending from the center | Varies (usually ferrite) | Autotransformers, impedance matching networks |
| Variable Inductor | Semi-circles with an arrow striking diagonally through the loops | Rectangle with an arrow striking diagonally through it | Ferrite or brass (for tuning) | Radio tuning, variable oscillators |
| Shielded Inductor | Standard symbol enclosed in a dashed or solid box (often omitted in modern CAD) | Standard symbol enclosed in a box | Ferrite with metallic shield can | High-density DC-DC converters, noise-sensitive environments |
Regional Standards: IEEE/ANSI vs. IEC Variants
Which standard applies to you depends on your region and the origin of the schematic you are reading. In the United States and Canada, schematics almost exclusively follow IEEE 315 / ANSI Y32.2 standards. This is the 'loopy' semi-circle style that most American-trained engineers and hobbyists recognize instantly.
Internationally, and in most modern European and Asian datasheets, the IEC 60617 standard dominates. IEC favors geometric simplicity: inductors are represented by a simple rectangle or a single loop, with core materials indicated by lines placed adjacent to the shape rather than running underneath loops. If you are reading a schematic from a European manufacturer like STMicroelectronics or Infineon, expect IEC rectangles. If you are reading a classic US military schematic or an older Arduino shield design, expect IEEE loops.
Rows People Get Wrong (And How to Fix Them)
Even experienced engineers misinterpret specific rows in the reference table, leading to blown MOSFETs or failed EMC certifications. Here are the most common mistakes:
- Iron vs. Ferrite Core Confusion: In older US schematics (pre-1990s), drafters sometimes used a solid line for both iron and ferrite cores, or omitted the dashes due to pen-and-ink limitations. If you assume a solid line means low-frequency iron, but the circuit is a 500kHz buck converter, you will specify the wrong part. Fix: Look at the switching frequency. If it is above 20kHz, the core must be ferrite or powdered iron, regardless of the solid line.
- The Missing Shield Box: Modern CAD libraries frequently drop the 'box' that denotes a shielded inductor to save schematic space. I've seen junior engineers blow up a $15 switching regulator because they mistook an unshielded drum core symbol for a shielded one, and the radiated EMI tripped the sensitive feedback pin. Fix: Never assume an inductor is shielded unless the BOM explicitly specifies a shielded series (e.g., Coilcraft MSS or Wurth WE-PD).
- Variable vs. Tapped: A variable inductor has an arrow that physically moves the core (changing inductance). A tapped inductor has a fixed wire connection. Swapping these in a resonant circuit will either lock your tuning range or short a winding.
Decision Tree: Selecting the Right Inductor for Your Build
Use this decision path to translate the schematic symbol into a physical component purchase. Follow the if-then logic to arrive at your concrete pick.
| Circuit Requirement | Schematic Symbol Clue | Core & Style Selection | Concrete Default Pick (MPN) |
|---|---|---|---|
| RF filtering, >100 MHz, low current | Air core (no line) or variable | Air core / Ceramic, unshielded, 0402 or 0603 package | Coilcraft 0603CS Series |
| Audio crossover, high current, <5 kHz | Iron core (solid line) | Laminated silicon steel or large gauge air-core wire | Erse Audio Super Q Coil |
| General DC-DC Buck/Boost (100kHz - 2MHz) | Ferrite core (dashed line) | Ferrite drum core, unshielded (if EMI isn't strict) | Coilcraft DO1608C Series |
| High-density DC-DC, noise-sensitive, >2A | Shielded Ferrite (box around symbol) | Shielded ferrite, molded construction | Wurth Elektronik WE-PD Series (e.g., 74477420) |
Real-World Verification When Markings Fade
What do you do when you are repairing a legacy board, the schematic is missing, and the physical markings on the inductor are faded or completely absent? You need a safe interpretation method to determine the core material and approximate value without desoldering the component.
First, use an LCR meter. Set it to measure inductance (L) at 100 kHz with a 1V RMS test signal. Take your baseline reading. Next, bring a strong neodymium magnet close to the inductor. If the inductance reading spikes wildly, drops significantly, or the LCR meter throws an error, you are dealing with a magnetic core (ferrite or powdered iron) that is being influenced by the external magnetic field. If the reading remains perfectly stable, it is likely an air-core or non-magnetic ceramic inductor.
For a deeper dive into calculating the exact saturation limits of unmarked ferrite cores on the bench, consult the Coilcraft inductor basics library or standard inductor theory tutorials. By combining the visual schematic symbol with bench-level LCR verification, you eliminate the guesswork and ensure your power stages survive their first power-on cycle.
