Inductor symbols represent components that store energy in a magnetic field, but the exact loops, lines, and arrows used on a schematic depend heavily on the governing standard. Whether you are reading a US-based IEEE 315 diagram or an international IEC 60617 print, the baseline representation is a series of half-circles (loops). Below is the complete reference for identifying these components in practice.
The Complete Inductor Symbol Reference Table
This table maps the physical component type to its standard schematic representations. Use this as your primary bench reference when tracing circuits or designing PCB footprints.
| Component Type | IEEE 315 (US/ANSI) Symbol | IEC 60617 (International) Symbol | Practical Application & Notes |
|---|---|---|---|
| Air Core Inductor | Continuous half-loops (no adjacent line) | Continuous half-loops or simple rectangle (S00542) | High-frequency RF circuits, VHF/UHF antennas. Low inductance, high Q-factor. |
| Ferrite / Iron Core Inductor | Half-loops with a solid straight line above/below | Half-loops with a solid straight line | Switch-mode power supplies (SMPS), buck/boost converters. E.g., Bourns SRP1260 series. |
| Powdered Iron Core | Half-loops with a dashed straight line | Half-loops with a solid line (often noted in text) | RF tuning, EMI filtering. Dashed line is an IEEE-specific distinction rarely seen in modern IEC prints. |
| Tapped Inductor | Loops with a perpendicular line and connection point | Loops with a perpendicular tap line | Impedance matching networks, autotransformers, multi-voltage output chokes. |
| Variable Inductor | Loops with a diagonal arrow crossing through | Loops with a diagonal arrow | Tunable RF filters, vintage radio IF transformers. Arrow indicates adjustable core or sliding contact. |
| Coupled Inductors (Common Mode Choke) | Two parallel sets of loops, often with phase dots | Two parallel sets of loops, sometimes enclosed in a box | EMI suppression on data/power lines. E.g., TDK ACM2012 series. Phase dots dictate winding direction. |
| Saturable Reactor | Loops with a solid line and a separate DC control winding | Similar to IEEE, with specific core saturation notations | Magnetic amplifiers, high-power AC control. The DC winding controls the AC impedance. |
Regional Standards: IEEE vs. IEC vs. Legacy UK
When interpreting electrical schematic symbols, knowing the origin of the drawing prevents costly wiring errors. The two dominant standards are IEEE 315 (ANSI Y32.2) and IEC 60617.
IEEE 315 (North America)
Dominant in the US and Canada, IEEE 315 relies heavily on pictorial representations. An inductor is always drawn as a series of semi-circles (loops). The core material is indicated by the line adjacent to the loops: solid for magnetic (iron/ferrite), dashed for powdered iron, and no line for air. This standard is highly visual, making it easy to read at a glance but physically larger on dense schematic sheets.
IEC 60617 (International / Europe / Asia)
The IEC symbol database standardizes schematic drawing globally. While IEC 60617 accepts the loop representation for inductors, it also permits a simple rectangle with the letter 'L' inside or adjacent to it (Symbol S00542) to represent a generic inductor coil. This rectangular style saves massive amounts of space on complex SMPS schematics but forces the reader to rely on the Bill of Materials (BOM) to determine the core type.
Legacy UK (BS 3939)
Older British schematics (pre-1980s) sometimes used a solid filled rectangle or a zig-zag line to denote chokes and inductors. If you are servicing vintage audio equipment or old industrial relay panels in the UK, do not confuse a BS 3939 zig-zag inductor with an IEEE resistor symbol.
Rows People Get Wrong (and Faded Marking Workarounds)
Mistake 1: Coupled Inductors vs. Transformers
The symbol for a coupled inductor (like a common-mode choke) and a standard isolation transformer look nearly identical—both feature two sets of loops. The distinction lies in the phase dots and the core line. A transformer symbol typically includes two parallel lines between the windings (indicating a laminated iron core) or a single line (ferrite), and the dots indicate voltage polarity. A common-mode choke symbol often omits the core lines entirely or places both windings on the same side of a single core line, with dots indicating current direction for magnetic flux cancellation. If the schematic lacks dots, check the BOM: part numbers starting with 'TCM' or 'ACM' are chokes; 'EE' or 'EF' cores are usually transformers.
Mistake 2: Air Core vs. Ferrite Core in IEC Prints
Because IEC 60617 allows a simple rectangle for all inductors, designers frequently omit the core type from the symbol, leaving it entirely to the BOM. If you are reverse-engineering a board from an IEC schematic, do not assume an air core just because the straight line is missing. Look at the physical component: a drum core or a shielded SMD cylinder (like a Wurth WE-PD) is ferrite/powdered iron. A literal bare copper coil on the PCB or a ceramic SMD chip is air/ceramic core.
Safe Interpretation of Faded Silkscreens and Missing Markings
On older PCBs, the 'L1' or 'L2' silkscreen often flakes off, leaving a two-pin magnetic component unidentified. To safely interpret it:
- Count the pins: Two pins = standard inductor. Four pins = likely a common-mode choke or a tapped inductor. Five+ pins = almost certainly a transformer.
- Measure DC Resistance (DCR): Use a multimeter. A standard power inductor (e.g., 10µH to 100µH) will read near 0Ω (typically 0.01Ω to 0.1Ω). If it reads open (OL), the internal wire is broken.
- Verify with an LCR Meter: A standard multimeter cannot measure inductance. Use a dedicated LCR meter (like the DER EE DE-5000 or Keysight U1733C) set to 1kHz or 100kHz. If it measures in the µH range, it is an inductor. If it measures in the mH range on a small SMD footprint, it is likely a common-mode choke.
Frequently Asked Questions
What is the difference between an inductor and a choke symbol?
Electrically, a choke is just an inductor designed specifically to block (choke) high-frequency AC while passing DC. In IEEE 315, there is no distinct symbol for a choke; it uses the standard inductor loops, sometimes enclosed in a box to denote a shielded component. In older schematics or specific military specs, you might see a choke drawn with a box around the loops or labeled explicitly as 'RFC' (Radio Frequency Choke) or 'CMC' (Common Mode Choke) next to the standard symbol. Always rely on the reference designator (L vs. T) and the BOM value to confirm its function.
How do you read inductor color codes if the schematic symbol is missing?
Through-hole molded inductors (like the Vishay IHLP or legacy RFC series) often use a 4-band color code identical to resistors, but the base unit is microhenries (µH), not ohms.
Example: An inductor with bands Brown - Black - Brown - Silver.
• Band 1 (Brown) = 1
• Band 2 (Black) = 0
• Band 3 (Brown Multiplier) = 10¹ (10)
• Band 4 (Silver Tolerance) = ±10%
• Result: 10 × 10 = 100 µH (±10%).
Note: Some RF inductors use a 3-dot system where the position of the dot on the body indicates the decimal point. Always cross-reference with an LCR meter if the physical markings are ambiguous.
Why does my inductor symbol have a box or shield around it?
A box drawn around an inductor symbol (or a dashed line enclosing the loops) indicates a magnetically shielded inductor. Unshielded inductors (like drum core types) leak significant magnetic flux, which can induce noise into adjacent sensitive traces or Hall-effect sensors. Shielded inductors (like the Coilcraft MSS1210 series) contain the flux within a ferrite sleeve or enclosed core. If your schematic shows a shielded symbol, you must use a shielded physical component to pass EMI/EMC compliance testing; substituting an unshielded part will likely cause radiated emissions failures.
