The base electrical inductor symbol is a series of four connected semi-circles (loops) under the US IEEE 315 standard, or a single semi-circle or rectangle under the international IEC 60617 standard. Core materials, taps, and adjustability are indicated by adjacent straight lines or diagonal arrows. If you are reading a schematic to source a physical component, the exact modifier on that symbol dictates whether you need a $0.05 surface-mount ferrite bead or a $12.00 shielded power drum core.
The Complete Electrical Inductor Symbol Reference Table
Use this table to translate schematic symbols into physical component requirements. The table covers the most common variants you will encounter in modern power supply, RF, and audio schematics.
| Component Type | IEEE 315 (US) Symbol | IEC 60617 (Intl) Symbol | Practical Meaning & Application |
|---|---|---|---|
| Basic / Air Core | Four connected semi-circles (humps) | Single semi-circle or empty rectangle | No magnetic core. Used in high-frequency RF circuits where core losses would degrade the Q factor. |
| Iron / Ferrite Core | Four humps with a solid straight line above/below | Single hump/rectangle with a solid straight line | Magnetic core increases inductance. Used in power filtering, chokes, and DC-DC converters. |
| Tapped Inductor | Core inductor with a perpendicular line intersecting the loops | Rectangle with a perpendicular tap line | Provides an intermediate voltage/inductance point. Common in autotransformers and impedance matching networks. |
| Variable Inductor | Core inductor with a diagonal arrow passing through the loops | Rectangle with a diagonal arrow passing through | Adjustable core (usually a threaded ferrite slug). Used for tuning RF tank circuits and IF filters. |
| Saturable Core | Core inductor with a secondary control winding symbol | Rectangle with a secondary control loop | Inductance is controlled by a DC bias current. Used in magnetic amplifiers and specialized lighting ballasts. |
| Coupled / Transformer | Two parallel sets of humps, often with core lines | Two parallel rectangles with core lines | Two or more inductors sharing a magnetic path. Used for isolation, step-up/step-down, and flyback converters. |
Regional Standards: IEEE 315 vs. IEC 60617 in Practice
The primary visual difference between the two dominant standards is the number of loops. The North American IEEE 315 standard relies on the 'four hump' representation, which visually mimics the physical winding of a coil. The international IEC 60617 standard simplifies this to a single semi-circle or a plain rectangle, relying entirely on the adjacent modifier lines to convey meaning.
In modern EDA tools like Altium Designer or KiCad, you can usually toggle between these standards in the symbol library settings. However, when reading legacy schematics or imported PDFs, you must identify the standard first. If you see a single semi-circle with a solid line above it, do not assume it is a capacitor or a basic coil; under IEC rules, that is a standard iron-core inductor. Conversely, a rectangle with no internal markings and a straight line above it is an IEC iron-core inductor, not a resistor.
Rows People Get Wrong: Misread Symbols & Faded Markings
Even experienced bench technicians misinterpret specific symbol variants, leading to incorrect part orders and circuit failures.
- Tapped Inductor vs. Autotransformer: A tapped inductor (Row 3) and an autotransformer share the exact same schematic symbol. The difference is purely functional and contextual. If the symbol is in an RF matching network, it is a tapped inductor (single winding, partial coupling). If it is in a mains voltage step-down circuit, it is an autotransformer. Always check the voltage ratings on the BOM before sourcing.
- Variable Inductor vs. Potentiometer: The diagonal arrow is the universal symbol for 'variable'. However, on a variable inductor, the arrow passes through the middle of the coil loops, indicating a moving core. On a potentiometer or variable resistor, the arrow points at the middle of the zig-zag line or rectangle, indicating a moving wiper contact. Mixing these up will result in ordering a trimmer pot when you need a tunable coil.
- Saturable Core vs. Standard Transformer: Row 5 looks remarkably like a standard two-winding transformer. The distinction is that a saturable core reactor's secondary winding is a DC control winding meant to intentionally drive the core into magnetic saturation, altering the AC impedance of the main winding. Do not substitute a standard isolation transformer here; it will saturate unpredictably and overheat.
Physical Markings: Safe Interpretation When Schematics Fail
When you are reverse-engineering a board or replacing a damaged component without a schematic, you must rely on physical markings. Through-hole molded inductors often use the EIA color code, which is identical to the resistor color code but read in microhenries (µH) instead of ohms. For example, a brown-black-black-silver banded inductor is 10 µH with a 10% tolerance.
Surface-mount (SMD) power inductors are notoriously difficult to identify. Many shielded SMD inductors have zero markings. Others use a cryptic 3-digit code. Unlike capacitors, where '103' means 10,000 pF, an inductor marked '100' typically means 10 µH (10 followed by zero zeros). An inductor marked '4R7' means 4.7 µH.
Never guess an unmarked SMD inductor's value based on physical size alone. Connect it to a bench LCR meter. For power supply inductors, test at 1 kHz with a 1Vrms test signal. For RF inductors, you must test at 100 kHz or higher to get an accurate reading, as low-frequency testing will not overcome the parasitic capacitance of the windings. Always measure the DCR (DC Resistance) as well; if the DCR is open (OL), the internal wire bond has failed due to thermal stress.
Decision Path: From Schematic Symbol to Concrete Part Number
Use this decision tree to translate the schematic symbol directly into a specific, purchasable component family. Do not default to generic 'inductors'; the core material and shielding dictate the part.
| Schematic Symbol & Context | Required Physical Characteristics | Concrete Part Recommendation |
|---|---|---|
| Air Core (No line) + RF circuit (>50MHz) | High Q factor, ceramic core, tight tolerance (±2%), SMD 0402 or 0603. | Coilcraft 0402HP-10NX (10nH air/ceramic core RF inductor) |
| Iron Core (Solid line) + DC-DC Buck Converter | High saturation current (Isat), shielded to prevent EMI, low DCR. | Würth Elektronik WE-PD 74477410 (10µH shielded power inductor, 3A Isat) |
| Variable (Arrow through) + IF Filter / Tank Circuit | Threaded ferrite slug, adjustable via non-magnetic tool, PCB mount. | Bourns 3256W Series (Trimmer inductor, specific value per tank frequency) |
| Ferrite Bead (Often drawn as a rectangle on a wire) + Power rail filtering | High impedance at target noise frequency, low DC resistance, rated for rail current. | Murata BLM18PG121SN1D (120Ω @ 100MHz, 2A rated ferrite bead) |
When sourcing the physical part, always verify the Saturation Current (Isat) and RMS Current (Irms) ratings against your circuit's peak and continuous loads. A schematic symbol only tells you the inductance value and core type; the datasheet tells you if the part will survive your specific current profile without saturating and turning into a dead short.
