A coil symbol on an electrical schematic represents a component that relies on electromagnetism. This includes passive components that store energy in a magnetic field (inductors and chokes) and electromechanical devices that convert electrical energy into physical movement (relays, contactors, and solenoids). Because a single physical property spans multiple component categories, the exact schematic symbol varies based on core material, mechanical function, and the drafting standard used by the engineer.
Complete Coil Symbol Reference (IEC & ANSI)
The table below maps the most common coil symbols to their physical counterparts. Read the IEC 60617 column for modern global and European schematics, and the IEEE/ANSI Y32.2 column for legacy North American industrial prints. Physical resistance values are provided as a bench-testing baseline for 24V DC variants.
| Component Type | IEC 60617 Symbol | ANSI/IEEE 315 Symbol | Typical Physical Part & 24VDC Coil Resistance |
|---|---|---|---|
| Inductor (Air Core) | Four contiguous half-loops (scallops) | Four contiguous half-loops | Murata 150uH RF choke (N/A - measured in mΩ) |
| Inductor (Iron/Ferrite Core) | Four half-loops with a solid straight line directly beneath | Four half-loops with a solid straight line beneath | Hammond 153 series filter choke (~1.2Ω DC resistance) |
| Variable Inductor | Four half-loops with a diagonal arrow crossing through them | Four half-loops with an arrow pointing at the center | Tunable IF transformer (adjustable ferrite slug) |
| Relay Coil (General) | Plain rectangle (Designated 'K' or 'KA') | Circle or single loop (Designated 'CR' for Control Relay) | Finder 55.34 14-pin (Approx. 1350Ω) |
| Contactor Coil | Rectangle with 'KM' inside or adjacent | Circle with 'M' or '1M' inside | Schneider TeSys LC1D09 (Approx. 14Ω for AC, varies by VA) |
| Solenoid Valve | Rectangle with a diagonal line/arrow indicating mechanical actuation | Circle with a specific valve actuator attachment symbol | ASCO 2-way brass valve (Approx. 48Ω) |
| Transformer Winding | Two sets of four half-loops facing each other, separated by core lines | Two overlapping circles or adjacent loop sets | Hammond 165 series isolation transformer primary |
Rows People Get Wrong (And How to Fix Them)
Even experienced technicians misinterpret specific coil symbols when transitioning from theory to the workbench. Here are the most common schematic-to-reality translation errors and how to resolve them.
1. Confusing a Relay Coil with a Contactor Coil
In IEC schematics, both look like simple rectangles. The distinction lies in the alphanumeric designation. A relay is labeled 'K' (or 'KA' for auxiliary relays) and is designed for logic-level switching, typically under 10A. A contactor is labeled 'KM' and is built to switch heavy motor loads, featuring arc chutes and higher mechanical endurance. If you install a 10A Finder relay in a circuit designated for a 'KM' contactor, the relay contacts will weld shut under motor inrush current.
2. Misidentifying Iron-Core vs. Air-Core Inductors in Power Supplies
Missing the solid line beneath the inductor loops is a critical error in switch-mode power supply (SMPS) design. An air-core inductor will not saturate at high currents, but it requires vastly more turns (and thus higher parasitic resistance) to achieve the same inductance as an iron-powder or ferrite core. If a schematic specifies an iron-core choke and you substitute an air-core coil of the same uH rating, the physical size and DC resistance will cause excessive voltage drop and thermal failure.
3. Safe Interpretation When Markings are Faded or Missing
In legacy industrial panels, you will frequently encounter 14-pin octal relays with completely faded coil voltage stamps and no accessible schematic. Never apply 120V AC to an unverified coil. Instead, use a multimeter to measure the DC resistance across the coil pins (typically pins 13 and 14 on a standard 14-pin base).
If your meter reads between 600Ω and 1500Ω, it is almost certainly a 24V DC coil. If it reads between 2,000Ω and 10,000Ω, it is likely a 120V AC coil. If the meter reads 'OL' (open loop), the internal coil wire has snapped or the thermal fuse inside the coil bobbin has blown. If it reads less than 50Ω on a standard 14-pin relay, the coil is shorted and will trip your power supply's overcurrent protection immediately upon energization.
Regional Standards: Which One Applies to You?
Schematic symbols are not universal. The drafting standard used depends on the region, the age of the facility, and the origin of the machinery.
- IEC 60617 (International / Europe / Modern Global): This is the current standard for almost all new equipment globally. It favors abstract, rectangular symbols for electromechanical coils. If you are working on modern PLC panels, European-imported CNC machines, or designing new circuits, use IEC symbols. Reference the IEEE 315 standard overview for cross-referencing legacy symbols to modern equivalents.
- IEEE 315 / ANSI Y32.2 (North America / Legacy): While modern US engineering firms are largely adopting IEC for international compatibility, older ANSI standards are still prevalent in legacy US manufacturing plants, military documentation, and older HVAC schematics. ANSI relies heavily on literal representations (drawing actual wire loops for inductors and circles for relay coils). You can review foundational inductor theory and legacy symbol representations via All About Circuits.
- Old UK BS 3939 (British Legacy): Superseded by IEC in the 1990s, BS 3939 is still found in the documentation of older British-built industrial machinery. It shares many similarities with old ANSI but includes unique variations for relay contacts and coil designations. Treat these schematics as historical artifacts and map them to IEC equivalents before modifying the circuit.
FAQ: Decoding Coil Symbols in the Field
What does a coil symbol with a diagonal arrow through it mean?
A diagonal arrow crossing through an inductor's loops indicates a variable inductor. In physical components, this is usually achieved via a threaded ferrite slug that can be screwed in or out of the coil's center using a non-magnetic tuning tool. This changes the magnetic permeability of the core, allowing you to fine-tune the inductance. In power electronics, a similar symbol with an arrow pointing to a specific tap on the coil represents a tapped inductor, used in autotransformers or specific boost converter topologies.
How do I identify a shaded-pole motor coil symbol?
A shaded-pole motor is a simple, single-phase AC motor commonly found in bathroom exhaust fans and small desk fans. On a schematic, its coil symbol looks like a standard inductor or motor winding, but it will feature a smaller, secondary loop or a thick shorted ring drawn adjacent to one side of the main coil poles. This secondary loop represents the copper shading coil that creates the phase shift necessary to start the motor. If you see this symbol, expect a low starting torque and a permanent, non-reversible rotation direction.
Why does my relay coil symbol have a box with 'K1' inside it?
In IEC 60617 nomenclature, the letter 'K' designates a relay or contactor coil. The number following it (e.g., K1, K2, K14) is the unique component identifier for that specific schematic. This allows the drafter to separate the coil from its associated contacts. The coil might be drawn on page 3 of the schematic in the power supply section, while its normally-open (NO) contacts are drawn on page 12 in the motor control section. Both will be labeled 'K1' to tell the technician they belong to the same physical component. For deeper component theory, consult Electronics Tutorials.
What is the difference between a coil symbol and a resistor symbol?
While a resistor restricts current flow and dissipates energy as heat (symbolized by a zigzag line in ANSI or a plain rectangle in IEC), a coil stores energy in a magnetic field and opposes changes in current. On a schematic, if you see a zigzag line, you are looking at a resistor. If you see contiguous half-loops (scallops) or a designated rectangle with a 'K' or 'L' prefix, you are looking at a coil. Mixing these up during troubleshooting will lead to incorrect assumptions about circuit behavior, particularly regarding inrush current and flyback voltage spikes when the circuit is de-energized.
