Relay Coil Symbol
Definition: The Relay Coil symbol represents the electromagnetic coil of a relay — drawn as a rectangle (IEC 60617) or a circle/rectangle (ANSI Y32.2 / IEEE 315) — that, when energised by a control voltage applied across terminals A1 and A2, produces a magnetic field to mechanically open or close one or more switch contacts; relay coils are designated K in schematics.
Also known as: relay coil symbol, electromagnet coil symbol, relay solenoid symbol, coil symbol schematic, K coil symbol, contactor coil symbol.
What the Relay Coil symbol means
The Relay Coil symbol in a circuit diagram marks the control side of a relay — the electromagnetic coil that acts as the actuating element. When a voltage within the relay coil's rated range is applied across its A1 (+) and A2 (−) terminals, current through the coil creates a magnetic field that pulls an armature, mechanically operating one or more sets of contacts elsewhere on the schematic. The relay coil symbol therefore represents the input (control) half of an electrically-operated switch.
Relay coil symbols appear paired with contact symbols (normally-open NO, normally-closed NC, or changeover CO contacts) that share the same designator letter and index, for example K1 for the coil and K1-1, K1-2 for its contacts. This pairing convention, standardised in IEC 60617-07 and ANSI/IEEE C37.2, allows engineers to trace control logic across large schematics without confusion between the energising circuit and the switched load circuit.
How to identify the Relay Coil symbol
In IEC 60617, the relay coil symbol is drawn as a plain rectangle (typically taller than it is wide) with a single line entering from the top (terminal A1) and a single line leaving from the bottom (terminal A2); no internal decoration appears inside the rectangle for a basic electromagnetic relay coil. In the ANSI Y32.2 / IEEE 315 convention, the coil is often shown as a circle or a rounded rectangle with the two terminals exiting left and right or top and bottom. In both standards the coil rectangle/circle is distinctly separate from — and not connected to — the contact symbols that the relay operates, which appear elsewhere on the drawing with a matching K-index reference.
Function in a circuit
The relay coil converts an electrical control signal (AC or DC at the coil's rated voltage, typically 5 V, 12 V, 24 V DC or 24 V, 120 V, 230 V AC) into mechanical motion via electromagnetism. When sufficient current flows through the coil winding, the resulting magnetic flux attracts a ferromagnetic armature, transferring mechanical force to one or more sets of contacts and either closing or opening them. This action allows a low-power signal — from a microcontroller output pin, a PLC output, or a thermostat — to switch a high-power load circuit without any direct electrical connection between the two circuits, providing both isolation and voltage-level translation.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 (switchgear, controlgear, and protective devices) defines the relay coil as a rectangle with terminals at opposite ends; the rectangle may contain a modifier symbol (e.g. a diagonal line for a reed relay or a delayed-action indicator) to denote relay type. The standard designator letter is K. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 (R1989) / IEEE 315-1975, section 3.5, shows the relay coil as a circle or rectangle; terminal identification uses A1/A2 in line with IEC practice when the two standards are used together, or numbered pin references in older ANSI drawings. The designator K is also used. |
| Key difference | The primary visual difference is shape: IEC uses a strict rectangle; ANSI/IEEE drawings historically used a circle or a rounded outline. Modern software-generated schematics often use the IEC rectangle globally. Functionally the symbols are identical. |
Terminals / pins
| Pin | Name |
|---|---|
| a1 | A1 |
| a2 | A2 |
Typical values
Relay coil rated voltages: 5 V DC, 12 V DC, 24 V DC (most common in control panels), 48 V DC, 24 V AC, 120 V AC, 230 V AC. Coil resistance: typically 50 Ω–500 Ω DC at rated voltage. Pick-up (operate) voltage: ~75–80% of rated voltage; drop-out (release) voltage: ~10–30% of rated voltage. Coil power consumption: 0.5 W–5 W depending on relay size.
Where the Relay Coil symbol is used
- PLC and industrial control panels — relay coils driven by 24 V DC output modules to switch 230 V AC loads
- Automotive wiring — 12 V relay coils switching starter motors, horn circuits, and lighting
- Home automation and HVAC — 24 V AC relay coils in thermostat control circuits switching compressor and fan contactors
- PCB-mount signal relays — 5 V or 3.3 V coils switching audio, RF, or instrumentation signal paths
- Latching relay circuits — relay coil symbol with SET/RESET annotation for bistable operation requiring no holding current
- Motor starter circuits — contactor coil symbols (functionally identical but larger current capacity) in star-delta or DOL starter schematics
- Safety interlock systems — relay coil used in safety relay modules to enforce machine guard logic
Example
In a simple 24 V DC PLC output circuit, a relay coil symbol labelled K1 is connected between PLC output Q0.1 and the 0 V rail; when the PLC energises Q0.1, current flows through the K1 coil, closing the normally-open contact K1-1 shown in the 230 V AC load circuit to power a conveyor motor — demonstrating the relay's role as a voltage-isolation and current-amplification interface between the low-voltage control system and the mains-voltage load.
Key facts
- The Relay Coil symbol is designated K in both IEC 60617 and ANSI Y32.2 / IEEE 315 schematics; individual coils are indexed K1, K2, etc.
- A relay coil symbol has exactly two terminals: A1 (positive/control supply) and A2 (negative/return), as defined in IEC 60617-07.
- IEC 60617 draws the relay coil as a plain rectangle; ANSI Y32.2 / IEEE 315 uses a circle or rounded rectangle for the same component.
- The relay coil symbol appears on the control circuit side of a schematic and is always electrically separate from the contact symbols it operates.
- Common relay coil rated voltages are 5 V DC, 12 V DC, 24 V DC, 24 V AC, and 230 V AC; the rated voltage is written alongside the K designator on the schematic.
- A relay coil with a diagonal line inside the rectangle (IEC modifier) indicates a reed relay, in which the coil surrounds a hermetically-sealed reed contact.
- Relay coils can be modelled with a flyback (freewheeling) diode across the A1/A2 terminals in DC circuits to suppress the inductive voltage spike when the coil de-energises — this diode is shown as a separate symbol in parallel with the coil.
- Contactor coil symbols are identical in form to relay coil symbols; the distinction is conveyed by the ampere rating noted in the parts list, not by a different glyph.
Diagrams that use this symbol
- ignition diagram
- relay diagram
- relay wiring diagram
- relay connection diagram
- 4 pin relay wiring diagram
- diagram what wires go to the starter solenoid
- 5 pin relay wiring diagram
- wire relay
Frequently asked questions
What does the relay coil symbol look like?
The relay coil symbol in IEC 60617 is drawn as a plain rectangle with one terminal (A1) entering from the top and one terminal (A2) leaving from the bottom. In ANSI Y32.2 / IEEE 315 schematics it is traditionally drawn as a circle or rounded rectangle with terminals at opposite sides. Both versions contain no internal decoration for a standard electromagnetic relay coil.
What does the relay coil symbol mean in a circuit diagram?
The relay coil symbol marks the electromagnetic coil of a relay — the component that, when energised by a control voltage, generates a magnetic field to mechanically operate one or more switch contacts. It represents the input (control) side of a relay; the output (switched) side is shown by separate contact symbols bearing the same K-index on the schematic.
What is the designator letter for a relay coil?
The standard designator letter for a relay coil is K, per IEC 60617 and ANSI Y32.2 / IEEE 315. Individual relays are indexed sequentially as K1, K2, K3, and so on. Contactor coils use the same K designator but may also be seen labelled KM (for motor contactor) on some European industrial schematics.
What is the difference between the IEC and ANSI relay coil symbol?
In IEC 60617 the relay coil is drawn as a rectangle; in ANSI Y32.2 / IEEE 315 it is traditionally drawn as a circle or rounded rectangle. Both represent the same component and use A1/A2 terminal labelling in modern practice. The functional meaning is identical — only the glyph shape differs between the two standards.
What voltage is used for a relay coil?
Relay coil voltages vary by application: 5 V DC and 12 V DC are common for PCB signal relays; 24 V DC is the dominant choice in PLC control panels; 24 V AC is standard in HVAC thermostat circuits; 120 V AC and 230 V AC coils are used where mains-level control signals are present. The rated coil voltage is always noted beside the K designator on a schematic.
How many terminals does a relay coil symbol have?
A relay coil symbol has exactly two terminals: A1 (the positive or control-supply connection) and A2 (the negative or return connection), as specified in IEC 60617-07. These two terminals represent the electrical input to the coil winding and are completely separate from the relay's contact terminals.
Why is a flyback diode shown across a relay coil symbol?
When a relay coil is de-energised, the collapsing magnetic field induces a brief high-voltage spike across the coil terminals (Lenz's law). In DC circuits, a flyback (freewheeling) diode is connected in reverse-parallel across the A1/A2 terminals to clamp this spike and protect the driving transistor or PLC output. The diode appears as a separate symbol in the schematic, placed in parallel with the relay coil rectangle.
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