Relay SPDT Symbol
Definition: The Relay SPDT symbol represents a Single-Pole Double-Throw electromagnetic relay — defined in IEC 60617-07 and ANSI Y32.2 / IEEE 315 — a five-terminal switching device with a coil (Coil+, Coil−) that electromagnetically transfers the common (COM) contact from the Normally Closed (NC) position to the Normally Open (NO) position when energised, providing both make and break switching in a single relay.
Also known as: SPDT relay, changeover relay, Form C relay, single-pole changeover relay, c-contact relay, CO relay.
What the Relay SPDT symbol means
The Relay SPDT symbol denotes an electromechanical relay with one input contact (COM) that can connect to either of two output contacts (NO or NC) depending on coil energisation state. This single-pole double-throw configuration makes the SPDT relay the most versatile basic relay type, capable of simultaneously opening one circuit while closing another.
In circuit schematics, the five pins serve distinct roles: Coil+ and Coil− carry the low-voltage energisation signal; NC is connected to COM at rest (coil off); NO connects to COM when the coil is energised; and COM is the common terminal through which the switched load current flows. The SPDT relay is depicted in its de-energised (rest) state in standard schematics, meaning COM connects to NC by default.
How to identify the Relay SPDT symbol
The Relay SPDT symbol is drawn as a rectangle enclosing a coil element (left side, Coil+ and Coil− pins) and a switching arm diagram (right side). The switching arm shows COM at centre, NC at the top (with the arm touching NC in rest state), and NO at the bottom (open in rest state). Some representations show the arm as a diagonal line pivoting from COM to either NC or NO position. The three contact pins (NC, COM, NO) exiting the right side of the rectangle are the key identifier.
Function in a circuit
The SPDT relay switches the COM terminal between the NO and NC positions in response to coil energisation. When the coil (Coil+ to Coil−) is de-energised, the armature spring holds COM against NC (circuit from COM to NC is closed). When coil current flows, the electromagnet pulls the armature, disconnecting COM from NC and connecting it to NO simultaneously. When coil power is removed, the spring returns COM to NC. This simultaneous break-before-make (or make-before-break, depending on design) action allows routing of a circuit to one of two destinations.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines the changeover contact (NC + NO with common) and the relay coil symbol (rectangle). The coil is drawn as a rectangle; the changeover contact shows the COM arm and both NO and NC positions. IEC 61810-1 specifies elementary relay performance requirements. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 Section 13 defines the Form C (SPDT) contact and the relay coil (rectangle with device designation). The coil and contact symbols are functionally identical to IEC; the Form C designation replaces IEC 'changeover contact' terminology. |
| Key difference | IEC 60617 and ANSI Y32.2 / IEEE 315 use nearly identical SPDT relay symbols — a rectangle for the coil and a pivoting arm for the Form C / changeover contact. The main difference is labelling: IEC uses NO/NC/COM; IEEE 315 uses Form A (NO), Form B (NC), and Form C (changeover) contact terminology. Both show the symbol in de-energised state. |
Terminals / pins
| Pin | Name |
|---|---|
| coil1 | Coil+ |
| coil2 | Coil- |
| nc | NC |
| com | COM |
| no | NO |
Typical values
Coil voltage: 5 V DC, 12 V DC, 24 V DC, 48 V DC common (pick-up voltage typically 75–80% of rated). Coil resistance: 50–2000 Ω depending on voltage/type. Contact rating: 10 A at 250 V AC, 10 A at 30 V DC (typical signal relay); up to 30 A for power relays. Operate time: 5–20 ms. Release time: 3–10 ms. Designator: K.
Where the Relay SPDT symbol is used
- Motor reversing circuits where SPDT relay contacts route either forward or reverse phase sequence to the motor
- Audio switching circuits routing a signal to either of two outputs or selecting between two input sources
- Logic circuits implementing mux/demux signal routing with relay switching
- Fail-safe control where COM on NC activates an alarm or backup circuit when the relay coil loses power
- Changeover transfer switches routing a load to either mains supply or generator supply
- Thermostat control circuits switching heating on NC and cooling on NO from a single relay output
Example
In a thermostat circuit schematic, the Relay SPDT symbol has Coil+ connected to the thermostat temperature sensor output and Coil− to ground; COM connects to the fan motor supply; when temperature is below setpoint and the coil is de-energised, COM connects to NC (heating element circuit closed); above setpoint the coil energises and COM transfers to NO (cooling compressor circuit closed), simultaneously disconnecting the heater.
Key facts
- The Relay SPDT has five pins: Coil+ and Coil− (energisation) and NC, COM, NO (Form C changeover contact); COM connects to NC when the coil is off and transfers to NO when the coil is energised.
- SPDT is also called Form C or changeover relay; it provides both a make (NO closes) and a break (NC opens) transition simultaneously on coil energisation.
- The relay is shown in its de-energised (rest) state in IEC 60617 and IEEE 315 schematics — COM touching NC by default means NC is the 'safe' or 'normal' circuit path.
- Coil polarity matters: Coil+ and Coil− must be observed for polarised relays and when using a flyback diode; non-polarised coils can be driven from either polarity.
- A flyback (freewheeling) diode wired in reverse-parallel across the coil (anode to Coil−, cathode to Coil+) is essential to suppress the inductive back-EMF spike when coil current is switched off, protecting the driving transistor or GPIO.
- The relay designator is K (e.g. K1); the three contacts are referenced as K1-COM, K1-NO, and K1-NC or drawn with a K1 label adjacent to the contact symbols on the schematic.
- SPDT relays are mechanically more complex than SPST relays and have a lower maximum switching current rating per contact compared to dedicated NO or NC-only power relays of the same coil size.
Frequently asked questions
What does the relay SPDT symbol mean in a circuit diagram?
The Relay SPDT (Single-Pole Double-Throw) symbol represents an electromagnetic relay with a single common contact (COM) that switches between a Normally Closed (NC) contact (when coil is off) and a Normally Open (NO) contact (when coil is energised). It has five pins: Coil+, Coil−, NC, COM, and NO.
What does the SPDT relay symbol look like?
The SPDT relay symbol is a rectangle containing a coil element on the left (Coil+ and Coil− pins) and a switching arm on the right showing three contact pins: NC at top, COM at centre, and NO at bottom. The arm is shown touching NC in the de-energised rest state, indicating COM connects to NC when the coil is off.
What is the difference between SPDT and SPST relays?
An SPST (Single-Pole Single-Throw) relay has one contact pair — either NO or NC only. An SPDT relay has a changeover contact with both NO and NC positions sharing a common terminal (COM), allowing it to switch a circuit to one of two paths. SPDT provides simultaneous make and break action; SPST provides only one switching action.
What is COM, NO, and NC on a relay?
COM (Common) is the centre terminal of the changeover contact through which switched current flows. NC (Normally Closed) connects to COM when the relay coil is off — this is the default active path. NO (Normally Open) connects to COM when the coil is energised. The COM terminal transfers from NC to NO on coil energisation.
What standard defines the SPDT relay symbol?
IEC 60617-07 defines the relay coil symbol (rectangle) and changeover contact symbol (pivoting arm with NC and NO positions). ANSI Y32.2 / IEEE 315 Section 13 defines the equivalent Form C contact. The SPDT relay electrical performance is specified in IEC 61810-1 (elementary relays) and IEC 60947-5-1 (auxiliary contacts).
Why do I need a flyback diode on a relay coil?
A relay coil is an inductor; when switched off, it generates a back-EMF voltage spike that can be many times the supply voltage (e.g. 50–200 V from a 12 V coil). This spike can destroy transistors or microcontroller GPIO pins. A flyback diode wired in reverse-parallel across the coil (anode to Coil−, cathode to Coil+) clamps this spike to approximately one diode forward voltage (0.7 V), protecting the drive circuit.
What is the schematic designator for an SPDT relay?
The schematic designator for any relay, including an SPDT type, is K per IEEE 315 and common international practice. The first relay is K1, the second K2, etc. The associated contacts are labelled K1-COM, K1-NO, and K1-NC on the schematic, or drawn with the K1 reference adjacent to the contact symbols.
Place the Relay SPDT symbol on a wiring diagram or schematic in the free online circuit diagram maker — no download required.