SPDT Relay Diagram

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An SPDT relay diagram shows a single-pole double-throw relay with three switch terminals — COM (common), NO (normally open), and NC (normally closed) — and the coil terminals that control which contact the common is connected to.

A single-pole double-throw (SPDT) relay is an electromechanical switch with two distinct electrical circuits: the coil circuit and the contact circuit. Understanding the diagram of each is fundamental to using relays correctly in control, automation, and switching applications.

The coil circuit consists of two terminals (labelled A1 and A2, or coil+ and coil−, or simply pins 1 and 2 depending on the relay format). When the rated coil voltage is applied across these terminals, the electromagnet inside the relay energises, attracting the armature and mechanically moving the common (COM) contact from its normal resting position.

The contact circuit has three terminals: - COM (common) — the moving contact that connects to either NO or NC - NO (normally open) — the contact that is open when the coil is de-energised; it closes when the coil energises - NC (normally closed) — the contact that is closed when the coil is de-energised; it opens when the coil energises

This creates two switching states: 1. Coil de-energised: COM connects to NC; NO is open 2. Coil energised: COM connects to NO; NC is open

The SPDT relay is also called a changeover relay because it changes the COM connection between NO and NC. This makes it useful for applications where a load must be switched between two different circuits — for example, switching a motor between normal and emergency power, switching between two sensors, or providing a form-C contact output for control systems.

In relay diagrams, the coil is represented as a rectangle or a circle with the coil terminals shown. The contact is drawn separately (IEC 60617 style): COM is the common line, with a diagonal switching line; the NC contact is shown with a diagonal slash indicating the normally-closed state; the NO contact is shown as an open gap.

Important: the coil and contact circuits are electrically isolated from each other. This isolation is the relay's key function — allowing a low-voltage, low-current control signal (e.g., 5 V from a microcontroller) to switch a completely separate high-voltage, high-current load (e.g., 230 V mains lighting).

A flyback diode (freewheeling diode) must always be connected across the relay coil when driven by a DC circuit — the inductive coil generates a large voltage spike when de-energised that can damage transistors, microcontrollers, or other driving electronics.

An SPDT (Single Pole Double Throw) relay has one common contact that switches between a normally closed (NC) and a normally open (NO) position, making it ideal for circuits that need to route a signal or power between two paths depending on whether the coil is energised. In automotive and industrial applications, pin 30 is the common, pin 87a is the normally closed output, and pin 87 is the normally open output; coil terminals are 85 and 86. This changeover capability lets you, for example, run a pump on a backup circuit when the primary signal is absent. Draw and annotate your SPDT relay wiring diagram free in the browser-based editor.

How to wire spdt relay diagram

Identify the relay type and confirm coil and contact ratings Verify the relay's coil voltage and coil resistance from the datasheet. Calculate the coil current (V / R) to ensure the driving circuit can supply it. Check the contact rating: maximum switching voltage, maximum switching current (resistive and inductive), and the electrical life at the intended operating current. Ensure the contact rating exceeds the load requirements with margin.
Identify the coil terminals (A1 and A2) and connect the drive circuit For a DC relay coil: connect A1 to the positive coil supply (through the switching transistor or control device) and A2 to the negative supply (ground). For an AC relay coil: connect A1 and A2 to the AC control voltage. The coil terminals are electrically isolated from the contact terminals — there is no direct connection between them. Fit a flyback diode across the coil for any DC relay driven by a semiconductor switch.
Add the flyback diode across the DC coil Connect a rectifier diode across the coil with the cathode (stripe end) connected to A1 (positive supply side) and the anode connected to A2 (negative supply side). This ensures the diode is reverse-biased during normal coil energisation and only conducts the flyback spike when the coil is de-energised. The diode should be rated for at least the coil supply voltage in reverse and 1 A forward current.
Identify COM, NO, and NC contact terminals Locate the three contact terminals on the relay body or PCB footprint. Consult the relay datasheet for the exact pin assignments — on PCB relays the pin assignments follow the datasheet pinout; on DIN-rail or socket-mounted relays, the terminal numbering follows a standard (often IEC 60664-1 or the relay manufacturer's numbering convention). Verify which pin is COM, NO, and NC using a continuity tester with the coil de-energised: COM-NC should show continuity; COM-NO should show open circuit.
Connect the load circuit through the appropriate contact Connect the load supply line through COM and the desired contact. For a load that should be ON when the relay is de-energised (fail-safe ON), connect the load between COM and NC. For a load that should be ON when the relay is energised, connect the load between COM and NO. If changeover switching is required (switching between two loads), connect each load to COM via its respective contact (NO or NC).
Verify isolation between coil and contact circuits Before powering up, use a digital multimeter to confirm there is no continuity between any coil terminal and any contact terminal. The relay should provide complete electrical isolation between these circuits. If continuity exists, the relay is faulty or the circuit has been incorrectly assembled.
Test switching operation under no-load conditions Apply the coil voltage (without load connected to the contact circuit). Use a continuity tester or multimeter on the contact terminals to confirm COM switches from NC to NO when the coil energises, and returns to NC when the coil de-energises. Listen for the click of the armature moving. Verify the flyback diode is not preventing coil energisation (incorrect diode orientation would short the coil supply).

Specifications

Contact configuration	SPDT — 1 form C changeover contact (COM, NO, NC)
Typical coil resistance (12 V DC relay)	Approximately 100–400 ohms (varies by relay; calculate coil current as V/R)
Typical coil current (12 V DC relay)	30–120 mA (varies by relay; verify from datasheet)
Typical contact rating (general-purpose miniature relay)	10 A at 250 V AC (resistive); 10 A at 30 V DC (resistive); derate significantly for inductive loads
Typical pick-up voltage (DC relay)	75% of nominal coil voltage (the relay coil energises at this voltage or higher)
Typical drop-out voltage (DC relay)	10–30% of nominal coil voltage (the relay releases at or below this voltage)
Coil terminal designation (IEC 60947-5-1)	A1 (coil positive, or line), A2 (coil negative, or neutral)
Contact terminal designation (IEC 60617 form C)	COM (common, moving contact), NO (normally open fixed contact), NC (normally closed fixed contact)

Safety warnings

Relay contacts switching mains voltage (120 V AC, 230 V AC) carry lethal electrical energy. Any circuit using an SPDT relay to switch mains loads must be designed, assembled, and tested by a qualified electrician or engineer. Mains-voltage wiring must comply with the applicable wiring regulations: NEC / NFPA 70 (USA), BS 7671 (UK), AS/NZS 3000 (Australia/New Zealand), or IEC 60364 (international). This content is for educational and reference purposes only.
Never exceed the relay contact's rated switching voltage, current, or power. Operating a relay beyond its rated contact capacity causes contact arcing, contact welding (contacts permanently fused closed), and accelerated contact erosion, all of which create safety hazards. Always select a relay with a contact rating that exceeds the load requirements.
Always include a flyback diode across any relay coil driven by a DC semiconductor switch. Omitting the flyback diode will eventually destroy the switching transistor, MOSFET, or microcontroller output port due to the inductive voltage spike generated when the coil is de-energised.
Relay contacts are not approved as a means of electrical isolation for safe maintenance. Before working on a load connected through relay contacts, isolate the load circuit at an upstream isolator or circuit breaker and verify dead with a calibrated voltage tester. A relay contact can arc closed (weld) under fault conditions and may not represent a reliable open circuit.
In applications where relay failure in the energised state (coil burned open) must be safe, use NC contact wiring so the load is energised by default and de-energises on relay failure. Conversely, where energising the load on relay failure is hazardous, use NO contact wiring so the load requires an active coil signal. Consider the fail-safe behaviour of both coil and contact failure modes in safety-critical designs.

Tools needed

Digital multimeter (for coil resistance, contact continuity, and voltage verification)
Oscilloscope (for observing flyback spike before and after diode installation)
Soldering iron and solder (for PCB relay assembly)
Screwdrivers (for relay socket terminal connections)
Breadboard or PCB for prototyping
Bench power supply (adjustable, for testing coil at rated voltage)
Continuity tester (for verifying COM/NO/NC contact states)

Common mistakes

Connecting the flyback diode in the wrong orientation (anode to positive supply side) — this creates a direct short circuit across the coil supply and will immediately damage the power supply or blow the supply fuse. The diode cathode (stripe) must connect to the positive coil supply terminal.
Confusing NC and NO terminals when wiring the load circuit — swapping NC for NO results in the load behaving in the opposite way to intended (ON when it should be OFF, OFF when it should be ON). Always verify the contact state with a continuity tester before connecting the load.
Using a relay with a coil voltage different from the control circuit supply — applying higher-than-rated voltage burns out the coil; applying lower-than-rated voltage may result in the relay failing to pick up reliably, especially at temperature extremes.
Driving a relay coil directly from a microcontroller GPIO pin without a transistor driver — most microcontroller GPIO pins are rated for 8–40 mA maximum. Relay coil currents are typically 50–200 mA. Connecting a relay coil directly to a GPIO pin will damage the microcontroller.
Not accounting for contact bounce in digital applications — mechanical relay contacts bounce (make and break multiple times) during switching transitions over a period of 1–20 ms. In digital input circuits connected to relay contacts, software debouncing or hardware filtering is required to prevent multiple edge detections.
Selecting a relay rated for resistive loads for an inductive load application — inductive loads (motors, solenoids, transformers) generate contact-damaging arcs when switched. For inductive loads, choose a relay rated for inductive duty (AC-1 or AC-3 classification for contactors), or use a snubber circuit across the contact.

Troubleshooting

Relay coil does not energise when the control signal is applied: Cause: Incorrect coil voltage applied, open circuit in the drive transistor, flyback diode installed in reverse (shorting the coil supply), or coil winding is open-circuit (burned out) Fix: Measure the voltage across the relay coil terminals with the control signal active. If voltage is present at the rated coil voltage, the coil is likely open-circuit — measure the coil resistance (should match the datasheet value, typically 50–1000 ohms). If no voltage is present, trace the drive circuit from the control signal through the transistor to the coil. Verify the flyback diode is correctly oriented (cathode to positive rail).
Relay contacts are welded (stuck in closed position and cannot be opened by de-energising the coil): Cause: Contact rating exceeded — load current was too high for the contact rating, causing arcing that fused the contacts permanently closed; or the load had a high inrush current that was not accounted for in the contact selection Fix: The relay must be replaced — welded contacts cannot be safely restored to service. Before fitting a replacement relay, calculate the actual load current and inrush current and select a relay with an appropriate contact rating. For motor or capacitive loads with high inrush, derate the relay contact current rating by 50–70% from the resistive rating, or use a contactor rated for the specific load type.
Microcontroller or GPIO pin is damaged after relay switching: Cause: Flyback diode omitted or incorrectly installed — the inductive voltage spike from the relay coil has damaged the transistor or microcontroller output Fix: Replace the damaged component. Install a correctly oriented flyback diode across the relay coil. Verify the diode is a general-purpose rectifier type rated for the coil supply voltage and at least 1 A. If using a transistor driver, also verify the transistor's Vce or Vds rating exceeds the coil supply voltage plus any expected spike before the diode clamps it.

Frequently asked questions

What does SPDT mean in relay terminology?

SPDT stands for single-pole double-throw. 'Single-pole' means there is one moving contact (COM). 'Double-throw' means the COM contact can connect to one of two fixed contacts — the normally-open (NO) or the normally-closed (NC). An SPDT relay provides a changeover function: in its resting state it connects COM to NC; when energised it connects COM to NO.

What is the difference between NO and NC contacts on a relay?

NO (normally open) contacts are open when the relay coil is not energised — no current flows through them in the default state. NC (normally closed) contacts are closed when the relay coil is not energised — current flows through them in the default state. When the coil energises, the positions swap: NO closes and NC opens. The 'normal' state always refers to the coil being de-energised.

Why is a flyback diode required across a relay coil?

A relay coil is an inductor. When the current through an inductor is suddenly interrupted (when the coil drive transistor switches off), the inductor's stored energy produces a large voltage spike in the reverse polarity — potentially hundreds of volts in microseconds. This spike can immediately destroy a transistor, MOSFET, or microcontroller I/O pin. A flyback diode connected across the coil (anode to coil negative, cathode to coil positive in a DC circuit) provides a path for this energy to dissipate safely.

Can I use the NO and NC contacts simultaneously in the same circuit?

Yes — this is one of the key advantages of an SPDT (changeover) relay. The COM contact switches between NO and NC, so both NO and NC are available simultaneously. For example, COM can supply power to a primary load via NC when the relay is de-energised, and automatically switch to a secondary load via NO when the relay energises. Both load circuits share the common COM terminal.

What coil voltages are typically available for SPDT relays?

SPDT relays are available with coil voltages including 3 V DC, 5 V DC, 6 V DC, 9 V DC, 12 V DC, 24 V DC, and 48 V DC for DC coils, and 12 V AC, 24 V AC, 120 V AC, and 230 V AC for AC coils. The coil voltage must exactly match the control circuit supply. Always verify the coil voltage, coil resistance (to calculate drive current), and pick-up and drop-out voltage specifications from the relay datasheet.

How do you wire an SPDT relay?

Wire pin 86 to your control signal (e.g. a switch or ECU output) and pin 85 to ground to form the coil circuit. Connect pin 30 (common) to your power source for the load. When the coil is de-energised, current passes from pin 30 to pin 87a (normally closed); when the coil energises, the contact switches and current flows from pin 30 to pin 87 (normally open). This lets one relay switch a load between two different states or circuits automatically.

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