DPDT Relay Diagram: Wiring, Pinout, and Circuit Applications Reference

DPDT Relay Diagram: Wiring, Pinout, and Circuit Applications Reference — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable dpdt relay diagram: download the diagram as SVG or open it and print to paper or PDF.

A DPDT relay diagram shows how a double pole double throw relay switches two separate circuits simultaneously using a single coil — the essential reference for motor reversing, changeover circuits, and interlocking applications.

A DPDT relay (Double Pole Double Throw) is an electromechanical switching device that uses a single electromagnetic coil to actuate two independent sets of switch contacts simultaneously. Each pole (set of contacts) has three terminals: Common (COM), Normally Closed (NC), and Normally Open (NO). When the coil is de-energised, each COM connects to its respective NC terminal. When the coil is energised, the armature moves and both COMs switch simultaneously to their respective NO terminals.

The DPDT relay therefore provides two simultaneous changeover switch actions with a single control signal. This distinguishes it from: - SPST (Single Pole Single Throw): one contact, two positions (on/off only) - SPDT (Single Pole Double Throw): one changeover contact (COM/NC/NO) - DPST (Double Pole Single Throw): two contacts, but each only opens or closes without a changeover position

The relay coil terminals are separate from the contact terminals. The coil must be energised at its rated voltage — commonly 5 VDC, 12 VDC, or 24 VDC for electronic/automation applications, or 24 VAC, 120 VAC for industrial relays. Applying significantly lower voltage means the relay may not pull in fully (chattering); applying higher voltage overheats the coil. A free-wheeling diode (flyback diode) connected across the coil in reverse polarity is essential in DC relay circuits driven by transistors or microcontrollers — when the coil is de-energised, the collapsing magnetic field generates a voltage spike that can destroy semiconductor switching devices without the diode.

The most important practical application of a DPDT relay is motor direction reversing. To reverse a DC motor, the polarity applied to the motor terminals must be reversed. A single DPDT relay can accomplish this: COM1 connects to the motor positive terminal, COM2 connects to the motor negative terminal. With the relay de-energised: COM1 → NC1 = +12 V (forward direction). With the relay energised: COM1 → NO1 = 0 V and COM2 → NO2 = +12 V (reversed direction). This makes both DPDT relays and H-bridge circuits (which are electronic equivalents of two DPDT relays) essential in motor control.

In safety interlock applications, DPDT relays provide both a normally-open output for the load circuit and a normally-closed contact for monitoring — without the possibility of both being closed simultaneously, which prevents a fault condition from being missed.

How to wire dpdt relay diagram

Identify the relay's coil voltage and contact ratings Before wiring, confirm the relay's coil voltage (5 V, 12 V, 24 VDC are common). Confirm contact ratings: maximum AC voltage (VMAX AC), maximum DC voltage (VMAX DC), and maximum switched current. The contact ratings for DC are usually lower than AC because DC arcs do not self-extinguish at zero crossings the way AC arcs do.
Identify the terminal numbering Locate the relay's datasheet or the diagram printed on the relay body. Identify coil terminals (often labelled A1/A2 on DIN-rail types, or Pin 1/Pin 8 on 8-pin octal relay bases). Identify the contact terminals for each pole. On a PCB relay, the pinout is typically documented with the relay viewed from the solder-side (bottom).
Connect the coil with a flyback diode (DC circuits) In DC circuits, connect the relay coil positive terminal to the switching device (transistor, MOSFET, microcontroller output through a transistor). Connect the coil negative terminal to ground. Place a 1N4007 (or equivalent) diode across the coil — anode to the coil negative (ground) side, cathode to the positive terminal. This clamps the inductive kickback spike.
Wire the load circuit on Pole 1 Connect the COM1 terminal to the first circuit's switching point. Connect NO1 to the load connection active in the energised state. Connect NC1 to the load connection active in the de-energised state. For a simple on/off application, leave the NC1 terminal unused and run the load between the supply and NO1, with COM1 connected to the supply return.
Wire the second circuit on Pole 2 Pole 2 (COM2, NC2, NO2) switches independently but simultaneously with Pole 1. In a motor reversing application, wire Pole 2 with opposite polarity to Pole 1 as described in the FAQ. In an interlock application, connect the NC2 contact to a monitoring or inhibit circuit that opens when the relay energises.
Test coil operation and contact switching Apply rated voltage to the coil terminals. Confirm the relay audibly clicks (the armature pulls in). Measure continuity across COM-NO (should now be closed) and COM-NC (should now be open). Remove coil voltage — relay should click back and COM-NC should be closed again. Measure contact resistance when closed — should be below 0.1 Ω on a new relay.

Specifications

Configuration	Double Pole Double Throw (DPDT) — 2 x COM, 2 x NC, 2 x NO
Typical coil voltages	5 VDC, 12 VDC, 24 VDC, 24 VAC, 120 VAC
Typical contact current rating (resistive load)	5 A, 10 A, or 16 A at 250 VAC (relay-specific)
Typical contact voltage rating	250 VAC / 30 VDC (resistive) — lower for DC inductive loads
Typical coil resistance (12 VDC, 10 A relay)	160–360 Ω (varies by relay; check datasheet)
Typical coil current (12 VDC relay)	30–75 mA
Contact resistance (closed, new relay)	< 0.1 Ω
Operate time (coil energise to contact close)	5–15 ms typical

Safety warnings

The switch contacts of a DPDT relay can carry mains voltage (120 VAC or 230 VAC) depending on the circuit they are switching. Any wiring connected to relay contacts that switch mains AC voltage must comply with your local wiring regulations and must be carried out by a licensed electrician. Never assume that because the relay coil is low-voltage DC, the contact wiring is also low-voltage.
Ensure the coil voltage matches the supply voltage exactly. A 12 VDC relay connected to 24 VDC will overheat the coil and fail — possibly causing a fire in enclosed equipment. A 24 VDC relay connected to 12 VDC may chatter or fail to pull in reliably.
Always fit a flyback diode across DC relay coils driven by transistors, MOSFETs, or microcontroller outputs. The inductive kickback spike from a relay coil without a diode is typically 50–300 V — more than enough to destroy a microcontroller I/O pin or a logic-level MOSFET in a single switching event.
Do not exceed the relay's rated contact current or voltage. Exceeding ratings accelerates contact wear, causes arcing, and can weld contacts closed — a particularly dangerous failure mode in safety interlock applications where a welded contact cannot open when commanded.

Tools needed

Multimeter (continuity and resistance functions)
Soldering iron and rosin-core solder (for PCB relay circuits)
Wire strippers and crimping tool (for wiring relay sockets)
Screwdrivers (flathead for screw-clamp relay socket terminals)
Power supply matched to relay coil voltage (for testing)

Common mistakes

Omitting the flyback diode when driving a relay coil from a transistor or microcontroller — the first switching cycle destroys the semiconductor. Always include the diode before powering the circuit.
Confusing the NC and NO contacts — wiring the load to the NC terminal when NO is intended means the load is active when the relay is de-energised and switches off when it is energised. Verify contact states with a multimeter with the coil both energised and de-energised.
Using a relay with a contact rating below the load current — contacts rated for 5 A resistive loads fail quickly when switching 5 A inductive loads (motors, solenoids). Use a relay rated for the actual load type — inductive load ratings are lower than resistive load ratings.
Connecting both NO terminals together at a common junction and both NC terminals together, then expecting this to behave as two separate changeover switches — this approach shorts the two circuits together on the NC side, which may not be the intended function.
Not verifying relay pull-in voltage under load conditions — a relay coil rated at 12 V will pull in reliably at 12 V but may fail to pull in at 10 V (the minimum coil voltage). In automotive applications where battery voltage can drop below 11 V during cranking, relay chattering can occur.

Troubleshooting

Relay coil energises (audible click) but contacts do not switch: Cause: Relay contacts are welded closed from previous overcurrent event; internal spring has failed; relay is incorrect type (DPST or SPDT installed instead of DPDT) Fix: Measure continuity across COM-NC and COM-NO with coil energised. If both show continuity, contacts are welded — relay must be replaced. Verify the relay part number matches the intended DPDT type.
Relay chatters rapidly instead of holding in the energised state: Cause: Coil supply voltage too low (below minimum pull-in voltage); excessive voltage drop on the coil supply wire; relay coil resistance too high due to a partial winding open circuit Fix: Measure voltage directly across the coil terminals with the relay attempting to pull in — must be within the rated voltage range. Check coil resistance matches the datasheet. Increase coil supply voltage if persistently low.
MOSFET or transistor driving the relay coil fails repeatedly: Cause: Flyback diode absent, installed in wrong polarity, or failed open; coil current exceeds transistor/MOSFET current rating Fix: Verify flyback diode is present, correctly oriented (anode to negative/ground side of coil, cathode to positive side), and measures correct diode drop in forward direction. Measure relay coil current — verify it is within the transistor/MOSFET Ic (collector current) rating.

Frequently asked questions

What do the terminals on a DPDT relay mean?

A DPDT relay has a coil (two terminals: coil+ and coil−) and two sets of switch contacts. Each set has three terminals: COM (common), NC (normally closed — connected to COM when coil is de-energised), and NO (normally open — connected to COM when coil is energised). On DIN-rail relays, terminals are typically numbered: 1-2 = coil, 11-12-14 = first set (11=COM, 12=NC, 14=NO), 21-22-24 = second set.

How does a DPDT relay reverse a DC motor?

Wire COM1 and COM2 to the two motor terminals. Wire NO1 and NC2 to the positive supply (+12 V), and NC1 and NO2 to 0 V (ground). With the relay de-energised: COM1 = +12 V (via NC1), COM2 = 0 V (via NO2) — motor runs in forward direction. With relay energised: COM1 = 0 V (via NO1), COM2 = +12 V (via NC2) — motor runs in reverse.

Why do I need a flyback diode across the relay coil?

When a relay coil is de-energised, the collapsing magnetic field induces a voltage spike — often several hundred volts for a brief instant — across the coil terminals with reversed polarity. This spike can destroy transistors, MOSFETs, or microcontroller I/O pins driving the coil. A diode connected in reverse polarity across the coil (cathode to positive, anode to negative of the coil supply) clamps the spike by conducting the inductive kickback current harmlessly.

What is the difference between a relay and a contactor?

A relay and a contactor are both electromagnetic switching devices, but designed for different current ratings. Relays typically switch currents up to 10–16 A and are used for control circuits and signal switching. Contactors are designed for motor load switching — 9 A to hundreds of amps — and have more robust contacts rated for inductive loads, with built-in arc suppression. Contactors are also typically used for 3-phase AC motor control.

Can a DPDT relay switch AC and DC simultaneously on the two poles?

Yes. The two poles of a DPDT relay are electrically isolated from each other and from the coil. One pole can switch an AC circuit while the other switches a DC circuit, as long as each circuit stays within the relay's contact rating for the relevant voltage and current type. Ensure the relay's contact ratings cover both circuit types.

Related diagrams

Free electrical calculators

Edit this diagram free in the online editor