Relay Wiring Diagram: How to Wire the Coil, COM, NO, and NC Terminals?

A relay wiring diagram shows two separate circuits. On the coil side, a low-voltage signal controls the relay through a transistor driver. On the contact side, the fused supply connects to COM, while the load connects to either NO or NC.

This guide starts with a practical example: an Arduino UNO controlling an SRD-05VDC-SL-C relay to switch a mains-powered lamp. It then explains how the wiring changes for different relay types.

In this guide, you will learn:

• Which wire goes to each relay terminal
• How to wire the coil driver, flyback diode, and contacts
• How to wire a mains load safely, step by step
• How to wire SPST, SPDT, DPST, DPDT, and SSR relays
• How to wire a relay module and an automotive relay

Figure: Relay wiring diagram showing an Arduino driving the coil through a transistor

Relay Wiring Diagram: Which Wire Goes to Each Terminal?

A 5-pin relay, such as the SRD-05VDC-SL-C, has two coil terminals and three contact terminals. The coil terminals connect to the low-voltage driver, while COM, NO, and NC connect to the load circuit.

Wire each terminal as follows:

• Coil terminals: Connect one to the positive coil supply, +5 V for a 5 V relay, and the other to the collector of the NPN driver transistor. A plain coil has no polarity.
• COM (common): Connect to the supply side of the load circuit: the fused live for an AC load, or the fused positive for a DC load.
• NO (normally open): Connect to the load so it turns on only when the coil is energized.
• NC (normally closed): Connects to the load and remains on until the coil is energized.

To identify the pins on an unmarked relay, use a multimeter with the coil unpowered. The meter shows continuity between COM and NC, and an open circuit between COM and NO; the coil of a 5 V SRD relay measures about 70 Ω.

The relay symbol guide covers how each of these terminals is drawn on a schematic.

Arduino Relay Wiring Diagram: UNO, 5 V Relay, and Mains Lamp

The master circuit drives an SRD-05VDC-SL-C coil from Arduino pin D7 via an NPN transistor, switching a mains lamp via the COM and NO terminals.

Figure: Relay wiring diagram schematic with Arduino UNO, NPN driver, flyback diode, fuse, and mains lamp.

Turn Your Relay Wiring Into a Working PCBA

Get a PCBA Quote

• Once your relay wiring is finalized, turn the circuit into a custom assembled PCB.
• JLCPCB can assemble your relay, driver components, diodes, resistors, connectors, and other parts on your board, so you can move from a tested circuit design to a finished PCBA.

Relay Coil Wiring: Transistor Driver, Flyback Diode, and Power Supply

The coil side needs three parts: an NPN transistor carries the coil current, a base resistor sets its drive, and a flyback diode absorbs the turn-off voltage spike.

Why an Arduino Pin Needs a Transistor to Drive a Relay Coil

At 5 V across 70 Ω, the SRD-05VDC-SL-C coil draws 71.4 mA. Arduino rates each UNO R3 pin at 20 mA, compared with the ATmega328P's absolute maximum of 40 mA, and each UNO R4 pin at 8 mA.

Connecting the coil directly to a pin exceeds both limits and can permanently damage the pin. With an NPN transistor as a low-side switch, the transistor carries the coil current, and the pin supplies only the base current.

Relay Driver Base Resistor Calculation

Size the base resistor so the transistor saturates at the full coil current:

1. Calculate the base current: (5 V − 0.7 V) / 1 kΩ ≈ 4.3 mA.
2. Calculate the forced current gain: 71.4 mA / 4.3 mA ≈ 17. This is well below the minimum hFE of an S8050 or 2N2222 at this current, so the transistor saturates.
3. Check the pin load: 4.3 mA is within the 8 mA rating of an UNO R4 pin and the 20 mA rating of an UNO R3 pin.

On a 3.3 V board, the same 1 kΩ resistor yields about 2.6 mA of base current, which still saturates the transistor at 71.4 mA. The 5 V coil still needs a 5 V supply.

Flyback Diode Orientation Across the Relay Coil

Place the diode directly across the coil, with the diode cathode (the banded end) on the +5 V side and the anode on the transistor collector.

When the transistor turns off, the coil's stored energy keeps current flowing. Without a diode, the collector voltage rises in a spike that can exceed the transistor's collector-emitter voltage rating. With the diode, the current circulates through the diode and the coil and then decays.

A diode fitted the wrong way round conducts from +5 V through the saturated transistor to ground, shorting the coil supply each time the relay turns on.

Figure: Relay coil current paths with the transistor on and off, showing flyback diode current and the spike without a diode.

Relay Contact Wiring: Connecting COM, NO, and NC to an AC Load

On the contact side, the fused live goes to COM, and the lamp connects between NO (or NC) and neutral. The neutral returns directly to the supply, so the relay switches only the live conductor.

Figure: Relay NO and NC wiring grid showing the lamp on NO and on NC.

• Relay NO Wiring for a Normally Off Load: Use NO for a load that should stay off if the controller resets or loses power, such as a heater, a pump, or most lighting control.
• Relay NC Wiring for a Normally Open Load: Use NC for a load that should run unless the controller turns it off, such as a ventilation fan. Move the lamp wire from NO to NC and keep the rest of the circuit unchanged.

Relay Wiring Safety Rules for Mains Loads

On a 120 V or 230 V load, switch and fuse the live conductor, derate the contacts for the load type, and separate and enclose the mains wiring. If you are not qualified for mains work, use a certified, enclosed relay product instead.

Figure: Safety comparison showing a switched neutral marked wrong and a fused, switched live marked right.

• Switch the live, not the neutral: A relay in the neutral turns the lamp off but leaves its socket live, so anyone replacing the lamp can still receive a shock.
• Fuse the live at the relay input: Size the fuse for the load and the wire gauge, and keep it within the relay's contact rating.

How to Wire a Relay Step by Step

Wire and test the coil side first, then the contact side with a low-voltage test load, and connect the mains last. This order separates coil faults from contact faults before any mains voltage is present.

1. Disconnect all power: Unplug the Arduino and the mains supply.
2. Fit the driver: Connect D7 through the 1 kΩ resistor to the transistor base, and connect the emitter to GND.
3. Connect the coil: Wire one coil terminal to +5 V and the other to the collector.
4. Fit the flyback diode: Place it across the coil, with the cathode on the +5 V side.
5. Test the coil side: Upload a sketch that toggles D7 once per second. The relay should click on each change.
6. Wire a test load: Connect a 12 V lamp and its own supply through COM and NO, and confirm the lamp follows the relay.
7. Wire the mains load: With the mains unplugged, connect the fused live to COM, NO to the lamp, and the lamp to neutral, then close the enclosure.
8. Power up and verify: Connect the mains and toggle D7 to confirm the lamp switches.

Relay Wiring Diagrams by Relay Types

Each relay type keeps the master circuit's coil side unchanged. What changes is the number of contact sets and how the load connects to them, while an SSR replaces the coil with an LED input.

An SPST relay wires exactly like the master circuit: a Form A relay uses COM and NO, and a Form B relay uses COM and NC in place of NO. Recalculate the base resistor if the coil current of a DPST or DPDT relay differs from the specified value.

SPDT Relay Wiring Diagram: 5-Pin Changeover

The SRD-05VDC-SL-C is already an SPDT relay, so the master circuit uses two of its three contact terminals. Add a second load on NC, and the relay switches between the two. The status lamp on NC lights when the coil is off, and the main lamp on NO lights when the coil is on. Both loads share a common COM, so they both run from the supply connected to COM.

Figure: SPDT relay wiring diagram with a status lamp on NC and the main lamp on NO.

SSR Wiring Diagram: Solid-State Relay for AC Loads

A DC-input, AC-output solid-state relay (SSR) uses an LED instead of a coil, so it requires no flyback diode.

Connect D7 to input + and GND to input −, then wire the two output terminals in series with the fused live, in place of COM and NO. Check the SSR's input current on its datasheet against the pin rating. If it is higher, drive the input through the master circuit's transistor stage.

• Off-state leakage: An AC SSR passes a small current while off, up to 10 mA at 200 VAC on the Omron G3NA series, for example, which can make an LED lamp glow faintly.
• Heat: The output has an on-state voltage drop of up to 1.6 V on the same G3NA series, so a 10 A load dissipates about 16 W in the SSR. Check the datasheet value and mount the SSR on a heatsink.
• AC loads only: A triac or SCR output turns off only when the load current crosses zero, so it cannot switch off a DC load. Use a DC-output SSR for DC loads.

Figure: SSR wiring diagram with Arduino D7

Relay Module Wiring Diagram

A relay module carries the master circuit's coil side on its own board: the transistor, base resistor, flyback diode, indicator LED, and often an optocoupler. Wire three pins to the Arduino, and the contact side stays the same.

Figure: Relay module wiring diagram connected to an Arduino UNO and a lamp

Automotive Relay Wiring Diagram: Pins 30, 85, 86, 87, and 87a

A 12 V automotive relay uses terminal numbers from DIN 72552 rather than COM, NO, and NC. On a standard ISO mini relay, pin 30 is the common contact, 87 is normally open, 87a is normally closed, and 85 and 86 are the coil.

A 4-pin relay omits 87a. To switch a 12 V lamp from a dashboard switch:

• Connect the battery positive through a fuse to pin 30.
• Connect pin 87 to the lamp, and the lamp's other terminal to chassis ground.
• Connect the fused +12 V through the dashboard switch to pin 86.
• Connect pin 85 to chassis ground.

Coil polarity matters on some relays. If the case diagram shows a diode across 85 and 86, wire the coil with the polarity the diagram marks, usually 86 to positive and 85 to ground, because a reversed suppression diode conducts directly across the coil supply. A coil with a suppression resistor, or with no suppression part, has no polarity.

To switch the relay from a 5 V microcontroller, keep 86 on fused +12 V and pull 85 to ground through an NPN transistor or logic-level MOSFET. Size it for the coil current on the relay datasheet. Fit a flyback diode across 85 and 86 with its cathode on 86 unless the relay already contains one, and connect the 12 V ground to the microcontroller GND.

Figure: Automotive relay wiring diagram with a fused 12 V lamp circuit.

Conclusion

Understanding a relay wiring diagram starts with knowing its two separate circuits: the coil circuit, which controls the relay, and the contact circuit, which switches the load through the COM, NO, or NC terminals.

For electromechanical relays, the basic coil-driving circuit remains similar, while the contact wiring changes depending on the relay type and how you want the load to operate.

Use NO for a normally off load and NC for a normally on load. When working with mains voltage, always fuse and switch the live conductor, use a relay rated for the load, and keep all mains connections safely enclosed.

FAQs About Relay Wiring Diagrams