How to Wire a Relay with Different Pin Diagrams?

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Author: Kynix

Published: 2021-10-14 | Last Updated: 2025-12-25

Contents

A Relay is an electrically operated switch. It allows a low-power signal (like one from a microcontroller or dashboard switch) to control a high-power circuit (like an electric motor, headlights, or industrial machinery). In essence, it provides complete electrical isolation between the control system (input loop) and the controlled system (output loop).

Used extensively in automotive systems, industrial automation, and modern Smart Home setups, the relay acts as an "automatic switch." It uses a small current to control a much larger one, offering crucial benefits like automatic adjustment, safety protection, and circuit conversion.

As of 2025, while Solid State Relays (SSRs) are gaining popularity for their silence and longevity, the traditional electromechanical relay remains the industry standard for high-current and cost-effective switching. This guide covers how to wire these essential components effectively.

Ⅰ Electrical Relay Structure & Basics

Electrical Relay Structure Diagram showing Coil, Armature, and Contacts

Figure 1. Electrical Relay Structure

To understand how to wire a relay, you must first understand its internal architecture:

Core Components: A relay consists of four primary parts: the coil, the magnetic circuit (core/yoke), the spring, and the contacts.
The Coil: When energized, the coil generates an electromagnetic field. This attraction pulls the armature, changing the state of the contacts.
Magnetic Circuit: Comprising an iron core, choke, and armature, this establishes the path for magnetic flux.
Air Gap: This is the critical distance between the armature and the core. When the coil is off, the gap is at its maximum (contacts in initial state). When on, the gap closes (contacts switched).
The Spring: Provides the resetting force. When the coil is de-energized, the spring pushes the armature back to its original position.
Contacts: These execute the control. They are divided into Normally Closed (NC) and Normally Open (NO).
- Energized: NC opens, NO closes.
- De-energized: Contacts reset to initial state.

Common Types of Relays:

Voltage Relays: High coil turns, thin wire. Connected in parallel with the load. (Most common).
Current Relays: Few turns, thick wire. Connected in series with the load.
Intermediate Relays: Used for signal transmission and controlling multiple secondary circuits.

Ⅱ How Do Relays Work?

An electromechanical relay is a switch operated by an electromagnet. When the coil receives current, the magnetic force pulls the "Common" (COM) contact arm from the "Normally Closed" (NC) position to the "Normally Open" (NO) position. When power is cut, a spring snaps it back.

In short: When a specific input (voltage, current, temperature) hits a set value, the relay changes the state of the output circuit to control or protect the system.

Example Analysis: Controlling a Light

8 Pin Relay Wiring Connection Schematic

Figure 2. 8 Pin Relay Wiring Connection

Relay Controls One Light Wiring Diagram

Figure 3. Relay Controls One Light

Wiring Logic:

To control a lamp using a relay, the power circuit is wired through the relay's contacts. The Neutral wire connects directly to the lamp. The Live (Hot) wire connects to the relay's Normally Open (NO) contact. When the relay is triggered, the circuit closes, and the light turns on.

Relay Controls Two Lights - Switching Logic

Figure 4. Relay Controls Two Lights (Toggle)

Dual Light Setup: By using both NC and NO contacts, you can toggle between two loads. When the coil is OFF, the NC light is ON. When the coil is ON, the NC light turns OFF and the NO light turns ON.

Ⅲ Relay Wiring with Different Pins

3.1 3-Pin Relay

What is a 3-Pin Relay?
These are commonly found in automotive applications as Flasher Units (for turn signals) or simplified horn relays. They work on electromechanical or thermal principles to cycle power on and off.

How to Wire a 3-Pin Relay:

3 pin relay wiring diagram

Figure 5. 3-Pin Relay Wiring Diagram

Standard configuration for a horn or load:

Pin 1 (Load): Connected to the device (e.g., horn).
Pin 2 (Battery/Power): Connected to the 12V power source (Common).
Pin 3 (Switch/Coil): Connected to the button (e.g., steering wheel button).

3.2 4-Pin Relay (SPST)

What is a 4-Pin Relay?
The 4-pin relay is the most common Single Pole Single Throw (SPST) relay used in automotive and general electronics to switch a single circuit on or off.

How to Wire a 4-Pin Relay:

4 pin relay wiring diagram

Figure 6. 4-Pin Relay Wiring Diagram

Pins 85 & 86 (Coil): These control the magnet. Connect one to ground and the other to your switch (+12V).
Pin 30 (Common): Connected to the high-power source (Battery +).
Pin 87 (Normally Open): Connected to the load (Fan, Light, Motor).

When the coil (85/86) is energized, Pin 30 connects to Pin 87.

12V 40A 4 Pin Relay

Figure 7. Standard 12V 40A 4-Pin Relay

Figure 8. Coil Pins (85 & 86)

Figure 9. Contact Pins (30 & 87)

3.3 5-Pin Relay (SPDT)

What is a 5-Pin Relay?
This is a Single Pole Double Throw (SPDT) relay. It allows you to switch power between two circuits (e.g., High Beam vs. Low Beam) or simply use the "Normally Closed" feature.

How to Wire a 5-Pin Relay:

5 pin relay wiring diagram

Figure 10. 5-Pin Relay Wiring Diagram

Pins 85 & 86: Coil (Control).
Pin 30: Common (Power In).
Pin 87a: Normally Closed (Power flows here when relay is OFF).
Pin 87: Normally Open (Power flows here when relay is ON).

3.4 6-Pin Relay

What is a 6-Pin Relay?
A 6-pin relay often functions similarly to a 5-pin but includes an extra terminal for internal bridging or specific DPDT signal configurations. In some automotive wiper relays, the extra pin handles parking logic.

Wiring Overview:

6 pin relay diagram

Figure 11. 6-Pin Relay Wiring Diagram

Typically, two pins act as the coil, and the remaining four form two pairs of switching contacts (or one complex changeover). Always check the specific datasheet, as 6-pin configurations vary more than standard 4/5-pin types.

3.5 8-Pin Relay (DPDT)

What is an 8-Pin Relay?
This is usually a Double Pole Double Throw (DPDT) relay. It effectively houses two 5-pin relays inside one shell, controlled by a single coil. It is ideal for reversing polarity on motors.

How to Wire an 8-Pin Relay:

8 pin relay diagram

Figure 12. 8-Pin Relay Wiring Diagram

Pins 2 & 7: Coil terminals (Power these to activate).
Pins 1 & 8: Common terminals (COM).
Pins 3 & 6: Normally Open (NO).
Pins 4 & 5: Normally Closed (NC).

3.6 Intermediate (Auxiliary) Relay

What is an Intermediate Relay?
Often used in industrial control panels (DIN Rail mounted), these relays transmit signals to control multiple larger contactors or actuators simultaneously. They are the backbone of classical automation logic.

Wiring and Safety (Flyback Diodes):

Intermediate Relay Wiring Diagram

Figure 13. Intermediate Relay Wiring Diagram

Standard industrial numbering (IEC):

13 & 14: Coil (A1/A2).
Contacts: Arranged in groups (e.g., 5-6-7-8 as NC, 9-10-11-12 as NO).

⚠️ 2025 Safety Tip: When using intermediate relays with DC currents, always install a Freewheeling (Flyback) Diode across the coil (Reverse biased: Cathode to Positive). When the coil turns off, the collapsing magnetic field creates a high-voltage spike (back EMF) that can destroy sensitive control electronics (like PLCs or Arduino boards).

Ⅳ FAQ: Relay Wiring in 2025

1. What is the difference between a Solid State Relay (SSR) and a Mechanical Relay?
Mechanical relays use moving parts (magnets/contacts) and make a "click" sound. They are cheaper and handle high surge currents well. SSRs use semiconductors (light/optical isolation), have no moving parts, are silent, and last much longer, but they generate heat and are generally more expensive.

2. What do the numbers on a standard automotive relay mean?
These are DIN standard numbers:
30: Common (Main Power Input)
85: Coil Ground
86: Coil Positive (Trigger)
87: Normally Open (Output when ON)
87a: Normally Closed (Output when OFF)

3. Does a Smart Home relay switch require a Neutral wire?
Yes. Unlike older mechanical switches that just cut the Live line, most modern 2025 Smart Relays (WiFi/Zigbee) need a Neutral wire to power their internal WiFi chip so they can stay connected even when the light is off.

4. What happens if I wire Pins 85 and 86 backwards?
On a standard mechanical relay without a diode, nothing happens—it will still work because the coil is not polarized. However, if the relay has a built-in suppression diode (common in modern cars), wiring it backwards will cause a dead short and blow your fuse.

5. What is an SPDT Relay?
SPDT stands for Single Pole Double Throw. It has one input (Common) and two outputs (NC and NO). It can route power to Circuit A when off, and switch to Circuit B when on.

6. Can I use a 12V relay on a 24V circuit?
No. You must match the Coil Voltage to your control system (e.g., 12V car vs. 24V truck). However, the contacts (switch part) can often handle higher voltages than the coil. Always check the rating printed on the case.

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How to Wire a Relay with Different Pin Diagrams?

Ⅰ Electrical Relay Structure & Basics