12v SPDT Relay Wiring Diagram – A Comprehensive Guide

If you’re looking for a comprehensive guide to understanding the 12v SPDT relay wiring diagram, then you’ve come to the right place. In this article, we’ll explain everything you need to know about SPDT relays and their wiring, including their working principles, advantages, and how to wire them in different applications.

What is a 12v SPDT Relay?

A Single Pole Double Throw (SPDT) relay is an electromagnetic switch that is activated by a low voltage input, such as 12 volts. It has three terminals: one common terminal (C) and two switched terminals (NC and NO). The NC terminal is normally closed (connected to C) when the relay is not energized, while the NO terminal is normally open (disconnected from C) in the same state. When the relay is energized, the NC terminal disconnects from C, while the NO terminal connects to C.

A 12v SPDT relay is designed to work with a 12 volt DC power source, and is commonly used in automotive, marine, and industrial applications. It is versatile and can be used for a variety of purposes, such as controlling lights, fans, motors, and other electrical devices.

Advantages of a 12v SPDT Relay

There are several advantages to using a 12v SPDT relay:

  1. Reliability: Relays are highly reliable and can switch high currents and voltages without damaging the control circuit.
  2. No electrical noise: Unlike solid-state relays (SSRs), SPDT relays do not produce electrical noise, making them ideal for sensitive applications.
  3. Easy to replace: Relays are easy to replace when they fail, which saves time and money compared to repairing a damaged circuit board or component.
  4. Low power consumption: Relays draw very little power from the control circuit, which reduces energy costs and extends battery life in mobile applications.

How to Wire a 12v SPDT Relay

Wiring a 12v SPDT relay is a simple process that requires a few basic tools and knowledge of electrical circuits. Here’s how to wire a 12v SPDT relay:

Step 1: Gather the necessary tools

To wire a 12v SPDT relay, you’ll need the following tools:

  • A 12v power source
  • A 12v SPDT relay
  • A multimeter
  • A wire stripper/cutter
  • A soldering iron and solder
  • A heat shrink tubing
  • A wiring diagram or schematic

Step 2: Identify the relay pins

Before wiring the relay, you need to identify the pins and their functions. A typical 12v SPDT relay has three pins: C (common), NO (normally open), and NC (normally closed). The pins are usually labeled on the relay body or on the datasheet. If you’re unsure, use a multimeter to check the continuity between each pin and the corresponding label.

Step 3: Connect the power source and load

The power source is connected to the C pin of the relay, while the load (the device that you want to control) is connected to either the NO or NC pin, depending on the desired operation. For instance, if you want to turn the device on when the relay is energized, connect it to the NO pin. On the other hand, if you want to turn the device off when the relay is energized, connect it to the NC pin.

Step 4: Connect the control circuit

The control circuit is connected to the control pins of the relay, which are usually labeled as COIL or + and -. The + pin is connected to the positive terminal of the power source, while the – pin is connected to the ground. When a voltage is applied to the coil, it generates a magnetic field that activates the relay contacts, causing the device to turn on or off.

Step 5: Test the circuit

Once you’ve wired the relay, it’s important to test the circuit to ensure that it works as intended. Use a multimeter to check the voltage across the load when the relay is energized and de-energized. If the voltage is within the acceptable range, then the circuit is working properly.

FAQ – Frequently Asked Questions

1. Can I use a 12v SPDT relay with a 24v power source?

No. A 12v SPDT relay is designed to work with a 12 volt DC power source only. Using a higher voltage can damage the relay contacts and other components in the circuit.

2. How much current can a 12v SPDT relay handle?

The current rating of a 12v SPDT relay depends on the specific model and manufacturer. Typical current ratings range from 10A to 30A. Be sure to check the datasheet or specifications before selecting a relay for your application.

3. Can I wire multiple devices to a single 12v SPDT relay?

Yes, but the combined current draw of all the devices must not exceed the maximum current rating of the relay. For example, if the relay is rated for 20A, and you want to control two devices that draw 10A each, you can wire them in parallel to the NO or NC pin of the relay. However, if you want to control devices that draw more than 20A, you’ll need to use a higher-rated relay or multiple relays.

4. How do I wire a 12v SPDT relay for inversion control?

Inversion control, also known as reverse control, is a method of controlling a device so that it operates in the opposite state when the relay is energized. To wire a 12v SPDT relay for inversion control, simply connect the power source and load to the NC and C pins, respectively. Then, connect the control circuit to the NO pin. When the relay is de-energized, the load will be on. When the relay is energized, the load will be off.

5. What is the difference between a SPDT and DPDT relay?

A Double Pole Double Throw (DPDT) relay is similar to a SPDT relay, but it has two sets of contacts instead of one. This allows it to switch two separate circuits simultaneously. DPDT relays are typically used for more complex applications that require multiple switch functions.


In summary, understanding the 12v SPDT relay wiring diagram is essential for any DIY enthusiast or technician who wants to control electrical devices with precision and efficiency. By following the steps outlined in this guide, you can wire a 12v SPDT relay for any application, from simple lighting circuits to complex motor controls. Be sure to check the datasheet and specifications of the relay before use, and always test the circuit before installation.