For a single circuit, either an SPST or SPDT relay will work. However, a project like building a control system for your solar and battery setup will require choosing an SPDT relay.
A power relay is one of those must-have accessories in a van or motorhome, yet many beginners either ignore it or don't know about it. It allows you to control regular accessories and appliances β such as lamps, pumps, heaters and fans β using a remote control or by setting them for automatic operation. However, for any of these projects to succeed, you must choose the right relay with the right features.
Why do we say this? Some relays are simply raw relay cubes containing only the internal coil and terminal contacts, meaning you'll need to create your own circuit board and wiring setup. Others are complete plug-and-play relays preinstalled on a circuit module, allowing you to use them without needing knowledge of complex circuitry. A few examples of such plug-and-play relays include:
- 6β36V Wireless Relay Switch with Remote Control
- 12V/40A Waterproof 5-Pin Automotive Relay
- 6β36V DC Leaded Wireless Relay with Remote Control
- 12V/30A 4-Pin Automotive Relay with Blade Fuse
- 9β265V Wireless Relay Switch with Remote Control
- 12V DC Wireless Relay Switch with Remote Control
- 12V DC/100A Heavy-Duty 4-Pin Relay
If you'd like to automate and remotely control all the electronics in your caravan, campervan or motorhome, this post covers the key details you need to understand plug-and-play relays. We'll discuss the common relay types used for these kinds of projects and provide a brief guide on how to choose the right one.
What Is a Power Relay and How Does It Work?
A relay is a special type of switch that you can use to automatically or remotely control an appliance or accessory in your adventure vehicle. Unlike standard switches, which are installed directly along the hot path of a circuit, a relay has its own low-current input circuit that runs on DC power, AC power, or both. Best of all, it can manage electrical paths carrying either low or high current.
When it comes to how a power relay works, it's not all about electromagnetism, as many "internet experts" claim. The actual operation of a relay depends on its core technology and configuration.
Electromechanical vs Solid-State Relays
There are two main technologies used to operate a power relay β electromechanical and solid-state. While they both serve the same purpose β switching your circuits on and off β they work very differently:
- An electromechanical power relay uses mechanical components to switch your circuits on and off. These "mechanical components" include an electromagnet, pivoted armature, return spring, movable contact(s) and fixed contact(s).
- A solid-state power relay uses electronic components (instead of moving parts) to switch your circuits on and off. These "electronic components" typically include an infrared LED (or similar component) and a phototransistor made from semiconductors.
Between the two, electromechanical relays are the most common in consumer applications. This is because they can be easily integrated into everyday systems β such as lighting, pumps, motors, heaters, fridges, chargers and more β without hassle.
You can also use solid-state relays to control common electronic devices, including lights, heating equipment, motors and pumps. However, many DIY vanlifers still prefer the mechanical option because of its cost-effectiveness, electrical flexibility and other advantages.
SPST, SPDT, DPST and DPDT Relay Configurations Explained
The configuration of a power relay refers to what the device does when electricity is supplied to it. Like regular switches, we use the term poles to describe the input points or how many separate circuits the relay controls, and throws to describe the output paths or how many different connections each circuit has.
- An SPST (Single Pole Single Throw) relay is the simplest configuration available, with a single set of contacts β NO and COM β that closes when the relay is actuated. Thus, you can only use it to connect or disconnect a single circuit.
- An SPDT (Single Pole Double Throw) relay is a simple but slightly more advanced configuration with a single set of contacts β NC, COM and NO β capable of controlling two different paths. It is the most popular design because of its five terminal pins, hence the name 5-pin relay.
- A DPST (Double Pole Single Throw) relay has a slightly more sophisticated configuration, featuring two sets of contacts β NO1, COM1 and NO2, COM2 β that open and close simultaneously. This allows you to control two completely separate circuits at the same time.
- A DPDT (Double Pole Double Throw) relay is the most sophisticated configuration available, containing two independent pathways within one unit. It also has two sets of contacts β NC1, NO1, COM1 and NC2, NO2, COM2 β each capable of controlling independent circuits. In other words, each trigger opens or closes two circuits simultaneously.
How Do You Identify a Relay's Configuration?
The four configurations above are limited to electromechanical power relays. They all feature a coil (wound around a magnetic core) with two terminal points for the input.
In addition to the coil terminals, a relay will also have a terminal point for each pole and common pin. The "common pin" (abbreviated as COM) is the shared terminal point on the secondary/switch side of the relay.
As for the relay pole terminal points, we name them after the type of output they represent. The NO (Normally Open) terminal is usually disconnected from COM, whereas the NC (Normally Closed) terminal is usually connected to COM by default. Therefore, you can easily identify the configuration of a power relay by the number of terminals it has:
| Coil Terminals | Switch Terminals | Total Terminals | |
|---|---|---|---|
| SPST Power Relay | 2 | 2 (NO & COM) | 4 |
| SPDT Power Relay | 2 | 3 (NC, COM & NO) | 5 |
| DPST Power Relay | 2 | 4 (Two SPST circuits) | 6 |
| DPDT Power Relay | 2 | 6 (Two SPDT circuits) | 8 |
What this means is that you can also identify a relay model by its number of terminals. For instance, here are the configurations of the relays highlighted earlier:
- 9β265V wireless relay switch β SPST power relay
- 6β36V DC wireless relay switch β SPST power relay
- 12V DC wireless relay switch β SPDT power relay
- 6β36V wireless relay switch β SPST power relay
- 12V/40A waterproof 5-pin automotive relay β SPDT power relay
- 12V/30A 4-pin automotive relay with blade fuse β SPST power relay
- 12V DC/100A heavy-duty 4-pin relay β SPST power relay
How to Choose a 12V Relay for Your Caravan, Campervan, RV or Motorhome
The most widely used power relays in the caravanning and RV industry are the mechanical types, specifically SPST and SPDT designs. They are simple to set up, even for non-techies, highly affordable, and ultimately versatile. Below are thirteen crucial factors to consider when choosing relays for controlling your van's appliances or accessories.
Your Needs
The very first step when shopping for a power relay for your van project is identifying your application. Once you outline why you need the device, it becomes easier to determine the other aspects β electrical rating, functionality, operating conditions, et cetera.
Application Category
The second factor to consider is the type of SPDT or SPST relay you need for your application. Generally, there are several designs available, but the two options best suited for caravanning are automotive and general-purpose relays.
Compatible Electricity
If your project calls for a general-purpose relay, you must decide whether you want it to work with DC power, AC power, or both. Otherwise, using a DC-only relay with an AC power supply is highly dangerous and may result in fatal electrocution, short circuits or electrical fires.
Maximum Voltage Rating
Any relay switch can only handle the voltage threshold its components are designed for β coil, contacts, PCB, resistors, diodes and the rest. If your power source voltage exceeds the maximum voltage rating, it can lead to overheating and arcing. This may damage the relay and connected load or cause electric shock and electrical fires.
Tip: Below is the voltage rating of our reference power relays:
| Relay | Type of Relay | Voltage Rating | Dimensions |
|---|---|---|---|
| 6β36V wireless relay switch | SPST (NO) | 6Vβ36V (12Vβ24V AC) | |
| 12V/40A waterproof 5-pin automotive relay | SPDT (NO & NC) | 12Vβ14V DC | |
| 6β36V DC leaded wireless relay | SPST (NO) | 6Vβ36V DC | 24 x 36 x 54 mm |
| 12V/30A 4-pin automotive relay with blade fuse | SPST (NO) | 12Vβ14V DC | |
| 9β265V wireless relay switch | SPST (NO) | 9Vβ72V DC / 85Vβ265V AC | 30 x 51 x 105 mm |
| 12V DC wireless relay switch | SPDT (NO & NC) | 12V DC | 26 x 38 x 42 mm |
| 12V DC/100A heavy-duty 4-pin relay | SPST (NO) | 12V DC | 45 x 45 x 77 mm |
Current Carrying Capacity
Similarly, you must ensure the power relay you choose can safely handle the current passing through it. If the current exceeds its rated capacity, you can expect issues such as damage to the relay and connected load, electrocution and electrical fires.
Switching Functionality
SPST relays can be NO (normally open) or NC (normally closed), whereas SPDT relays offer both features. Therefore, when buying a general-purpose or automotive relay, you must decide what you want it to do within your circuit.
PS: A NO relay only switches on your accessory or appliance when activated, whereas an NC relay keeps your load powered when the relay is off.
Construction Quality
The construction quality of a relay also matters, as it determines how long the device will serve you before breaking down. Remember, your camper will be exposed to high vibrations on tarmac and off-road terrain. So, choose a relay made with a heavy-duty housing, solid terminals and high-quality internal components.
Ingress Protection
Different manufacturers design SPST and SPDT relays for different environments. If you need to mount your relay in an area exposed to the elements, choose one with good sealing against dust and moisture. This is especially important for automotive relays that may be mounted in the van's engine bay.
Temperature Rating
Besides ingress protection, your relay must also be able to operate under the temperature conditions of its mounting area. Therefore, check the temperature rating to ensure it suits your installation location β whether it's a hot engine bay or a cold outdoor environment.
Ease of Installation
Generally, SPST and SPDT power relays are straightforward to install. However, some have lead terminals, whereas others have screw-on terminals. You must also ensure the device has a mounting hole or bracket so it can be securely installed in your desired location.
Ease of Operation
Your power relay must also be easy to configure and operate. Of course, this usually isn't a problem with automotive relays, since they work automatically when your vehicle's electrical system sends the activation voltage. However, for general-purpose relays, it's beneficial to have an easily accessible setup button, and even better if the relay includes a remote control for wireless operation.
Physical Size
The physical dimensions of a relay make a huge difference when it comes to space-saving and electrical capability. A larger relay usually contains larger components that can handle more electrical power and dissipate heat more effectively. Consequently, a larger relay will also require more mounting space.
Therefore, when choosing a power relay for your caravan, campervan, RV or motorhome project, you must ensure it fits within the available mounting space while also handling the supplied electricity effectively.
Pricing and Value
Lastly, it's important to ensure the power relay you choose is worth the price you pay. For instance, our popular general-purpose wireless relays range from $32β$42 and include a remote-control fob. On the other hand, our bestselling automotive relays are available from $20 and feature a rugged flame-retardant housing, oxygen-free copper wiring and many other useful features. Would you consider our relays expensive, or worth every penny?
Can I Install Power Relays on My Own?
If you've been planning to automate or modernise your camper's electrical setup, we hope this guide has helped. As mentioned earlier, relays are essentially specialised switches with their own control circuit. Once you understand this, they become much easier to choose, wire into your circuits and operate.
That said, don't forget that the Australian/New Zealand Electrical Wiring Framework still limits DIY electrical work, especially when it involves hardwiring. You can only legally install these relays yourself when working with DC power circuits.
If you want your power relay hardwired into a 240V AC distribution system, you'll need a licensed electrician for the project. This is to ensure compliance with standards such as AS/NZS 3000, which govern general wiring requirements for wire sizing, circuit protection, isolation features, etc.
Frequently Asked Questions
What is the difference between a 4-pin and a 5-pin relay?
A 4-pin relay is an SPST design with two coil terminals plus NO and COM contacts, so it simply connects or disconnects a single circuit. A 5-pin relay is an SPDT design with NC, COM and NO contacts, letting one trigger switch between two different electrical paths.
How do I know if a relay is normally open or normally closed?
A NO (normally open) relay only switches on your accessory or appliance when the relay is activated, whereas an NC (normally closed) relay keeps your load powered when the relay is off. SPST relays are one or the other, while SPDT relays offer both in a single unit.
Can I install a 12V relay in my caravan myself?
Yes β under the Australian/New Zealand electrical wiring framework you can legally install relays yourself on DC power circuits. If the relay is to be hardwired into a 240V AC distribution system, you'll need a licensed electrician to ensure compliance with standards such as AS/NZS 3000.
How much does a power relay for a caravan cost?
Our popular general-purpose wireless relays range from $32β$42 and include a remote-control fob, while our bestselling automotive relays start from $20 and feature a rugged flame-retardant housing and oxygen-free copper wiring.
Ready to automate your setup? Check out our 6β36V wireless relay switch with remote control and start controlling your van's accessories the easy way.
