Relays in the Automotive World: What They Are, What They Do, and Why You Need Them
Relays are an essential yet often overlooked component in the automotive world. Whether you’re building a custom vehicle, modifying existing systems, or simply maintaining your car, relays play a crucial role in managing the electrical systems that power modern vehicles. But what exactly are relays, what do they do, and why are they so important? Let’s dive into everything you need to know about automotive relays.
What Are Automotive Relays?
An automotive relay is an electrically operated switch. Relays allow a small electrical current to control a larger one, enabling the safe and efficient operation of various systems in a vehicle. Typically, relays consist of an electromagnet (coil), a spring-loaded switch, and a set of contacts. When a small current is passed through the coil, it creates a magnetic field that moves the switch and connects the contacts, completing a larger electrical circuit.
In simpler terms, a relay lets you use a low-power signal to control a high-power circuit without risking damage to your control switch or wiring.
How Do Relays Work?
Here’s a step-by-step look at how a basic automotive relay operates:
- Low Power Input: A low-current signal (often from a switch or control module) is sent to the relay’s coil.
- Electromagnetic Field: This current energizes the coil, creating an electromagnetic field.
- Switching Action: The electromagnetic field pulls a metal arm (or switch) to close or open a set of contacts.
- High Power Output: Once the contacts close, they allow a higher current to flow through, powering devices like lights, fans, or fuel pumps.
- Deactivation: When the low-current signal stops, the magnetic field collapses, and the contacts open again, turning off the high-power device.
Types of Relays
- Normally Open (NO): The relay’s default state is “off” (the circuit is open). When the relay is energized, it closes the circuit.
- Normally Closed (NC): The relay’s default state is “on” (the circuit is closed). Energizing the relay opens the circuit.
- SPST (Single Pole Single Throw): This is the simplest relay type, controlling just one circuit with one output.
- SPDT (Single Pole Double Throw): This relay has one input and can toggle between two different outputs.
- DPDT (Double Pole Double Throw): Controls two independent circuits, useful for complex systems.
Why Do You Need Relays?
Protection from Overcurrent: Relays prevent small switches and control modules from being overloaded by high-current devices. If you tried to power headlights or a starter motor directly through a dashboard switch, you’d likely fry the wiring or the switch itself. A relay handles the high-current load, while the switch safely controls it.
Reduce Wiring Complexity: Relays allow you to control high-power circuits with low-current wiring, reducing the need for thick, heavy gauge wiring all the way from the control switch to the device. Instead, you can use lightweight wiring for the switch and route the high-current wiring through the relay, which is closer to the power source.
Power Efficiency: Since relays can switch large amounts of current with a small trigger signal, they reduce the electrical strain on control systems, improving overall efficiency.
Control High-Current Devices: Relays are essential when controlling high-current devices like starter motors, headlights, fuel pumps, cooling fans, and air conditioning compressors. Most of these devices require more current than the control system can provide directly, making relays the only safe option.
Automating Functions: Relays enable automation, such as turning on auxiliary lights when the engine starts or turning off accessories when the ignition is off.
When Do You Need Relays?
When Controlling High-Power Components: If you’re adding powerful fog lights, an electric radiator fan, or a winch, you’ll need relays to manage the high current required to operate them.
For Aftermarket Modifications: If you’re installing new equipment like air horns, additional lights, or sound systems, relays are needed to ensure proper power delivery without overloading your vehicle’s existing electrical system.
Starter Circuits: The starter motor pulls a significant amount of current. A relay helps manage this by allowing a small ignition switch to control the large amount of power needed to start the engine.
Automated Circuits: If you want to set up automated features, such as turning off auxiliary power when the ignition is off, or switching between low and high power circuits based on certain conditions, relays make this possible.
Protecting Sensitive Electronics: Modern cars have numerous sensitive control systems, including Engine Control Modules (ECM), Body Control Modules (BCM), and more. Relays can isolate these systems from power-hungry devices, reducing the risk of electrical surges or malfunctions.
How to Use Relays in Automotive Applications
Using a relay involves wiring the appropriate terminals correctly. A standard 4-pin relay will usually have the following labels:
- Pin 85: Connected to the ground.
- Pin 86: Connected to the control signal, such as a switch or control module.
- Pin 30: Connected to the power source (battery or alternator).
- Pin 87: Connected to the device you are powering (lights, fan, etc.).
For relays with 5 pins (SPDT types), the additional terminal is:
- Pin 87a: Normally connected to Pin 30 when the relay is not energized.
Here’s a practical example: If you’re installing auxiliary lights on your off-road vehicle, the switch inside the cabin would connect to Pin 86, Pin 85 would go to ground, Pin 30 to the battery, and Pin 87 to the positive terminal of the lights. When you flip the switch, the relay activates, sending power from the battery to the lights without routing high current through the switch.
Whether you’re wiring up high-power devices or ensuring your car’s control systems operate safely, relays are indispensable. By understanding how relays work, when to use them, and how to install them correctly, you can make smarter decisions when modifying or maintaining your vehicle’s electrical systems.
If you have more questions about relays, feel free to call our techs at (800) 423-9696! Or click here to see our full line up of relays to make your wiring upgrade PAINLESS!
