You can find all sorts of automotive relays in nearly every car, truck, and even boats. Relays are mainly used to let a low-power circuit control a higher-power one, like when you switch on your headlights. If you tried to hook up your headlights directly to the switch, you'd probably overload the switch, melt wires, and risk starting a fire! Relays are also handy for switching multiple things with one output. By connecting a single output to several relays, you can open and/or close circuits all at once. For example, you could turn on your radio while the antenna pops up. This article will dive into the parts and features of automotive relays and some important design and usage considerations.

Automotive Relay Circuit Design

Most automotive relays you'll come across are either Single-Pole, Single-Throw (SPST) or Single-Pole, Double-Throw (SPDT), and they draw very little current—usually under 200 milliamps. These relays have either normally closed or normally open contacts that can handle up to 30 or 40 amps. They work using a control circuit with a coil, and a load circuit with a switch. When you apply power to the coil, it creates a magnetic field that either opens or closes the switch. The diagrams below show how this all works.

The Snubber

When you disconnect power from the relay and the magnetic field collapses across the coil, a voltage spike happens. This spike is often several hundred volts, and it goes in the opposite direction of the normal current flow. Since automotive relays are usually controlled by sensitive electronic circuits, this reverse voltage needs to be controlled and dissipated. The most common way to do this is by placing either a resistor or a diode across the relay coil. This component is usually called a snubber.

Diode Snubbers

As we discussed in our article about diodes, when a diode is in reverse bias, it doesn't let current flow through. So, when the voltage direction is normal, no current passes through the diode. But when the reverse voltage happens, the diode gets forward biased, and excess voltage can flow through it, completing the circuit and going to the other end of the coil. This current keeps circulating in the diode and coil circuit until the voltage spike is gone.

Resistor Snubbers

Resistor snubbers work by letting only a limited amount of current flow through the resistor during a voltage spike. According to allaboutcircuits.com, "Resistor-Capacitor snubbers have been used in cars for years in engine ignition systems to minimize the arcing across switch contact ‘points’ in the distributor with a small capacitor, called a condenser. Any mechanic can tell you that the life of the distributor's ‘points’ depends on how well the condenser works."

Life Expectancy

Like anything else, automotive relays will eventually fail. Some even come with a rating for how many times they can close a circuit before they burn out. (Typical relays are rated for about 100,000 operations.) The good news is that ISO automotive relays are standardized, which means a lot of manufacturers make the same style. So, replacing them is usually cheaper than swapping out a switch!

Types of Automotive Relays

Electromechanical Relays

An electromechanical relay is a traditional type of relay that uses mechanical connections to open or close a circuit when it gets an electrical signal. It's made up of an electromagnet that moves contacts to build or detach the circuit. In automotive applications, these relays are known for being reliable and long-lasting. They're commonly used to switch high-current circuits like cooling fans, lamps, and starters. Electromechanical relays are perfect for applications that need high switching capacity and reliable handling of heavy electrical loads.

electromechanical relay

Time Delay Relays

A time delay relay is a special kind of relay that delays opening or closing the circuit in response to a control signal. It's often used in cars for things like headlight or interior light delay circuits, or in delay start circuits to prevent a big surge of current all at once.

time delay relay

Micro Relays

Micro relays are just smaller versions of electromechanical relays, made to save space in today’s cars where there’s not much room. Even though they’re tiny, they still give you performance similar to larger relays. You’ll find them used in places like power window controls, door locks, and signal switching in car electronics—basically, any situation where size and weight reduction matter.

Micro relays

Power Relays

Power relays are built for high-current loads, making them ideal for operations where you need to distribute and manage large amounts of electrical energy. Automotive systems like power steering, electric cooling fans, and heated seats all use power relays. They make sure that electrical systems run reliably and are protected when subjected to high loads. With their tough design and ability to handle high current, power relays are a vital part of modern car design.

Power relays

Advantages of Automotive Relays

High Current Capacity

Car relays are designed to handle large currents—way more than a regular electrical switch can manage. This means the relay can take care of power-hungry parts like motors, heaters, and other high-demand components without worrying about overheating or power loss. With its ability to effectively manage high current, the relay ensures the car’s electrical system runs smoothly and efficiently over time. This boosts overall reliability and safety.

Benefits:

Safety: Helps prevent overheating and damage while making sure high-current equipment runs safely.

Durability: Reduces stress on the switching mechanism, so electrical parts last longer.

Efficiency: Allows for better energy distribution with minimal energy loss, keeping the car’s system working at its best.

Isolation

Relays provide electrical isolation between the control circuit and the load circuit. This means that any disruptions or issues in the load circuit won’t affect the control circuit, and vice versa. It also helps reduce electromagnetic interference (EMI), protecting sensitive parts like sensors and control modules in the vehicle. Plus, it safeguards against voltage spikes caused by other electrical components.

Benefits:

Protection: Stops interference or power surges from damaging delicate electrical equipment.

Reliability: Keeps control circuits running smoothly by isolating them from load circuit issues.

Compliance: Helps meet important standards for electromagnetic compatibility (EMC), which is essential for vehicle safety and reliability.

Compact Size

Modern automotive relays are designed to be small and lightweight, taking up less space in today’s crowded car compartments. Their compact size allows for flexible installation in tight spaces, so they can be integrated into complex vehicle systems without sacrificing performance.

Benefits:

Space Saving: Maximizes use of vehicle space, especially important in smaller or electric vehicles.

Weight Reduction: Helps cut down the overall weight of the vehicle, improving fuel efficiency and performance.

Design Flexibility: Gives car designers more room to create innovative layouts without compromising relay functionality.

Versatility

Automotive relays are super flexible and can handle a variety of electrical systems in a vehicle. They can switch between different types of loads—resistive, inductive, and capacitive—making them perfect for controlling lights, motors, solenoids, heating elements, and more. Their adaptability lets them meet the specific needs of a wide range of automotive systems.

Benefits:

Adaptability: Works for many applications, from simple lighting circuits to advanced engine management systems.

Enhanced Functionality: Lets modern vehicles incorporate more advanced features and functionalities.

Cost Efficiency: Cuts down on the need for specialized switches or controls by offering a single solution for multiple applications.

Applications of Automotive Relays

Lighting Systems

Relays are key to controlling lighting systems in cars. They act as switches to control the power going to the interior lights, fog lights, headlights, and taillights. These lights use a lot of current, which the relays handle to keep the car’s wiring from overheating and ensure everything works efficiently.

Examples:

Headlights: Relays help manage the high current needed for headlights, ensuring they work reliably and shine bright.

Fog Lights: They let you turn on the fog lights separately from the main lights, which is especially useful in bad weather.

Interior Lighting: The relay controls the lights inside the car, like the roof lights and instrument panel lights, making sure everything’s lit up when needed.

Heating and Cooling Systems

Relays also control the heating and cooling systems in the car, like electric heaters, air conditioning compressors, and radiator fans. By managing these components, the relay helps keep the engine and passenger areas at the right temperature, improving both engine efficiency and passenger comfort.

Examples:

Radiator Fans: The relay turns the fan on and off based on engine temperature to prevent overheating.

Air Conditioning Compressors: It helps switch the AC compressor on and off, keeping the cabin temperature steady and boosting fuel efficiency.

Electric Heaters: Relays control the electric heater that warms the seats or defrosts the windows, making sure you stay comfortable on chilly days.

Engine Management

The relay is a crucial part of the engine management system. It controls important components like the fuel pump, ensuring the engine runs smoothly, efficiently, and reliably in all driving conditions.

Examples:

Fuel Pump: The relay turns on the fuel pump when the engine starts and keeps fuel pressure steady while the engine runs, so the engine always gets the right amount of fuel.

Ignition System: It controls the ignition module or coil, making sure the spark plugs fire at the right time for efficient combustion.

Engine Cooling Fans: The relay turns the cooling fan on and off as needed to maintain the right engine temperature, preventing overheating.

Power Windows and Door Locks

Relays are used in power window and door lock systems to control the motors that raise and lower windows or lock and unlock doors. They manage the large currents these motors need, giving you easy access and control.

Examples:

Power Windows: Relays activate the window motors to raise and lower windows at the touch of a button, making life easier.

Door Locks: They control the door lock actuators, letting you lock or unlock all doors from the driver’s seat or with a keyless entry system.

Fuel Systems

In vehicle fuel systems, relays are crucial for controlling parts like the fuel pump and injectors. They make sure fuel is delivered efficiently, improving engine performance and fuel economy.

Examples:

Fuel Injectors: The relay controls when the fuel injectors open and close, giving each cylinder the right amount of fuel for optimal combustion.

Fuel Pump: The relay keeps the fuel pump running, making sure fuel pressure stays steady for consistent fuel delivery.

Safety and Security Systems

Relays are vital in car safety and security systems, managing functions like airbag deployment. They also help activate systems like the anti-lock braking system (ABS) and alarms, ensuring quick and reliable responses to keep you safe and protect your car from theft.

Examples:

Airbag System: Relays trigger the airbags in the event of an accident, inflating them quickly to protect the occupants.

ABS System: They control the ABS solenoid valve, adjusting brake pressure during emergency braking to maintain steering control and prevent wheel lockup.

Alarm System: Relays activate the car’s alarm if there's an attempted break-in, warning the owner and deterring thieves.

Frequently Asked Questions

What is the most common automotive relay?

Even though there are different types of relays, the most common ones in low-voltage auto and marine applications are electro-mechanical relays. These work by activating an electromagnet to pull a set of contacts, either making or breaking a circuit.

What is an automotive relay?

An automotive relay is an electromagnetic switch that lets an electrical signal or a high current flow from the battery to a receiver. This could be things like dipped headlights, turn signals, or other actuators in your car.

Are all 12V car relays the same?

No, not all relays are the same. If you install a relay for the wrong application, it could cause a short (like in the internal relay circuit), and that could lead to functionality issues or even damage your car’s computer systems.

How to tell if a relay is bad?

To check if a relay is bad, listen for a click when the electromagnetic coil activates and the internal switch closes. You can also use a multimeter set to continuity mode. Listen for the beep as you test for continuity between the switch pins. If there’s no beep, it means the switch pins are stuck open, and your relay is faulty.

Do automotive relays need to be grounded?

Yes, automotive relays need both 12V positive and ground to trigger the relay. This is done through terminals 85 and 86. The relay can be triggered on either side of the circuit, but you need both a positive and negative connection to make it work. A good rule of thumb is that terminal 86 is usually positive.

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