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What role do relay boxes play in a car?

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What is a Relay?

A relay is essentially an electrically operated switch. It uses a small amount of current to control a much larger current, enabling the safe and efficient operation of various electrical components in your car.

Relays consist of a set of input terminals for single or multiple control signals and a set of operating contact terminals. The switch may have any number of contacts in a variety of contact forms, such as on contacts, off contacts or combinations thereof.

What is a Fuse?

In electronics and electrical engineering, a fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current. It is a sacrificial device; once a fuse has operated it is an open circuit and must be replaced or rewired, depending on its type.

Curious about the different types of fuses in your car and how to identify a blown one? Check out our comprehensive guide, ‘A Guide to Automotive Fuses‘.

Common Relay Types and Their Automotive Roles

Common Version (Easy to Understand)

  • SPST-NO (Single-Pole Single-Throw, Normally-Open)

This simple relay acts like a basic on/off switch. When activated, it closes the circuit, allowing current to flow. Think of it like the relay that turns on your headlights or fog lights.

  • SPST-NC (Single-Pole Single-Throw, Normally-Closed)

This one works in reverse. It keeps a circuit closed until activated, then opens it to stop the current. Imagine it like the relay that cuts power to your starter motor once the engine is running.

  • SPDT (Single-Pole Double-Throw)

This versatile relay acts like a two-way switch, directing current to one of two paths depending on its state. Think of it like the relay that controls your windshield wipers, switching between high and low speeds.

  • DPST (Double-Pole Single-Throw)

Imagine two SPST relays combined into one. This type controls two separate circuits simultaneously, either both on or both off. This could be used to control your high beams and fog lights together, for example.

  • DPDT (Double-Pole Double-Throw)

The most complex of the bunch, this relay is like having two SPDT relays in one, offering intricate control over multiple circuits. Think of it like the relay managing your power windows, controlling both up and down movement for each window.

Specialized Version (Complex)

  • SPST-NO (Single-Pole Single-Throw, Normally-Open)

This type features a single Form A contact, also known as a “make” contact. It has two terminals that can be connected or disconnected by the relay action. Including the two terminals for the coil, an SPST-NO relay has four terminals in total.

  • SPST-NC (Single-Pole Single-Throw, Normally-Closed)

This type uses a single Form B contact, also known as a “break” contact. Like the SPST-NO relay, it has four terminals in total, but the contact behavior is opposite: the circuit is closed when the relay is not energized and opens when the relay is activated.

  • SPDT (Single-Pole Double-Throw)

This type utilizes a single set of Form C contacts, known as “break-before-make” or “transfer” contacts. It has a common terminal that connects to either of the other two terminals, but never both at the same time. This functions like a selector switch, directing the current flow to one of two paths. Including the coil terminals, an SPDT relay has five terminals.

  • DPST (Double-Pole Single-Throw)

This is essentially equivalent to two SPST switches controlled by a single coil. It has six terminals, including the coil terminals. The poles can be either Form A (NO) or Form B (NC), or one of each. The specific configuration should be indicated using the NO and NC designations for clarity.

  • DPDT (Double-Pole Double-Throw)

This type is like having two SPDT switches controlled by a single coil. It has two sets of Form C (transfer) contacts and eight terminals, including the coil terminals. This allows for more complex switching possibilities for controlling multiple circuits.

Circuit Symbols of Relays

Understanding Terminal Labels in Automotive Relays

When working with automotive relays, you’ll often encounter numbers on the relay terminals. These numbers aren’t random; they follow a standard called DIN 72552, which ensures consistent labeling for easier identification and troubleshooting. Commonly Used Terminal Labels:

  • 85 & 86

These terminals connect to the relay coil, which is like the “brain” of the relay. Terminal 85 typically connects to the negative (-) side of the control circuit, while terminal 86 connects to the positive (+) side.

  • 87

This terminal is the “output” of the relay and connects to the load that you want to control, such as a headlight or a fuel pump. This is typically a normally open (NO) contact, meaning it’s open when the relay is not energized and closes when the coil is energized.

  • 87a

This terminal is also connected to the load but is normally closed (NC). This means it’s closed when the relay is not energized and opens when the coil is energized.

  • 30

This terminal connects to the positive (+) side of the battery or power source.

Terminal Labels

How do Relays Work?

The electromagnet starts energizing when the current flows through the control coil and then intensifies the magnetic field. The electromagnet becomes connected to the power source through the contacts to the load and a control switch. The upper contact arm becomes attracted to the lower fixed arm and then closes the contacts that result in a short circuit. The contact then moves in the opposite direction and creates an open circuit once the relay has been de-energized.

The movable armature will return to its initial position when the coil current is off. The force that causes its movement will be almost the same as the half strength of the magnetic force. Spring and gravity provide this force.

The Relay’s Inner Workings

  • The Coil and the Electromagnet

At the heart of a relay lies a coil of wire. When a small electrical current flows through this coil, it creates a magnetic field, turning the coil into an electromagnet.

  • Magnetic Attraction

This magnetic field attracts a movable arm, called an armature, towards it. The armature is connected to a set of contacts, which are like tiny switches.

  • Completing the Circuit

As the armature moves, it closes the contacts, completing a circuit and allowing a much larger current to flow. This is how a small signal controls a powerful component.

  • Breaking the Circuit

When the current to the coil stops, the magnetic field disappears. The armature, helped by a spring, returns to its original position, opening the contacts and breaking the circuit.

Think of it this way the small current is like your finger flicking a light switch and the relay is like a powerful mechanical arm that flicks a huge lever that controls the current flowing to the large appliance.

Relay Design

Why are Relays Used in Cars?

Relays are essential components of your car’s electrical system, acting as safeguards and control units for various functions. Let’s delve into why they are so crucial:

  • Headlights

These high-draw components require robust switching. Relays handle the amperage load with ease, ensuring crisp transitions between high and low beams without compromising the delicate headlight switch.

  • Starter Motor

Cranking the engine demands a surge of power. Relays act as the intermediary, using a low-current control signal to engage the starter’s high-amperage circuit, preventing wear and tear on the ignition switch.

  • Fuel Pump

Consistent fuel delivery is crucial for engine performance. Relays maintain a steady and controlled power supply to the fuel pump, optimizing fuel efficiency and engine responsiveness.

  • Cooling Fans

Keeping your engine cool under pressure is vital. Relays activate the cooling fans based on temperature thresholds, preventing overheating and potential engine damage during demanding drives.

  • Air Conditioning Compressor

Relays manage the air conditioning compressor clutch, delivering refreshing cool air while minimizing strain on the A/C control system.

  • Power Windows, Seats, and More

From power windows and seats to sunroofs and complex entertainment systems, relays ensure smooth and reliable operation of these convenience features, adding to your driving pleasure.

What is the Purpose of Automotive Fuse and Relay Boxes?

Because the relays are changed and closed frequently, they need their own fuses to protect them. In practical application, it is placed in the same position as the fuse. That’s why the fuse and relay are sometimes in the same box. Fuses are used in automobiles to limit current flow and prevent fires. A relay is a switching device that uses a small current to control a much larger current in a second circuit.

  • Centralized Protection Hub

Fuse boxes act as the central nervous system of your vehicle’s electrical safety, housing multiple fuses that guard against overcurrent and short circuits, preventing damage to components and potential fire hazards.

  • Organized Accessibility

Relay boxes provide a dedicated space for relays, eliminating the need to hunt for individual components scattered throughout the engine bay. This organization facilitates easy diagnostics and replacement when needed.

  • Relay Safeguards

Relays, with their frequent switching actions, are susceptible to overloads. Dedicated fuses within the fuse box provide an additional layer of protection for these workhorses of the electrical system.

  • Streamlined Maintenance

Fuse and relay boxes simplify troubleshooting and repair, allowing you to quickly identify and replace faulty components, getting you back on the road with minimal downtime.

  • Enhanced Vehicle Safety

By preventing electrical overloads and short circuits, fuse and relay boxes are essential for maintaining the safety and integrity of your vehicle’s electrical system.

Mastering your car’s electrical system starts with understanding the roles of relays and fuses. This knowledge empowers you to troubleshoot issues, perform maintenance, and appreciate the intricate engineering that keeps your vehicle running at its peak.

Power Up Your Ride with Daier Relay Boxes

Daier offers high quality relay boxes for automotive applications, designed to meet the needs of modern vehicles and provide reliable performance in a variety of conditions.

DIY Relay Box

RB-R6F6 6 Way Universal Fuse Relay Block Kit


RB-R6F6 Automotive 6 Way Universal Fuse Relay Block Kit

The 6 Way Universal Fuse Relay Block Kit is 6 slots relay panel design, 5 slots for 5-pin Bosch style relays, and 1 slot for RTT7121 4-pin relay.
RB-R10F15 Fuse and Relay Box


The 15 Way Universal Fuse and Relay Box is 10 slots relay panel design, that includes 50×relay terminals, and 30×fuse terminals.

Each slot is precisely cut and has a curved outlet on the edge to keep all your wiring neat.

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Relay Box Kits

RB-R6F6S-W1 12V Waterproof 6 Way Automotive Fuse And Relay Box Part#RB-R6F6S-W1

The automotive fuse and relay box includes 6-way relay housing + relay terminal + insurance insert terminal + mounting bracket and waterproof adhesive pad, with neutral 4-pin relay and insert fuse with wire.

Crafted from durable nylon 66 material with an IP65 waterproof rating, our relay box is built to withstand harsh conditions and provide long-lasting performance.

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RB-R6F6-W2 Universal Pre-wired 6 Slots 12 Volt Auto Fuses and Relays Box Part#RB-R6F6-W2

The auto fuses and relays box is a 6-slot relay and 6-slot fuse design that includes 6×Bosch-style 4-pin relays, 6×ATC/ATO fuses, and pre-wired.

The 12-volt relay box is equipped with clearly labelled terminals for relays and fuses, making it easy to install and ready to use, even for DIY enthusiasts.

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RB-R4F12-W1 8 Way Relay Fuse Block Box Part#RB-R4F12-W1

The Waterproof 8 Way Relay Fuse Block includes 4×12V 40A 4-pin relays, 12×blade fuses, 20×relay terminals, 24×fuse terminals, and wires.

The relay base can use both relay and fuse at the same time, with high performance and stable performance.

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RB-R6F11-W1 11Way Pre-wired Relays and Fuses Relay Block Box Part#RB-R6F11

The 11-way relays and fuses relay block box includes 6×4-pin or 5-pin relays, 11×blade fuses, 30×relay terminals, 22×fuse terminals with pre-wired.

This 12v relay box will also come with terminals for relays and fuses, which will enable you to install and use the fuse and relay box at once.

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Whether you’re a seasoned car enthusiast or just looking to add some extra functionality to your ride, our relay boxes are the perfect solution.


By understanding the role of relays and fuse boxes, car owners can better appreciate the complex electrical systems that keep their vehicles running smoothly and safely. These often-overlooked components are the unsung heroes behind many of your car’s functions, from starting the engine to powering your favorite accessories.

Eager to delve deeper into the world of automotive electrical components? Explore our informative resources on fuse relay boxes, fuse holders, and fuse blocks by clicking the links. For a comprehensive overview of our offerings, download our 2024 Automotive Parts Catalogue.

Need specific product information or pricing? Reach out to our team at sales1@chinadaier.com. We’re always happy to assist you!

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