An automotive relay is like a remote-controlled switch that lets a low-current circuit control a high-current one—perfect for safely powering things like headlights, horns, fans, or fuel pumps without running heavy wires through your dashboard.
⚙️ How It Works
An automotive relay has two circuits:
- Control (coil) circuit: Low current, activated by a switch or signal
- Load (power) circuit: High current, powers the actual device
When voltage is applied to the coil terminals (usually pins 85 and 86), it creates a magnetic field that pulls a switch closed inside the relay. This connects the power input (pin 30) to the output (pin 87), allowing current to flow to your accessory.
Common Relay Pinout (DIN 72552 Standard)
| Pin | Function |
|---|---|
| 85 | Coil ground (negative) |
| 86 | Coil power (positive) |
| 30 | Common power input |
| 87 | Normally open (power output) |
| 87a | Normally closed (optional) |
In a 4-pin relay, you’ll use 85, 86, 30, and 87. In a 5-pin relay, 87a is added for circuits that need power when the relay is off.
🛠️ How to Wire a 4-Pin Relay (Example: Headlights)
- Pin 85 → Ground
- Pin 86 → Trigger switch (e.g., headlight switch)
- Pin 30 → Fused 12V power from battery
- Pin 87 → Headlight positive wire
When the switch sends power to pin 86, the relay activates and sends battery power from pin 30 to pin 87—lighting up your headlights. 🧠 Why Use a Relay?
- Prevents voltage drop and switch burnout
- Allows thin wires and small switches to control big loads
- Adds safety and reliability to your electrical system
