Fog Light Circuit: Trailer Connection & Relay Control

Description

In numerous nations, it is now mandated or, at a minimum, strongly advised to possess a rear-facing auxiliary lighting system on a vehicle trailer. Furthermore, the regulatory framework necessitates that when the trailer is attached to the vehicle, the rear-facing auxiliary lighting of the towing motor vehicle must be deactivated. The circuit presented below is exceptionally well-suited for this particular application. The circuit is strategically positioned in proximity to the vehicle’s rear fog light. The 12-V power supply to the lamp is diverted and instead connected to relay contacts 30 and 87A (K1, K3). The rear fog light of the automobile will continue to function without interruption when activated.

Circuit diagram:

Should a trailer equipped with a fog light be connected to the trailer connector (7- or 13-way, K2), a current will flow through L1. L1 represents a coil consisting of approximately 8 turns, meticulously wound around reed contact S1. S1 will engage due to the current passing through L1, thereby energizing relay Re1 and the rear fog light of the automobile is automatically switched off. The trailer’s fog light will, naturally, remain illuminated. The dimensions of L1 are determined by the specifications of reed contact S1. The fog lamp is rated at 21 W, resulting in a current of 1.75 A at 12 V. L1 is sized to accommodate a current between 1.0 and 1.5 A, ensuring a reliable contact closure. The wire gauge should be approximately 0.8 mm. The relay Re1 is a standard automotive relay designed to manage the flow of lamp current.

More about components

Relay Re1 is an automotive relay that is capable of switching the lamp current. This type of relay is commonly used in automotive applications to control high-power circuits, providing a reliable and electrically isolated switching mechanism. It is designed to handle the surge currents associated with the operation of the fog light, ensuring safe and efficient operation of the circuit. Typical automotive relays for this purpose operate at 12V DC and can handle current loads up to 30A, depending on the specific model and manufacturer specifications. They are characterized by their robust construction, moisture resistance, and long lifespan, making them suitable for the demanding environment of a vehicle's electrical system. The specific model used in this circuit would be a common 12V DC automotive relay, often featuring a latching mechanism for easy operation. The relay's contacts are designed to be normally open, ensuring that the fog light is off until activated by the relay. The mechanical operation of the relay is typically achieved using a small solenoid, which is energized by the current flowing through L1. This solenoid pulls a mechanical lever, closing the contacts and allowing current to flow to the fog light. The time delay incorporated in the relay's design helps prevent nuisance switching due to electrical noise or transient voltage spikes.