Fog Lamp Switch Circuit

In many countries, it is now mandatory or at least recommended to have a rear fog light on a trailer. There is an additional requirement that when the trailer is connected to the vehicle, the rear fog light of the towing vehicle must be turned off. The circuit presented is highly suitable for this application. It is positioned near the rear fog light of the vehicle. The 12-V connection to the lamp must be interrupted and instead connected to relay contacts 30 and 87A (K1, K3). When the rear fog light is activated, it will continue to function normally.

The described circuit utilizes a relay system to manage the operation of the rear fog light in conjunction with a trailer. The primary function is to ensure compliance with regulations that require the towing vehicle's rear fog light to be deactivated when a trailer is attached.

The circuit is designed with a relay that operates based on the status of the rear fog light switch. When the switch is activated, the relay is energized, allowing current to flow to the rear fog light. However, when a trailer is connected, the relay's configuration ensures that the power supply to the rear fog light of the towing vehicle is interrupted. This is achieved by routing the 12-V supply through the relay contacts 30 and 87A, which are normally closed contacts.

In this setup, the relay is triggered by the vehicle's rear fog light switch, which maintains the operation of the rear fog light as long as the trailer is not connected. When the trailer is attached, the relay opens the circuit, effectively cutting off power to the rear fog light of the towing vehicle. This ensures that only the trailer’s rear fog light is illuminated, adhering to safety standards and preventing confusion for other drivers.

The circuit should be installed close to the rear fog light assembly to minimize voltage drop and ensure reliable operation. Proper gauge wiring must be used to handle the current load of the rear fog light, and all connections should be secured to prevent any moisture ingress or corrosion, which could affect the circuit's performance. Additionally, it is advisable to include a fuse in the circuit to protect against potential short circuits or overload conditions.

Overall, this relay-based circuit provides a practical solution for integrating a trailer's rear fog light system while ensuring that the towing vehicle's rear fog light is disabled, thereby enhancing road safety in adverse weather conditions.In most countries it is now mandatory or at least recommended to have a rear fog light on a trailer with the additional requirement that, when the trailer is coupled to the car, the rear fog light of the towing car has to be off. The circuit shown here is eminently suitable for this application. The circuit is placed near the rear fog light of the car. The 12-V connection to the lamp has to be interrupted and is instead connected to relay contacts 30 and 87A (K1, K3).

When the rear fog light is turned on it will continue to operate normally.. 🔗 External reference