# Fog Lamp Switch Schematic Circuit Diagram

In many nations, it has become obligatory or highly advised to install a rear fog light on trailers, accompanied by the stipulation that. When the trailer is attached to the vehicle, the vehicle’s rear fog light must be switched off. The circuit presented here is ideal for this purpose. It is positioned near the car’s rear fog light. The existing 12-V power supply to the lamp needs to be disconnected and redirected to the relay terminals 30 and 87A (K1, K3). This arrangement ensures that the rear fog light functions as usual when activated.

### Understanding Trailer Fog Light Circuitry

When a trailer equipped with a fog light connects to the trailer connector (7- or 13-way, K2), current flows through coil L1, wound around reed contact S1. S1 closes due to this current, activating relay Re1. Consequently, the car’s rear fog light turns off, while the trailer’s fog light illuminates. The size of L1 is contingent on S1’s specifications. As the fog lamp draws 21 W at 12 V (resulting in 1.75 A), L1 is designed for a current between 1.0 and 1.5 A, ensuring reliable contact closure. A wire of approximately 0.8 mm diameter is necessary. The automotive relay Re1 efficiently handles the lamp current, with negligible voltage drop across L1.

### Electric Potential Difference: The Essence of Voltage

Voltage, also known as electric potential difference, electric pressure, or electric tension, signifies the difference in electric potential between two points. It denotes the work required per unit of charge to move a test charge between these points. In terms of dimensions (M L² T⁻³ I⁻¹), voltage is derived from the ratio of energy to charge. Expressed in SI units (kg⋅m²⋅s⁻³⋅A⁻¹), voltage serves as the driving force within an electrical circuit, propelling charged electrons (current) through conductive paths to accomplish tasks like illuminating lights. In essence, voltage equals pressure and is measured in volts (V).

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