What is the principle of rear window defogging?
2 Answers
Rear window defogging works by heating to remove fog. Its working principle involves supplying electricity to the resistors on the rear window, generating heat to eliminate frost or prevent moisture condensation on the rear window, thereby avoiding fogging and frosting. When driving in winter, or on cloudy or rainy days, water vapor inside the car condenses on the windows due to the cold, forming a layer of fog or even frost. Since warm air cannot reach the rear window glass, and the driver cannot wipe off the fog or frost on the rear window while driving, it can obstruct the rear view. Therefore, most modern cars are equipped with a rear window defogging function. The defogging resistors on the rear window are conductive heating coatings printed and adhered to the inner surface of the rear glass. These conductive coatings consist of a set of parallel lines, with a distance of about 4 centimeters between adjacent lines. The ends of these lines are connected, forming a parallel circuit. They are connected to the car's power supply circuit via a switch or relay, with most household cars operating at 12 volts.
I've been driving for many years, and the principle behind rear window defogging actually utilizes heating technology. The rear window glass has tiny embedded resistance wires. When you press the defog button, electric current passes through these wires, generating heat. This heat directly warms the glass surface, and the increased temperature evaporates the condensed moisture or fog on the window, restoring clear visibility. This method is much faster than traditional wiping approaches, especially practical during rainy or cold weather. The principle is similar to the front windshield defogging system, but rear windows typically have a simpler design relying on DC electrical systems. It's important to regularly check if the rear window is clean, as dirt or dust can hinder heating efficiency, slowing down defogging. From a safety perspective, this prevents obstructed views when reversing or backing up, reducing accident risks. I always remind new drivers to develop the habit of checking defogging functions as soon as they get in the car.
