The Working Principle of Siemens Washing Machine's Electronic Switch (Electronic Door Lock)
Source: Beijing Siemens Washing Machine After-sales Service Phone Number, http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3432932.
The electronic switch, also known as the electronic door lock, is connected in series in the power circuit and can control and protect the washing machine. When the washing machine door is closed properly, the door handle hook pushes the movable plate, connecting the power supply. At this point, the PTC heating element inside the electronic door switch is powered and heats up, causing a rapid rise in temperature and entering a high-resistance state. The bimetallic strip deforms due to heat and bends upwards, pushing the moving contact sheet, which connects the moving contact point with the stationary contact point. At the same time, the small pin on the moving contact sheet also moves upward and inserts into the small square hole between the fixed plate and the movable plate, ensuring that the washing machine door remains closed and cannot be opened.
A PTC heating element is a positive temperature coefficient thermistor. When the PTC element is powered, its temperature rises to a certain point (the Curie point), at which the resistance increases by thousands or even tens of thousands of times. When the power is cut off, it takes about 2 minutes for the PTC heating element to recover from the high-resistance state to the low-resistance state. During this time, the temperature of the PTC element drops, the bimetallic strip returns to its original shape, the moving contact point disconnects from the stationary contact point, the plastic small pin resets, the movable plate becomes free to move, the door handle hook resets, and the door can then be safely opened.
Front-loading drum washing machines' door switches are also equipped with water level interlocking mechanisms to prevent water overflow when the door is opened while there is visible water inside the machine.