outboard motor thermostat stuck open.What is the overcurrent protection principle of self restoring fuse PTC?

　　Due to the characteristic of instantaneous resistance jump, this type of PTC is often used for overcurrent protection in electrical products. We know that for a conductor, its resistance value is R. When the current flowing through the conductor is I and the duration is t, the heat generated by the conductor Q can be calculated using Joule's law: Q=I ^ 2 * R * t. It can be seen that the heat generated by the conductor is proportional to the square of the current flowing through the conductor I, that is, as long as the current increases a little, the heat generated Q will sharply increase. During normal operation, due to the small working current IH, the heat generated by the current flowing through PTC is not sufficient to reach the Curie point. PTC has always shown a zero power resistance Rmin at the factory, usually at the milliohm level (specific data varies depending on the PTC series), which is equivalent to the closed state of a switch. Once the current flowing through PTC It is twice the normal working current IH, the heat accumulated inside PTC reaches the Curie point, The resistance value will immediately jump to Rmax. From a macro perspective, it is equivalent to a switch opening. Of course, if the switch truly opens, its resistance value is ∞, and usually the ratio of Rmax/Rmin is at least 10 ^ 4 times greater. Due to the non polarity of PTC, when connected in series in the load circuit, the rapid and rapid increase in PTC resistance will effectively limit the working current of the entire circuit, thus effectively protecting the load from being burned out. When the power supply is disconnected, the current in the entire circuit disappears. After about 1 minute, the heat inside the PTC dissipates and cools down, and it automatically returns to the initial resistance value Rmin. Therefore, PTC is also known as a recoverable fuse, or a self restoring fuse, a self resetting overcurrent protector, etc.

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