A thermal fuse, also called a thermal circuit breaker, is a temperature-sensing circuit disconnection device. A thermal fuse senses overheating generated during abnormal operation of electrical and electronic products, thereby disconnecting the circuit to prevent fire. Commonly used in: hair dryers, irons, rice cookers, electric stoves, transformers, motors, water dispensers, coffee makers, etc. A thermal fuse cannot be reused after use; it only operates once at its melting temperature. There are several types of thermal fuses. The following describes three common types: Type 1: - Before Composed of a sliding contact, a spring, and a fusible element (electrically nonconductive thermal pellet). Before the thermal fuse activates, current flows from the left lead to the sliding contact, through the metal casing, and to the right lead. When the external temperature reaches the predetermined temperature, the fusible element melts, compressing the spring. That is, the spring expands, and the sliding contact separates from the left lead. The circuit is opened, and the current between the sliding contact and the left lead is interrupted. Second type: - Before This type consists of axisymmetric leads, a fusible metal compound (thermal element) that is fusible at a specified temperature, a special compound to prevent oxidation, and a ceramic insulator. As the ambient temperature rises, the specific resin compound begins to liquefy. When it reaches its melting point, the resin melts, generating surface tension. The metal compound connecting the two leads then melts and moves towards the leads, permanently breaking the circuit. Third type: Square-shell type thermal fuse A fusible alloy wire is connected between the two leads of the thermal fuse. This wire is coated with special resin, allowing current to flow from one lead to the other. When the ambient temperature rises to the fuse's operating temperature, the fusible alloy melts and, under the influence of surface tension and the special resin, contracts into a spherical shape, attaching to the ends of the two leads. This permanently breaks the circuit. The following are essential guidelines for ensuring proper fuse operation: i) Each thermal fuse has a rated current and voltage, fusing temperature (Tf), operating temperature (Th), and maximum temperature (Tm). It must be used within the specified parameters. ii) When selecting the fuse installation location, care must be taken to prevent stress transfer to the fuse due to vibrations within the finished product or displacement of other components. iii) The fuse must be installed in a location where the temperature will not rise above the maximum operating temperature after it blows. iv) It cannot be used in machines where liquids or humidity levels are maintained above 95%. v) The thermal fuse must be installed in a location where only the heat source capable of sensing the fuse can detect its heat. If this is unavoidable due to structural limitations, thermal barriers must be installed. For example, when installing it in a heater, avoid direct connection to prevent the hot wire from heating the fuse. vi) Connecting fuses in parallel or continuously passing them overcurrent or overvoltage to increase current will damage the internal contacts of the fuse, affecting its normal operation. Therefore, it should not be used under these conditions. While thermal fuses are designed for high reliability, the capacity of a single thermal fuse to handle abnormal situations is ultimately limited. Furthermore, damage to the thermal fuse due to human error or unforeseen force majeure events can prevent it from functioning properly, thus hindering the timely circuit cutoff in the event of a machine malfunction. Therefore, in situations where the machine overheats, erroneous actions directly impact human health, there is no circuit cutoff device other than the fuse, or high safety requirements necessitate the use of two or more thermal fuses with different melting temperatures. Thin thermal fuses are specifically developed for over-temperature protection of lithium-ion batteries. Their working principle is similar to the second type of thermal fuse mentioned above. A low-melting-point alloy connects the circuit, surrounded by fluxing resin. The low-melting-point alloy is connected to a metal conductor and sealed with plastic material. When a short circuit or other cause leads to an internal temperature rise to a certain level, the thermal fuse, located close to the battery cell, melts rapidly, cutting off the external circuit and preventing battery explosion and personal injury. As handheld electronic devices become increasingly portable, the size and thinness of thermal fuses need to be as small and thin as possible. The thinnest thermal fuses currently available are only 0.65mm thick and 2.7mm wide. Their internal resistance is also very low, around 8mΩ, resulting in minimal power consumption in standby mode and significantly increasing the standby time of lithium-ion batteries. However, they also have drawbacks. First, they are unrecoverable, unlike PTC fuses. Even if a PTC trips due to an accidental malfunction, the battery can continue to function once the fault is cleared. Once a thermal fuse trips, the entire battery ceases to work. Of course, as the quality of lithium-ion batteries improves, the probability of such accidental malfunctions decreases. The fact that a faulty fuse prevents the battery from continuing to function becomes an advantage. After all, the cost of a battery is negligible compared to the risk of personal injury. It's better to be safe than sorry. If a mobile phone battery has experienced even one high-temperature malfunction, who would dare keep this potential time bomb nearby? Secondly, it cannot be welded and processed using ordinary methods. For example, PPTC can withstand short-term high temperatures, so it can be reflow soldered or directly injection molded into batteries. However, thermal fuses can only be welded locally using spot welding, laser welding, or other methods to prevent them from melting. Currently, the most commonly used are 98℃ and 92℃ thin thermal fuses. At a heating rate of 1℃/min, the melting point of a 98℃ thin thermal fuse is 94±3℃, and the melting point of a 92℃ thin thermal fuse is 89+3/-4℃, i.e., 85℃~92℃. Thermal fuses can be categorized as follows: By material: Metal casing, plastic casing, oxide film casing By temperature rating: 73°C, 99°C, 77°C, 94°C, 113°C, 121°C, 133°C, 142°C, 157°C, 172°C, 192°C, 216°C, 227°C, 240°C, 70°C, 77°C, 84°C, 92°C, 95°C, 105°C, 110°C, 115°C, 121°C, 128°C, 130°C, 139°C, 141°C, 144°C, 152°C, 157°C, 169°C, 184°C, 185°C, 192°C, 216°C, 227°C, 228°C, 240°C, 250°C, 280°C, 320°C
