Thermal Cutoffs and Thermal Fuses Applications - Electric Kettles - Electric Toasters - Electric Blankets - Heat Lamps - Wave Flow Solder Machines - Electric Kilns - Heaters Contact us for technical data and full product information Extract from an article by Mituharu Murase. Engineering Department Uchihashi Estec Co.,Ltd Thermal Cutoffs Thermal cutoffs are the component for overheat protection. It detects the heat produced by excess current which is created by short circuit or any other troubles of the components in circuits. As for the components of overheat protection, thermostats and PTC thermistors are also applied. These components function with abnormal heat and reset when the heat is cooled down. However, thermal cutoffs are named as the last protection components and are applied to prevent critical accidents that may cause fire. Thermal cutoffs do not reset, once they function. That point is similar to current fuses but they function differently. Current fuses break the circuits when the fuse elements are cut off by self-heating. However, thermal cutoffs break circuits when fuse elements are cut off by the temperature rise of surroundings, since the internal resistance of thermal cutoffs are very low. Small current increase which can not be detected by the current fuse will raise the temperature of equipment. Thermal cutoffs are required for that kind of application. Thermal cutoffs are not usually replaced by consumers though current fuses are. Hence, they are not popular for the consumers but are protecting many kinds of equipment from overheat accidents. Sorts and Features of Thermal Cutoffs The construction of typical thermal cutoff is shown in the figure 1. Low melting point alloy is welded between two lead wires, and is covered by rosin based special compound and inserted into the ceramic tube and is sealed by epoxy resin in order to maintain air-tightness. How it functions is as follows: When the surrounding temperature reaches melting point, the alloy melts away toward two lead wires and becomes spherical by the surface tension effected by the special compound. Various temperatures are available depend on the structures of alloys. However, limited alloys are applied for thermal cutoffs because of the stability of cut off temperature and the reliability for long term application. At present, the product series with various shapes and the temperature range of 76 deg to 187 deg C are available. A part of our products are shown in table 1. Figure 1. Construction of Thermal Cutoffs Eutectic Thermal Cutoffs 1. Since the construction is simple and small, they are easy to be installed. 2. The elements are made of alloys, they are very stable and less deterioration for long term application. 3. Since the bodies are made of the ceramic or plastic that have good electrical insulation, insulation work is not required. 4. They are in small packages, they are very sensitive and fast functioning. 5. They are available at reasonable prices compared with other protective components. Table 1 Eutectic Thermal Cutoffs Features Of Thermal Cutoffs Main features of thermal cutoffs are as follows: Functioning Temperature It is the fusing off temperature when the thermal cutoff is placed in the heat chamber in which the temperature is raised with specific ratio. As you see in the table 2, the functioning temperatures have little variations and are very accurate, when the whole part of thermal cutoffs are heated gradually. Table 2. Response Time In figure 2, you will see the reference data of the response time. Thermal cutoffs are put in silicon oil of certain temperature and response times until functioning are measured. They function at about 10 seconds even at a little over the functioning temperature. Figure 2. Response Time Aging Deviation It is the deviation caused by deterioration when the thermal cutoffs are exposed in high temperature ambient for a long time. As you see in the figure 3, it is stable even for a long time exposure. Figure 3. Aging Deviation Technology Utilized As you may understand, the heater and thermal cutoffs are thermally connected, and consequently thermal cutoffs function and break the circuits at specified temperature to protect the equipment. In figure 4, you will see the sample diagram that thermal cutoffs are mounted on the surface of the coils of motors or transformers through the insulation films and will detect heat created by the hang up of motors or the secondary short-circuit of transformers. In the figure 5, you will see the application to protect the inrush preventive resistors in switching power supplies from overheating. In normal condition, the resistors are opened by the thyristors. When they become abnormal in the circuit, the thyristors become open and make the resistors heated abnormally. The thermal cutoffs detect that heat and break the circuits. The thermal cutoffs connected with the resistors or the thermal cutoffs with resistors built into the cemented containers are applied. Figure 4. Overheat Protection of Transformers Figure 5. Overheat Protection of Inrush Preventive Resistors in Switching Power Supplies Items To Mind For Applications of Thermal Cutoffs 1. In order to utilize the thermal cutoffs' sensitive feature, apply the lead wires longer. In case they are applied in the coil-windings of motors or transformers, they must be installed at the places where are thermally conductive and are able to detect the heat of coils. Also the body and two lead wires must be evenly heated. 2. When finished products are designed, do not install thermal cutoffs at the places where sever vibration exists or vibration continues. 3. It must be designed that abnormal heat does not pass the maximum temperature limits of thermal cutoffs even with overshoot of the heater. 4. Install the thermal cutoffs at the places where they function only by the abnormal heat. 5. Though thermal cutoffs are reliable components, there is a limitation for one component to handle. If the thermal cutoffs are damaged by any chance, they may not function at abnormal conditions. If body injury or accidents are expected when the circuit is not broken, it is recommended to apply more than one thermal cutoff of different functioning temperatures. Engineering Trend As reflection of safety trend these days, thermal cutoffs are required to meet the various applications. In addition to the existing requirements of larger capacity and varied functioning temperatures, more requirements are made for thermal cutoffs, such as easier installation at production process, the shape for easier work and also additional function like a current fuse. For example, they are recently applied for the overcharge protection of highly efficient rechargeable batteries, such as lithium-ion or nickel metal hydride batteries that are rapidly expanding the applications for cellular phones or video camcorders. The large capacity in the small package is required for these batteries. Hence, they require for the thermal cutoffs not only the electrical features but also the smaller packages and the easier shape for terminal connection. To meet these requirements, we developed the miniature thermal cutoffs 6 Series (1.4Dx 4.5mmL) and the slim packaged thermoprotectors TA Series with less than 1mm thickness. (as shown in picture 2) picture 2> We have also developed the thermal cutoffs with resistors mounted on the ceramic substrates and covered by resin. They are applied to protect the heater wires of blankets from overheating. (as shown in picture 3) In the future, safety requirements for equipment will be increased and designing to prevent accidents will be required. Hence, thermal cutoffs will be applied together with other protection devices such as surge protectors, excess voltage or current protectors. Thermal cutoffs are required to suit with the features of these devices and also to have longer life. picture 3> Battery Protection Devices ELCUT thermoprotectors are assisting with various batteries protection. Especially, they are applied for the overheat or overcharge protection of Lithium-Ion Batteries. Notice of a marking change on the products. In compliance with DENAN law, the markings on Thermal Cutoffs and Thermal Cutoffs/Resistor assemblies have been changed since those products were started manufacture on Apr. 1st, 2003. Instead of conventional DENTORI mark, therefore, the mark of < PS >E is put on the products. Notice of the CCC approval in Peoples Republic of China. CCC approved for Uchihashi products. Source: Uchihashi Home Page. Contact us for full Technical Specifications, product types, sizes and availability