This invention relates to a temperature responsive electric fuse or switch of the non-reset type, operable to open a circuit at a specific temperature. More particularly, the invention relates to an improvement of a temperature sensitive pellet for a thermal fuse, the reliability of which is improved by a sensitive and rapid temperature response characteristic at a specific temperature.
It is known to use overheating prevention devices to open a circuit when the temperature of an electric apparatus exceeds a given range, to increase the safety of the apparatus. Conventionally, such overheating preventing devices include two well-known types. One type is the non-reset type thermal fuse, employing temperature sensitive materials fusible at a specific temperature. Another type is self-resetting and uses bimetal thermocouple means. The bimetal-type is disadvantageous, in that even when it once functions to open the circuit, a drop in the ambient temperature allows the device to reset and start the current flow again. The switch-off and switch-on operation may thus be repeated unless the power supply is cut off, or the cause of the fault is eliminated. Thus, heat may be gradually stored in the bimetal device, whereby the temperature response of the device may be changed.
Recently, the non-reset type thermal fuse which employs an organic temperature sensitive pellet has been widely used due to the advantage of a constantly stable function over long periods of time in the absence of external changes in comparison with thermal fuses employing fusible metals. Such organic pellet fuses do not store any heat due to its non-reset characteristic, whereby the desired safety results.
This kind of thermal fuse of the non-reset type is described, for instance, in U.S. Pat. No. 3,519,972. FIG. 1 shows such a conventional thermal fuse with a cylindrical housing 10, made of a metal having a good electrical and heat conductivity. A first lead-in wire 12 is connected to one end of the housing 10. The switching elements include a temperature sensitive pellet 14, metallic disks or plates 16 and 18, two types of compression spring means 20 and 34 arranged within the housing 10, and a resilient slidable contact member 22, the peripheral portion 24 of which abuts against the inner wall of the metallic housing 10. These elements and an insulator 28 are enclosed by means of a housing edge 26. A second lead-in wire 30 passes through a further insulator 36 which hermetically seals the housing. Two different compression springs 20 and 34 are combined so that one of the springs has a greater resiliency than the other, while the other spring has a greater restoring force than the former. The second lead-in wire 30 has a head portion 32 forming a contact. A more detailed description of the prior art fuse and its function may be had from the above patent.
If the temperature at which the fuse is supposed to operate is, for example, 130.degree. C, such a substance as fructose is used for the pellet. If the temperature is 145.degree. C, glucose is used and if the temperature is 155.degree. C, salicylic acid is used for the pellet, to give but a few examples. The organic substance, which is used to prepare the temperature sensitive pellets, is first pulverized into a powder which is then molded into pellets by a press molding.
However, the thermal conductivity of the pellel 14 is not always good. Moreover, the transmission of thermal energy always occurs from the outside of the pellet inwardly and through the cylindrical housing 10. Thus, even if the outside of the pellet should melt, its center may remain solid. As a result, it happens sometimes that the slidable contact member is not separated from the head portion of the second lead-in wire and the electric circuit is not interrupted. Moreover, since the melting of the temperature sensitive pellet occurs gradually from the outside inwardly, the slidable contact member is in an unstable state during the meeting. Thus, even if the slidable contact member is still separated from the second lead-in wire, the distance between the contacts is small enough to cause sparks, and the current is never fully interrupted. This is a serious disadvantage of the conventional temprature temperature pellet.
The conventional temperature sensitive pellet of the above fuse is made of an insulating organic material and molded under the proper pressure to form, for example, a cylinder of fine grains of said organic materials which are disclosed in U.S. Pat. No. 2,934,628. This kind of material must be selected in accordance with the particular use for which the thermal fuse is intended. Besides, it is difficult to reduce the costs of this prior art, small-sized thermal fuse, because it is hard to prepare the selected fine grains of organic materials.