Dual protection device for circuits

[0001] The present invention relates to a dual protection device that includes two independent protections to ensure that the electric appliance will not be damaged when the protection device is overload.

[0002] A conventional protection device for electric appliance uses fuses which are broken when the fuses are overheated due to overload. After the fuses are broken, the circuit is cut off to prevent the electric appliance from being burned. In the modern design of the electric appliance, several circuits are involved and each circuit has a switch or protection device so as to achieve multiple layers of protection.

[0003] As shown in FIG. 1, a latest protection device includes a bi-metallic plate 201 which has a first end fixed to a first terminal 202 and a second end is a free end which has a first contact point 203 connected to an underside thereof. A second contact point 205 is fixed on a second terminal 204 and when the first and second contact points 203, 205 are in contact with each other, the circuit is in “ON” status. When the circuit is overloaded, as shown in FIG. 2, the high temperature makes the bi-metallic plate 201 bend upward so that the first and second contact points 203, 205 are separated and the circuit is in “OFF” status. By this way, the appliance is protected from being burned. However, it is a difficult task to ensure and to manufacture each bi-metallic plate having the same physical characteristics. Some of the bi-metallic plates do not bend at desired temperature and the range of the temperature is too wide to precisely set the bi-metallic plates to have the same characteristics. Furthermore, some bi-metallic plates are not so sensitive to the temperature and do not react as desired. Besides, when the bi-metallic plate bends less than as desired, the small gap between the two contact points might generate sparks to burn the whole appliance.

[0004] The present invention intends to provide a dual protection device that includes a bi-metallic plate and a contact plate which is connected to a carrier of one of the contact point by low melting metal which melts at desired temperature to cut off the circuit.

[0005] The present invention relates to a protection device that comprises a bi-metallic plate having a first end fixed to a first terminal and a second end of the bi-metallic plate has a first contact point. A carrier has a second contact point connected on a first end thereof and the second contact point is located beneath the first contact point. A contact plate has a first end in contact with a second terminal and a second end of the contact plate is forced to connect to a second end of the carrier by a low melting point metal. The low melting point metal melts to separate the second end of the contact plate and the carrier when the protection device is overloaded.

[0006] The present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a preferred embodiment in accordance with the present invention.

[0016] Referring to FIGS. 3 and 5, the protection device of the present invention comprises a first protection device and a second protection device 12. The first protection device includes a curved bi-metallic plate 11 which has a first end fixed to a first terminal 13 by a rivet 131. A second end of the bi-metallic plate 11 is a free end which has a first contact point 111 connected to an underside thereof. A carrier 15 is clamped between two positioning members 152, 153 and has a second contact point 151 connected on a first end thereof. The second contact point 151 is located beneath the first contact point 111.

[0017] The second protection device 12 includes a contact plate 121 that has a first end connected to a second terminal 14 by another rivet 141 and a second end of the contact plate 121 is forced to be connected to a second end of the carrier 15 by a low melting point metal 122. The first and second terminals 13, 14 extend out from a box 10. The second end of the contact plate 121 tends to be separated from the carrier 15 when the low melting point metal 122 is not yet connected between the carrier 15 and the contact plate 121.

[0018] As shown in FIG. 4, when the protection device is overloaded, the bi-metallic plate 11 is bent upward so that the first and second contact points 111, 151 are separated so as to cut off the circuit. This is the first stage of protection for the circuit.

[0019] As shown in FIG. 6, if the bi-metallic plate 11 is not bent as expected when the protection device is overloaded, the low melting point metal 122 melts due to the temperature and, according to the nature of the contact plate 121, the second end of the contact plate 121 is separated from the carrier 15 to cut off the circuit.

[0020] Therefore, there are two protection devices to ensure that the circuit is cut off when the protection device is overloaded.

[0021] As shown in FIG. 7 which shows another embodiment of the second protection device, wherein the second terminal 14 includes an upward portion and a spring 123 is connected between the second terminal 14 and the contact plate 121′. Low melting point material 122′ is used to connect the two ends of the contact plate 11, the upward portion of the second terminal 14 and the carrier 15. When the protection device is overloaded, as shown in FIG. 8, the bi-metallic plate 11 is bent upward so that the first and second contact points 111, 151 are separated so as to cut off the circuit. If the bi-metallic plate 11 is not bent as expected, the low melting point material 122′ melts so that the spring 123 pushes the contact plate 121′ away from the carrier 15 and the upward portion of the second terminal 14 as shown in FIG. 9.

[0022] While we have shown and described the embodiment in accordance with the present invention, it should be clear to those skilled in the art that further embodiments may be made without departing from the scope of the present invention.