Current breaker, safety circuit and secondary battery pack

1. A current cut-off device comprising:

a plate-shaped terminal piece having a contact;

a movable piece including an elastic portion formed in a plate shape so as to be elastically deformed and a movable contact arranged at one end portion of the elastic portion and having the movable contact so as to be pressed against and in contact with the contact; and

a thermally actuated element which biases the movable piece by deforming in accordance with a temperature change and causes a state of the movable piece to be shifted from a conductive state in which the movable contact is in contact with the contact to a cut-off state in which the movable contact is separated from the contact, wherein

an end portion of the movable contact and an end portion of the contact overlap,

when the state of the movable piece is shifted from the conductive state to the cut-off state, as the movable contact moves, it snaps and moves above the contact so that positional relationship between the end portions of the movable contact and the contact switches, and

a direction in which the movable piece presses the movable contact to the contact and a direction in which the thermally actuated element biases the movable piece match.

2. The current cut-off device according to claim 1 , wherein when the state of the movable piece is shifted from the conductive state to the cut-off state, the movable contact moves from a first surface side to a second surface side opposite to the first surface of the terminal piece.

3. The current cut-off device according to claim 1 , wherein the thermally actuated element biases the movable piece in a direction of pushing when the state of the movable piece is shifted from the conductive state to the cut-off state.

4. The current cut-off device according to claim 1 , wherein

the device comprises a case for accommodating the contact, the elastic portion, the movable contact, and the thermally actuated element, and

the terminal piece and the movable piece are embedded in the case.

5. The current cut-off device according to claim 4 , wherein

the case includes a first case provided with a hole portion for accommodating the contact, the elastic portion, the movable contact, and the thermally actuated element, and

a second case configured to be attached to the first case so as to close the hole portion.

6. The current cut-off device according to claim 4 , wherein

the device has a movable body movably provided with respect to the case, and

the movable body is pressed onto the movable piece in the cut-off state so as to get the state of the movable piece back to the conductive state.

7. The current cut-off device according to claim 5 , wherein

the device has a movable body movably provided with respect to the case,

the movable body is pressed onto the movable piece in the cut-off state so as to get the state of the movable piece back to the conductive state,

the second case has formed therein a through hole for allowing part of the movable body to stick out of the case, and

the movable body is provided so as to freely protrude and retract from the through hole.

8. The current cut-off device according to claim 1 , wherein

a slit extending in a longitudinal direction of the movable piece is formed in at least one of the contact and the movable contact.

9. A safety circuit for electrical equipment comprising the current cut-off device according to claim 1 .

10. A secondary battery pack comprising the current cut-off device according to claim 1 .