1. Field of the Invention
The present invention relates to a switching device suitable as a microswitch for position detection and so on.
2. Description of the Related Art
In a well-known type of a switching device as a microswitch for position detection, a normal-close contact and a common contact are normally electrically connected to each other, and when a slider is depressed by a predetermined stroke, a normal-open contact and the common contact are electrically connected to form another circuit.
FIG. 3 is a cross-sectional view of such kind of conventional switching device.
Referring to the figure, there is provided a case 1, a slider 2 slidably held in the case 1 to be depressed downward in the figure, a clip-shaped movable contact 3 fixed to the slider 2, a return spring 4 for urging the slider 2 in the direction reverse to the depressing direction, a normal-close contact 5, a common contact 6, and a normal-open contact 7. These fixed contacts 5, 6 and 7 are arranged at almost regular intervals to project from the case 1, and respectively connected to terminals 8, 9 and 10. In other words, a metal plate having the normal-close contact 5 and the terminal 8, a metal plate having the common contact 6 and the terminal 9 and a metal plate having the normal-open contact 7 and the terminal 10 are embedded apart from one another in the case 1 made of a synthetic resin material.
In the switching device thus constructed, when a working portion 2a of the slider 2 is not depressed, the normal-close contact 5 and the common contact 6 are electrically connected through the movable contact 3, and the normal-close contact 5 is held in the ON state. However, since the common contact 6 is not electrically connected to the normal-open contact 7, the normal-open contact 7 is held in the OFF state. In other words, since the slider 2 of this switching device is normally pushed up to the uppermost stroke position by the return spring 4, when it is not depressed, a circuit in which the terminals 8 and 9 are connected and the terminals 9 and 10 are not connected is formed. On the other hand, when the working portion 2a of the slider 2 is depressed by a predetermined stroke, the common contact 6 and the normal-open contact 7 are electrically connected through the movable contact 3 and the normal-open contact 7 is switched from the OFF state to the ON state after the normal-close contact 5 and the common contact 6 are disconnected, thereby selecting another circuit in which the terminals 8 and 9 are not electrically connected and the terminals 9 and 10 are electrically connected. Furthermore, when the depressing force exerted on the slider 2 is released in this state, since the slider 2 is pushed up to the uppermost stroke position by the resilience of the return spring 4, the normal-close contact 5 returns to the ON state, and the normal-open contact 7 returns to the OFF state.
Therefore, if an unillustrated object to be detected is set to depress the slider 2 when moved to a predetermined position, it can be detected that the object is placed at the predetermined position, in response to the change of a signal in the circuit switching.
In such a microswitch for position detection, if the normal-close contact 5 and the normal-open contact 7 are promptly switched from ON to OFF and OFF to ON, respectively, by depressing the slider 2, in other words, if the OFF area of the normal-close contact 5 and the ON area of the normal-open contact 7 are sufficiently allocated with respect to the total stroke of the-slider 2, the timing of detection is speeded up, thereby increasing convenience. However, in the above conventional switching device, since the common contact 6 is placed between the normal-close contact 5 and the normal-open contact 7 along the depressing direction of the slider 2, the circuit switching can be performed only when the slider 2 is firmly depressed. Therefore, it is difficult to speed up the timing of detection. In other words, since the switching device having the contact structure shown in FIG. 3 cannot perform circuit switching at a desired timing according to the usage thereof, there is little latitude in designing the device, and it is also difficult to improve the response of the device as a microswitch for position detection.