Electric switch system for a power tool

#The present invention relates to an electric switch system for DC motor powered power tools, such as electric screw drivers, electric power drills, and so forth, and in particular to such an electric switch system which can control the on-off of the power supply to the motor, the rotational direction of the motor, the rotational speed or the torque output of the motor, and the supply of electric power to the control circuit for the motor as a single unit which can be actuated as a single continuous manual operation.

#Conventionally, electric switches of this type consisted of a number of two-pole switches having independent contact units which are adapted for individual actuation for achieving the on-off control, the speed control, the reversal control of the rotational direction, and the quick stop of the motor for such a power tool. For instance, a rotary switch having a rotatable knob for switching over the polarity of the electric current supplied to the motor rotor is used for reversing the rotational direction of the motor in addition to the main control switch for controlling the on-off of the motor and, in many cases, the rotational speed of the motor.

#However, the use of a large number of contact units tended to increase the number of component parts for the switch unit and made its structure highly complex, and this factor has been detrimental to the simplification of the assembly process and the reduction of the overall size and the cost of the switch structure.

#Now the present invention is described in the following with reference to the appended drawings, in which:

#FIG. 1 is an exploded perspective view of an essential part of a DC motor powered power tool to which an embodiment of the switch system according to the present invention is applied;

#FIG. 2 is a longitudinal sectional side view of this DC motor powered power tool;

#FIGS. 1 through 3 generally illustrate an electric switch structure for a DC motor powered power tool which, for instance, may consist of an electric screw driver, and this electric switch structure, which may be incorporated, for instance, in the grip of a power tool, comprises a control circuit unit 11 for a DC motor (not shown in the drawings), an operation lever 12 for manually controlling the motor, and a switch casing 13 which support them.

#The control circuit unit 11 comprises a power transistor 14, a heat radiation cover 15 which covers this power transistor 14 from above, a printed circuit board 17 which carries a control circuit and fixedly secured to the switch casing 13, a moveable block 20 which is made of synthetic resin material and is slidably guided along the fore-and-aft direction within the switch casing 13, a return spring 21 for this moveable block 20, a resistor board 18 of a small thickness which is provided as a part of a variable reference voltage generator and securely attached to the lower surface of the printed circuit board 17, a brush 19 which consists of an electroconductive elastic piece received in the moveable block 20 so as to slide over the surface of the resistor board 18 as the moveable block 20 is moved along the fore-and-aft direction as described hereinafter.

#The power transistor 14 is generally rectangular in shape, and is provided with three leads 22 which project from one side face thereof and are bent downward to be connected to appropriate parts of the control circuit as described hereinafter. The heat radiation cover 15 is shaped as a box having an open bottom, and is mounted integrally over the switch casing 13 surrounding the printed circuit board 17 with the power transistor 14 attached to the interior of the heat radiation cover 15 with a small screw 23.