Abstract:
In order to prevent the grip portion of a power tool from heating up, a brake resistor in which a regenerative current generated by a motor flows is disposed in a tail portion of the handle. The brake resistor is composed of two resistor elements connected in parallel with each other. The two resistor elements are mounted on different surfaces of a circuit board to effectively utilize an interior space of the handle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to an electrically powered tool (hereinafter referred simply to as “power tool”), and more particularly to an arrangement of heat generating resistors used in a motor control circuit. 
         [0003]    2. Description of the Related Art 
         [0004]    A conventional power tool and a motor control circuit used therein are described, for example, in Japanese Patent Application Publication No. 2002-281777.  FIG. 1  is a partial cross-sectional view of a conventional impact driver  10 , and  FIG. 2  is a circuit diagram of the motor control circuit  20  used in the impact driver  10  shown in  FIG. 1 . 
         [0005]    The motor control circuit  20  shown in  FIG. 2  includes a speed controllable switch  2 , a diode bridge  3 , a forward/reverse switching circuit  5 , a DC motor  6 , and a brake circuit  4 . The speed controllable switch  1  is connected to a commercial AC power supply  1  and operates to control a voltage to be applied to the DC motor  6  depending upon how deep a trigger switch  7  (see  FIG. 1 ) of the impact driver  10  is pulled. The diode bridge  3  performs either a half-wave or full-wave rectification of the AC voltage and supplies a DC voltage to the DC motor  6  through the forward/reverse switching circuit  5 . 
         [0006]    The brake circuit  4  is configured from a normally closed contact  4   a  (hereinafter referred to as “NC contact  4   a ”) and a resistive coil  4   b . The NC contact  4   a  and the resistive coil  4   b  are connected in series across positive terminal A and negative terminal B of the DC side so that a closed loop is formed by the motor  6 , the resistive coil  4   b , and the NC contact  4   a  when the NC contact  4   a  is closed. 
         [0007]    When the operator removes his or her finger from the trigger switch  7  to stop driving the impact driver  10 , the speed controllable switch  2  turns off and the NC contact  4   a  of the brake circuit  4  turns on, allowing a regenerative current or a brake current Ix to flow in the closed loop formed by the motor  6 , the resistive coil  4   b , and the NC contact  4   a.    
         [0008]    As shown in  FIG. 1 , a board  8  on which the resistive coil  4   b  is mounted is disposed inside of the grip portion  11   a  and behind the trigger switch  7 . When consecutively driving a number of small-size screws into a workpiece with the impact driver  10 , the trigger switch  7  is repeatedly turned on and off. Each time the trigger switch  7  is turned off, the brake current flows in the resistive coil  4   b  and generates heat. Due to the heat generated by the resistive coil  4   b , the grip portion  11   a  of the handle  11  can be heated up to a degree that the operator cannot keep on grasping the impact driver  10 . 
       SUMMARY OF THE INVENTION 
       [0009]    In view of the foregoing, it is an object of the invention to obviate the problems involved in the conventional power tools. 
         [0010]    To achieve the above and other objects, there is provided a power tool that includes a motor; a main housing in which the motor is accommodated; a handle; a switch circuit; and a brake circuit. The handle has a tail portion and a grip portion. An operator grips the grip portion when driving the power tool. The grip portion has one end integral with the main housing and another end integral with the tail portion. The switch circuit is connected between a power supply and the motor and controls the motor. The brake circuit includes a resistor and is connected to the motor for allowing a regenerative current generated by the motor to flow when a driving current flowing in the motor is interrupted, wherein the resistor is disposed in the tail portion to which the operator&#39;s hand does not normally extend when the operator grasps the grip portion. Accordingly, heat generated from the resistor of the brake circuit does not cause the grip portion to heat up. 
         [0011]    It is desirable that the resistor of the brake circuit is made up of a first resistor element and a second resistor element connected in parallel with each other. With the parallel-connection of two resistors, a resistance value can be decreased, thereby decreasing the amount of heat generated from the resistors. In this case, in terms of effectively utilizing an internal space of the handle, it is further desirable to mount the first and second resistor elements on front and rear surfaces of a circuit board, respectively. 
         [0012]    It is desirable that the brake circuit further include a contact that is held open when the driving current flows in the motor and is closed when the driving current flowing in the motor is interrupted. 
         [0013]    It is also desirable that the tail portion be protruded outwardly from the grip portion in an axial direction of the main housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]    The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a cross-sectional view showing a part of a conventional impact driver; 
           [0016]      FIG. 2  is a circuit diagram showing a motor control circuit used in the impact driver shown in  FIG. 1 ; 
           [0017]      FIG. 3  is a cross-sectional view showing a part of an impact driver according to an embodiment of the invention; 
           [0018]      FIG. 4  is a circuit diagram showing a motor control circuit used in the impact driver shown in  FIG. 3 ; 
           [0019]      FIG. 5A  is a rear view showing a circuit board for mounting a brake resistor according to the embodiment of the invention; 
           [0020]      FIG. 5B  is a front view showing the circuit board for mounting another brake resistor according to the embodiment of the invention; and 
           [0021]      FIG. 6  is a side view showing a grip portion of the impact driver. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    An embodiment of the invention will be described with reference to  FIGS. 3 to 6 , in which the invention is applied to an impact driver. In the circuit diagrams in  FIGS. 2 and 4 , the same or corresponding components are designated by the same reference numerals. 
         [0023]    As shown in  FIG. 3 , the impact driver  30  includes a main housing  31  and a handle  11  extending substantially orthogonal to an axial direction of the main housing  31  but slightly slanted toward the rearmost end of the main housing  31 . The handle  11  has a grip portion  11   a  joined at one side to the main housing  31  and is integral with the main housing  31 . Also, the handle  11  has a tail portion  11   b  integral with the grip portion  11   a.    
         [0024]    A DC motor  6  is accommodated in the rear portion of the main housing  31 . Although not shown in  FIG. 3 , an impact mechanism is also accommodated in the main housing  31  for generating impact and drive force when driving power is supplied from the DC motor  6 . With the impact and drive force, a fastener, such as a screw, is tightened into a workpiece. A switch circuit board  32  is disposed in the upper part of the grip portion  53   a  and behind a trigger switch  7 . 
         [0025]    As shown in  FIG. 6 , the grip portion  11   a  is a portion where an operator grasps when driving the impact driver  30 . The grip portion  11   a  has a size fitted to the general size of a human hand. When the operator grasps the grip portion  11   a , the operator&#39;s hand does not generally extend to the tail portion  11   b . The tail portion  11   b  is protruded outwardly from the grip portion  11   a  in an axial direction of the main housing  31  so that the impact driver  30  does not easily fall down when the impact driver  30  is held by the operator upside down. 
         [0026]    As shown in  FIG. 4 , a motor control circuit  40  includes a switch circuit configured from a speed controllable switch  2  and a diode bridge  3 . The speed controllable switch  2  is configured from two normally open contacts (hereinafter referred to as “NO contacts”)  2   b  and  2   c , and a thyristor  2   a  connected in parallel with the NO contact  2   b . The NO contacts  2   b  and  2   c  operate in ganged with a trigger switch  7  (see  FIGS. 3 and 6 ). The diode bridge  3  performs a half-wave or a full-wave rectification of an AC output from an AC power supply  1  and applies a DC power to the DC motor  6 . 
         [0027]    A forward/reverse switching circuit  5  is interposed between the diode bridge  3  and the DC motor  6 . The forward/reverse switching circuit  5  is configured from forward/reverse switching contacts  5   a  and  5   b  for switching a rotational direction of the DC motor  6 . Further, a capacitor  9  is connected across the AC power supply  1  for suppressing noises generated when operating the impact driver  30 . 
         [0028]    The motor control circuit in  FIG. 4  further includes a brake circuit  4 . The brake circuit  4  is configured from an NC contact  4   a  and two resistors  4   c  and  4   d  connected in parallel with each other. The NC contact  4   a  and the parallel-connected resistors  4   c  and  4   d  are connected in series across the positive and negative terminals of the diode bridge  3 . When the NC contact  4   a  is closed, the brake circuit  4  forms a closed loop with the motor  10  and the forward/reverse switching circuit  5 . The NC contact  4   a  is closed when the two NO contacts  3  and  4  of the speed controllable switch  2  are simultaneously opened. This occurs when the operator removes his or her finger from the trigger switch  7 . 
         [0029]    In operation, when the operator pulls the trigger switch  7  to a level less than the maximum, the NO contact  2   b  is closed and the NO contact  2   c  is held open. Thus, a driving current flows in the motor  6  through a path including the diode bridge  3  and the contact  5   a  of the forward/reverse switching circuit  5 , and the current flowing out from the DC motor  6  flows in a path including the contact  5   b  of the forward/reverse switching circuit  5 , the diode bridge  3 , and the thyristor  2   a  and the NO contact  2   c  of the speed controllable switch  2 . The driving current flowing in the DC motor  6  corresponds to the level or depth of the trigger switch  7  pulled. 
         [0030]    When the operator pulls the trigger switch  7  up to the maximum or to the full depth, both the NO contacts  2   b  and  2   c  of the speed controllable switch  2  are closed. In this case, the diode bridge  3  performs a full-wave rectification of the AC voltage, so that the driving current is approximately doubled as compared with the case where the trigger switch  7  is pulled to a half way. More specifically, in addition to the driving current flowing in the path described above, the driving current further flows in the DC motor  6  through a path including the NO contact  2   b , the diode bridge  3 , the contact  5   a  of the forward/reverse switching circuit  5 , and the current flowing out from the DC motor  6  flows in a path including the contact  5   b  of the forward/reverse switching circuit  5 , the diode bridge  3 , and the NO contact  2   b.    
         [0031]    When the operator removes his or her finger from the trigger switch  7 , the NO contacts  2   b  and  2   c  are simultaneously opened, and at the same time, the NC contact  4   a  of the brake circuit  4  is closed. As a result, a regenerative current or brake current generated by the DC motor  6  flows in the closed loop formed by the brake circuit  4 , forward/reverse switching circuit  5 , and the DC motor  6 . Due to the parallel-connection of two resistors  4   c  and  4   d , the brake current flowing in each of the resistors  4   c  and  4   d  is reduced and heat generated therefrom is also reduced as compared with the case where a single resistor is employed. 
         [0032]    As shown in  FIGS. 5A and 5B , one of the two resistors  4   c  and  4   d  is mounted on a rear surface of a circuit board  33  and the counterpart resistor  4   d  is mounted on a front surface of the same circuit board  33 . The circuit board  33  on which the two resistors  4   c  and  4   d  are mounted is disposed in the tail portion  11   b  of the handle  11 . According, the heat generated from the resistors  4   c  and  4   d  does not substantially rise the temperature of the grip portion  11   a.    
         [0033]    While the invention has been described in detail with reference to a specific embodiment thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein. For example, the present invention can be applied not only to an impact driver but also to all kinds of power tools using a DC motor.