Abstract:
A protection circuit for a lithium battery pack that serves as a power source of a power tool is arranged in the lithium battery pack and connected among a reversal/forward rotation circuit, a speed variation circuit, and a motor of the power tool. During a discharging process, the protection circuit detects battery voltage and temperature and when abnormal events occur, the protection circuit cuts off the discharge current. The protection circuit has a simple structure, low costs, and is easy to incorporate to the currently existing pressing-control circuit of the power tool.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    (a) Technical Field of the Invention 
         [0002]    The present invention relates generally to a protection circuit for a lithium battery pack that serves as a power source of a power tool, and in particular to a battery protection circuit that, as compared to the conventional battery protection circuit, features simple structure, low costs, being easy to match currently existing depressing-controlled circuit of power tools, and functions detect battery voltage and temperature in the discharging process of the battery for immediately cutting off the discharge current when abnormal events occur. 
         [0003]    (b) Description of the Prior Art 
         [0004]    Lithium ion batteries, lithium polymer batteries, and other lithium based chemical cells have generally different discharging characteristics from other types of secondary cells. The lithium based batteries or cells are easily subject to damage when they are over-discharged or being of over-temperature. Further, over-temperature may also cause explosion of the lithium based batteries, especially when a number of lithium batteries are connected in series and/or in parallel to effect high current charging and discharging for power tools that requires much larger output power than a single cell can provide. In such applications, the lithium batteries are much more easily subject to damages caused by over-discharging and the costs are much higher when the batteries are so damaged. Also, explosion of the batteries is more powerful, if it happens. Apparently, a protection circuit is often employed to detect voltage and temperature of the lithium battery during the operation thereof and to immediately cut off the discharge current at the time when abnormal events occur. Such a protection circuit must also ensure little leakage current when a power tool having such as protection circuit is put in non-operation condition. However, when a protection circuit is incorporated in a battery pack, it often mismatches a reversal/forward rotation circuit and a speed variation circuit that are commonly present in the power tool. 
         [0005]    A regular nickel-cadmium battery or nickel-metal-hydride battery is taken as an example to explain the circuit of a power tool that is a combination of a reversal/forward rotation circuit, a speed variation circuit, and a motor of the power tool, and the battery.  FIG. 1  of the attached drawings illustrate the example circuit, which comprises a reversal/forward rotation circuit  10  having a pair of single-pole-double-throw switches  11 ,  12 , which include externally-located control members for switching the switches  11 ,  12 . The two throw terminals of the two switches  11 ,  12  are coupled to each other respectively and are connected to terminals of a motor  30  respectively. A common terminal of the single-pole-double-throw switch  11  is connected to an anode of a battery  40 , while a common terminal of the single-pole-double-throw switch  12  is connected to a third throw terminal of a single-pole-triple-throw switch  21  that constitutes in part a speed variation circuit  20 . 
         [0006]    Besides the single-pole-triple-throw switch  21 , the speed variation circuit  20  also comprises a pulse width modulation (PWM) generation circuit  22 , a power transistor  23 , a switch  24  that is closed by the switching operation of the single-pole-triple-throw switch  21 , and a diode  25 . The switch  21  has a common terminal connected to a cathode of the battery  40 , a first throw terminal that is open-circuited, and a second throw terminal that is connected to a grounding terminal of the PWM generation circuit  22  and also connected to an emitter of the power transistor  23 . The third throw terminal, which as mentioned above, is connected to the common terminal of the single-pole-double-throw switch  12 . The PWM generation circuit  22  has an input terminal that is connected to the anode of the battery  40  and an output terminal connected to a base of the power transistor  23 . A collector of the power transistor  23  is connected to the third throw terminal of the switch  21 . The switch  24  has a terminal connected to the anode of the battery  40  and an opposite terminal connected to the third throw terminal of the switch  21  so that the switch  24  is induced to close by the operation of switching the switch  21  to the first throw terminal. The diode  25  has a negative terminal connected to the anode of the battery  40 , and a positive terminal connected to the third throw terminal of the switch  21 . 
         [0007]    When an operation button or actuation button is manually depressed, the switch  21  is driven away from the first throw terminal and thus opening the switch  24 . When the switch  21  reaches the second throw terminal thereof, the emitter of the power transistor  23  is put in electrical connection with the cathode of the battery  40  and the PWM generation circuit  22  is actuated to supply a PWM signal from the output terminal thereof to the base of the power transistor  23 . The tool operation button, when depressed or actuated, varies a variable resistor, in such a way that the extent that the operation button is depressed corresponds to the PWM signal ratio, and thus, the rotational speed and output torque of the motor  30  are increased in accordance therewith. When the operation button is depressed to the bottom most position, the switch  21  is driven to the third throw terminal and the power transistor  23  is bypassed. In this condition, the motor  30  is directly driven by the closed third throw terminal of the switch  21 . When the operation button is no longer depressed and is thus released, the switch  21  is biased back to the first throw terminal and power supplied to the motor  30  is cut off and the switch  24  is controlled by such a backward switching operation to close, which applies a positive voltage from the battery  40  to both terminals of the motor  30  and thus inducing braking operation of the motor  30 . This stops the operation of the motor  30 . Further, since, in this situation, the switch  21  breaks the connection between the cathode of the battery  40  and the power transistor  23  and the grounding terminal of the PWM generation circuit  22 , the leakage current is reduced. 
         [0008]    As shown in  FIG. 2 , by incorporating the above discussed reversal/forward rotation circuit  10  and the speed variation circuit  20  in a power tool that is powered by a lithium battery pack  70 , together with a protection circuit  50  arranged within the battery pack  70 , to effect cutting off the discharge current at the time when abnormal events occur, a discharge circuit of a lithium battery set  71 , which is a circuit that is in connection with the previously discussed switch  21 , must be connected in series with a power transistor MOS 1 . Such an arrangement, when incorporated to the reversal/forward rotation circuit  10  and the speed variation circuit  20 , raises a problem where the power supplied to the PWM generation circuit  22  will be cut off by the power transistor MOS 1  and the PWM generation circuit  22  fails to properly function. 
         [0009]    An example solution to such a problem is illustrated in  FIG. 3 , wherein the lithium battery pack  70  is further provided with two transmission lines, of which one is branched off the connection between the cathode of the lithium battery pack  71  and the speed variation circuit  20 , which line forms a terminal that provides an electrical signal labeled V-, the power transistor MOS 1  being arranged on the line, and the other line is coming from a terminal of the protection circuit  50  that receives a positive electrical signal labeled VDD, a switch  51  being arranged on the line. If necessary, a temperature detection terminal Vth can be further added for detecting the temperature during a charging process. When the switch  21  is slightly depressed, the switch  51  is closed to provide VDD to drive the protection circuit  50  and conducting on the power transistor MOS 1  to have the common terminal of the switch  21  connected to the anode of the battery pack  70  to thereby form an operation circuit. 
         [0010]    The known circuit, however, suffers insufficient space inside a power tool for arrangement of the switching circuit. In other words, the power tool has to be redesigned with an increased size. In addition, two power transistors connected in series make efficiency low and costs increased. 
         [0011]    The present invention is aimed to solve the drawbacks of the conventional protection circuit employed in power tool that uses a lithium battery pack as a power source. 
       SUMMARY OF THE INVENTION 
       [0012]    The primary purpose of the present invention is to provide a protection circuit for a lithium battery pack that serves as a power source of a power tool to solve the above discussed drawbacks and being simple in construction with low costs and being easy to incorporate a reversal/forward rotation circuit and a speed variation circuit. 
         [0013]    To achieve the above objective, in accordance with the present invention, a protection circuit for a lithium battery pack that serves as a power source of a power tool is provided, which is arranged in the lithium battery pack and is connected among a reversal/forward rotation circuit, a speed variation circuit, and a motor of the power tool. During a discharging process, the protection circuit detects battery voltage and temperature and when abnormal events occur, the protection circuit cuts off the discharge current. The protection circuit has a simple structure, low costs, and is easy to incorporate to the currently existing pressing-control circuit of the power tool. 
         [0014]    The foregoing object and summary provide only a brief introduction to the present invention. To fully appreciate these and other objects of the present invention as well as the invention itself, all of which will become apparent to those skilled in the art, the following detailed description of the invention and the claims should be read in conjunction with the accompanying drawings. Throughout the specification and drawings identical reference numerals refer to identical or similar parts. 
         [0015]    Many other advantages and features of the present invention will become manifest to those versed in the art upon making reference to the detailed description and the accompanying sheets of drawings in which a preferred structural embodiment incorporating the principles of the present invention is shown by way of illustrative example. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will be apparent to those skilled in the art by reading the following description of preferred embodiments thereof, with reference to the attached drawings, in which: 
           [0017]      FIG. 1  is a conventional circuit diagram illustrating the combination of a reversal/forward rotation circuit, a speed variation circuit, a motor, and a battery for a power tool powered by a nickel-cadmium battery on a nickel-metal-hydride battery; 
           [0018]      FIG. 2  is a unworkable arrangement of circuit corresponding to that shown in  FIG. 1  with the battery being replaced by a lithium battery pack and incorporating a battery protection circuit therein; 
           [0019]      FIG. 3  is a not-working-well arrangement of circuit corresponding to that shown in  FIG. 1  with the battery being replaced by a lithium battery pack and incorporating a battery protection circuit therein; 
           [0020]      FIG. 4  is a circuit diagram of a protection circuit in accordance with the present invention for protecting a lithium battery pack that powers a power tool; and 
           [0021]      FIG. 5  is a circuit diagram of another embodiment of a battery protection circuit in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    The following descriptions are of exemplary embodiments only, and are not intended to limit the scope, applicability or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing exemplary embodiments of the invention. Various changes to the described embodiments may be made in the function and arrangement of the elements described without departing from the scope of the invention as set forth in the appended claims. 
         [0023]    With reference to the drawings and in particular to  FIG. 4 , which illustrates a circuit diagram of a protection circuit in accordance with the present invention for protecting a lithium battery pack, generally designated  70 , that powers a power tool, the protection circuit, generally designated with reference numeral  60 , comprises an abnormality control integrated circuit (IC)  61 , a first resistor  62 , a second resistor  63 , a power transistor  64 , a first diode  65 , and a second diode  66 . A power input terminal of the abnormality control IC  61  is connected to an anode of a lithium battery set  71  that constitutes the battery pack  70 , and is connected to a first detection terminal thereof via the first resistor  62 . The first detection terminal is also connected via the second resistor  63  to a negative terminal of the protection circuit  60 , which is further connected to an emitter terminal of the power transistor  64 . The power transistor  64  also has a base terminal that is connected to an output control terminal of the abnormality control IC  61 , and a collector terminal connected through the first diode  65  to the anode of the lithium battery set  71 . The second diode  66  is arranged between the emitter and collector of the power transistor  64 . The connection of a reversal/forward rotation circuit  10 , which originally constitutes in part a control circuit of the power tool, with a corresponding throw terminal (the third throw terminal) of a single-pole-tripe-throw switch  21  of a speed variation circuit  20 , which is also parts of the control circuit of the power tool, is, in accordance with the present invention, switched to connect to the collector of the power transistor  64 . The abnormality control IC  61  also has a second detection terminal that is connected to a corresponding terminal of a PWM generation circuit  22  that constitutes in part the speed variation circuit  20  of the power tool. The negative terminal of the protection circuit  60  is connected to second and third throw terminals of the single-pole-triple-throw switch  21  of the speed variation circuit  20 . Consequently, the power transistor  23  that constitutes in part the conventional speed variation circuit  20  is not omitted. 
         [0024]    As a consequence of the above described arrangement, the power transistor  64  and the first diode  65  function to supply power to the motor  30 ; the abnormality control IC  61  detects battery voltage of the lithium battery set  71 , switching status, and temperature of the battery and controls the operation of the motor  30 ; and a voltage division circuit constituted by the first and second resistors  62 ,  63  provides a signal indicating battery voltage (alternatively, each circuit detecting each single cell). The operation of the circuit will be described as follows: 
         [0025]    (1) When the power tool is not in operation, and the single-pole-triple-throw switch  21  is not depressed or actuated, the grounding line of the protection circuit  60  and the cathode of the lithium battery set  71  are separated, namely open-circuited, so that the leakage current is extremely small. 
         [0026]    (2) When the operation button of the power tool is moderately depressed, the single-pole-triple-throw switch  21  is moved to the second throw position where the grounding terminal of the protection circuit  70  is connected to the cathode of the lithium battery set  71  for operation. 
         [0027]    (3) The abnormality control IC  61  detects the battery voltage through the first detection terminal thereof, and detects the position of the switch  21  by the second detection terminal and conducts the power transistor  64  on through the output control terminal thereof. 
         [0028]    (4) At the time when the operation button of the power tool is moderately depressed to move the single-pole-triple-throw switch  21  to the second position, the PWM generation circuit  22  generates and applies a PWM signal to the abnormality control IC  61  through the second detection terminal and the abnormality control IC  61  conveys the PWM signal, in the same phase, through the output control terminal thereof to conduct the power transistor  64  on/off for driving the motor  30 . 
         [0029]    (5) The extent that the single-pole-triple-throw switch  21  is depressed corresponds to the conduction ratio (or ratio of working duration) of the PWM signal, which indirectly corresponds to the output power of the motor  30 . 
         [0030]    (6) When the single-pole-triple-throw switch  21  is depressed to the bottommost position, which corresponds to the third throw position, due to the fact that the second and third throw terminals of the single-pole-triple-throw switch  21  are shorted with each other, the power supply through the protection circuit  60  is not altered. The conventional drawback that when the conventional switch  21  is depressed to the bottommost position, the PWM signal cannot effect full-cycle conduction-on can be overcome by a single-chip processor of the abnormality control IC  61  where a built-in program detects the movement of the switch  21  to the bottommost position, and conducts the power transistor  64  on. 
         [0031]    (7) When the switch  21  is no longer depressed and released, the power transistor  64  is switched from a full-cycle conduction-on condition back to a pulse width modulating condition. Once the switch  21  returns back to the original position, namely the first throw position, the grounding terminal of the protection circuit is separated from the cathode of the lithium battery set  71  and is connected to the anode of the lithium battery set  71  through the switch  24  of the speed variation circuit  20  that is originally present in the power tool. This, under the protection secured by the second diode  66  connected across the power transistor  64 , supplies positive voltage to both terminals of the motor  30  and thus inducing braking on the motor  30 . 
         [0032]    (8) In charging the lithium battery pack  70 , the connection of the reversal/forward rotation circuit  10  that is originally present in the power tool with the corresponding terminal of the single-pole-triple-throw switch  21  of the speed variation circuit  20  is not coupled to a charger circuit, while the connection of the protection circuit  60  to the reversal/forward rotation circuit  10  and the speed variation circuit  20  are coupled to the charger circuit so that when the power tool is connected to a charger, the grounding terminal of the protection circuit  60  is connected to a grounding terminal of the charging power source and the second detection terminal serves as input/output means for the charger. 
         [0033]    The protection circuit for lithium battery pack that serves as a power source of a power tool can be embodied in another manner, which is illustrated in  FIG. 5 . The circuit shown in  FIG. 5  is substantially the same as the counterpart circuit shown in  FIG. 4  with additional components, including voltage regulation IC  67 , a capacitor  68 , a third resistor  69 A, a fourth resistor  69 B, a fifth resistor  69 C, a sixth resistor  69 D, a seventh resistor  69 E, and a temperature sensing resistor  69 F. The anode of the lithium battery set  71  is connected to the IC  67 , which can be for example IC  7805 , and further connected, through an operation terminal of the IC  67 , to the power terminal of the abnormality control IC  61 . The operation terminal of the IC  67  is also grounded through the capacitor  68  and is additionally grounded through the seventh resistor  69 E that is in series connection with the temperature sensing resistor  69 F. A node point between the seventh resistor  69 E and the temperature sensing resistor  69 F is connected to a third detection terminal of the IC  61 . The second detection terminal of the IC  61  is connected in series with the third resistor  69 A and then connected to the PWM generation circuit  22  shown in  FIG. 4 . The second detection terminal is also connected through the fourth resistor  69 B to the ground. The fifth resistor  69 C is connected between the base of the power transistor  64  and the output control terminal of the IC  61 . The base of the power transistor  64  is also connected through the sixth resistor  69 D to the grounding terminal of the IC  61 . 
         [0034]    It is apparent from the above description that the protection circuit for a lithium battery that serves as a power source for a power tool can effectively cut off discharge current to protect the battery when over-temperature occurs in a charging and/or discharging process. Thus, the charging process can be controlled at the optimum condition for each lithium battery, ensuring that none of the lithium battery in a battery pack will be over-discharged to exceed a preset lower limit and also allowing a battery pack constituted by serially-connected batteries to be charged with the maximum current. Consequently, the battery pack can be of extended service life and short charging time. 
         [0035]    It will be understood that each of the elements described above, or two or more together may also find a useful application in other types of methods differing from the type described above. 
         [0036]    While certain novel features of this invention have been shown and described and are pointed out in the annexed claim, it is not intended to be limited to the details above, since it will be understood that various omissions, modifications, substitutions and changes in the forms and details of the device illustrated and in its operation can be made by those skilled in the art without departing in any way from the spirit of the present invention.