Abstract:
The speed control circuit in accordance with the present invention mainly includes a switch circuit and a voltage-detection circuit. The voltage-detection circuit is actuated depending upon high or low voltage of power supply to thereby determine the switch circuit to output high or low operating voltage to a driver circuit of the motor in response to changes of the voltage of the power supply. The motor is adjusted and operated at desired speeds by a rated range of input voltage according to the high or low operating voltage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is related to a speed control circuit for a dc brushless motor and more particularly to a switch circuit and a voltage-detection circuit changing motor speed in various speed features of the dc brushless motor in response to changes of voltage of power supply.  
           [0003]    2. Description of the Related Art  
           [0004]    Currently, fans are used for forced air cooling of the electronic devices, such as power supply, CPU of computers, display card, and computer components etc. Recently, there is a trend to adjustably operate fans at high or low speed depending upon ambient conditions. In high temperature ambiance, a fan must operate at high speed to reduce or remove heats produced by electronic components so as to ensure these electronic components being operated in normal and avoid these heats resulting in damage to these electronic components. In low temperature ambiance, the fan must operate at low adequate speed for saving power and decreasing noise.  
           [0005]    [0005]FIG. 1 illustrates a schematic view of a traditional driver circuit for a single phase dc brushless motor.  
           [0006]    Referring to FIG. 1, the driver circuit for the single phase dc brushless motor includes a coil L 1 , a Hall element IC 1 , a driver element IC 2 , and a capacitor C 1 . The Hall element IC 1  is used for detecting changes of a permanent magnetic field of a rotor to thereby output weaker Hall voltage. And then the Hall voltage is magnified by the driver circuit IC 2  to further output alternative signals (square waveforms) from two terminals O 1  and O 2  to the bridging transistors Q 1  through Q 4 . The coil L 1  is actuated in turn to change current in direction so as to rotate the rotor. FIG. 2 illustrates a schematic view of a traditional driver circuit for a double phase dc brushless motor.  
           [0007]    Referring to FIG. 1, the driver circuit for the double phase dc brushless motor includes a first coil L 1 , a second coil L 2 , a capacitor C 1 , a Hall driver element IC 3 , a first resistor R 1 , a second resistor R 2 , and a transistor Q 1 . The resistors R 1  and R 2  and transistor Q 1  are consisted of a rotational speed detective circuit. The Hall driver element IC 3  is used for detecting changes of a permanent magnetic field of a rotor and then the Hall voltage is magnified to output alternative signals (square waveforms) from two terminals O 1  and O 2  to the coils L 1  and L 2  respectively. The coils L 1  and L 2  are actuated in turn to change current so as to rotate the rotor.  
           [0008]    In cooling fan system, there is generally providing two voltage levels to a driver circuit for various desired amounts of forced air. For example, a high voltage level 12V controls a fan operating at 3000 rpm, and a low voltage level 6V at 1500 rpm. Therefore, the cooling fan system operates at different speeds to provide cooling capacity depending upon the ambient conditions.  
           [0009]    [0009]FIG. 3 illustrates a voltage diagram for speed of a traditional dc brushless motor. Between two specific voltages, ranged from 12V to 6V for example, have high speed, low speed and nonlinear feature. When the cooling fan system is input 12V, the motor is operated at high speed 3000 rpm; if when the cooling fan system is changed to input 6V, the motor is operated at low speed 2000 rpm according to the nonlinear feature. Accordingly, the cooling fan system fails to certainly operate at predetermined low speed 1500 rpm and to meet the system demand.  
           [0010]    The motor structure and the nonlinear feature limit the traditional dc brushless motor whose speed cannot be adjusted depending upon the system needs. In order to improve these limitations of the traditional motor to perform desired speeds, it is necessary to be redesigned.  
           [0011]    The present invention intends to provide a speed control circuit comprising a switch circuit and a voltage-detection circuit used for adjusting an operating voltage depending upon speeds of the motor. The operating voltage of desired speed of the motor is determined by the rated range of input voltage in such a way to mitigate and overcome the above problem.  
         SUMMARY OF THE INVENTION  
         [0012]    The primary objective of this invention is to provide a speed control circuit for a dc brushless motor comprising a switch circuit and a voltage-detection circuit used for adjusting operating voltage depending upon speeds of the motor.  
           [0013]    The secondary objective of this invention is to provide a speed control circuit for a dc brushless motor using a switch circuit to determine an operating voltage of low speed.  
           [0014]    The present invention is a speed control circuit for a dc brushless motor. The speed control circuit includes a switch circuit and a voltage-detection circuit. The voltage-detection circuit is actuated depending upon high or low voltage of power supply to thereby determine the switch circuit to output high or low operating voltage to a driver circuit of the motor in response to changes of the voltage of the power supply. The motor is adjusted and operated at desired speeds by a rated range of input voltage according to the high or low operating voltage.  
           [0015]    Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The present invention will now be described in details with references to the accompanying drawings herein:  
         [0017]    [0017]FIG. 1 is a schematic view of a traditional driver circuit for a single phase dc brushless motor;  
         [0018]    [0018]FIG. 2 is a schematic view of a traditional driver circuit for a double phase dc brushless motor;  
         [0019]    [0019]FIG. 3 is a voltage diagram for speed of a traditional dc brushless motor;  
         [0020]    [0020]FIG. 4 is a block diagram of a speed control circuit for a dc brushless motor in accordance with the present invention;  
         [0021]    [0021]FIG. 5 is a voltage diagram for speed of a speed control circuit in accordance with the present invention;  
         [0022]    [0022]FIG. 6 is a schematic view of a speed control circuit in accordance with a first embodiment of the present invention applied in a driver circuit for a single phase dc brushless motor; and  
         [0023]    [0023]FIG. 7 is a schematic view of a speed control circuit in accordance with a second embodiment of the present invention applied in a driver circuit for a double phase dc brushless motor. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    Referring now to the drawings, there are two embodiments of the present invention shown therein, which include generally a driver member and a speed control member.  
         [0025]    [0025]FIG. 4 illustrates a block diagram of a speed control circuit for a dc brushless motor in accordance with the present invention.  
         [0026]    Referring initially to FIG. 4, the speed control circuit  10  includes a switch circuit  11  comprising an adjusting resistor and a voltage-detection circuit  12 . The switch circuit  11  is adapted to control power supply to output a high or low operating voltage to a driver circuit  1  of the motor. And the voltage-detection circuit  12  is adapted depending upon high or low voltage of the power supply to actuate the switch circuit  11  in response to changes of the voltages of the power supply.  
         [0027]    [0027]FIG. 5 illustrates a voltage diagram for speed of a speed control circuit in accordance with the present invention. Referring to FIGS. 4 and 5, when the system is input high voltage 12V, the switch circuit  11  is actuated to be closed by the voltage-detection circuit  12  and thus the motor can be input a high operating voltage 12V from the switch circuit  11 . The motor is capable of operating and maintaining at high speed 3000 rpm until the input operating voltage is changed to a low voltage. When the system is changed to be input a low voltage 6V, the switch circuit  11  is actuated to be opened by the voltage-detection circuit  12  and thus the motor can be input a predetermined low operating voltage xV dropped by the adjusting resistor. The motor is capable of operating and maintaining at low speed 1500 rpm.  
         [0028]    [0028]FIG. 6 illustrates a speed control circuit in accordance with a first embodiment of the present invention applied in a driver circuit for a single phase dc brushless motor. Referring to FIGS. 1 and 6, reference numerals of the first embodiment have applied the identical numerals of the traditional single phase dc brushless motor. The driver circuit of the first embodiment has the similar configuration and same function as the traditional driver circuit and the detailed descriptions are omitted.  
         [0029]    Referring now to FIG. 6, the speed control circuit  10  in accordance with the first embodiment of the present invention is connected to the driver circuit  1  of the single phase dc brushless motor. The speed control circuit  10  includes a switch circuit  11  consisted of a transistor Q 2 , a resistor R 3 , and a resistor R 4 . The resistor R 4  is an adjusting resistor Rx. When the transistor Q 2  is cut off, the voltage of the power supply Vcc is passed the resistor R 4  and dropped. And then this voltage drop is output to the driver circuit  1  of the motor.  
         [0030]    Referring again to FIG. 6, the speed control circuit  10  further includes a voltage-detection circuit  12  consisted of a transistor Q 3 , a resistor R 5 , and a resistor R 6 . When the transistor Q 3  is saturated, a current from the power supply Vcc can pass a base of the transistor Q 2  and the resistor R 3  to form a bias current between a collector and an emitter of the transistor Q 3 . Consequently, the transistor Q 2  is saturated, and the power supply Vcc is output to the driver circuit  1  through an emitter and a collector of the transistor Q 2  so as to output a high operating voltage. This high operating voltage is input into the driver circuit  1  and thus the motor (coil L 1 ) is operated at high speed, as shown in FIG. 5. When the transistor Q 3  is cut off, the voltage of the collector of the transistor Q 3  is risen that cuts off a bias current between an emitter and a base of the transistor Q 2 . Consequently, the transistor Q 2  (bias current) is cut off, and the power supply Vcc is input into the resistor R 4  and dropped so as to output a low operating voltage. This low operating voltage is input into the driver circuit  1  and thus the motor (coil L 1 ) is operated at low speed, as shown in FIG. 5. The resistors R 5  and R 6  consisted in a voltage divider. The voltage divider divides the voltage of the power supply Vcc to output a high voltage level (0.6-0.7V) to saturate the transistor Q 3  when the power supply is risen above a reference voltage (xV). The transistor Q 2  is actuated to saturate by means of switching to saturate the transistor Q 3 . By contrast, the voltage divider outputs a low voltage level to cut off the transistor Q 3  when the power supply is fallen below the reference voltage. Similarly, the transistor Q 2  is actuated to cut off by means of switching to cut off the transistor Q 3 .  
         [0031]    [0031]FIG. 7 illustrates a speed control circuit in accordance with a second embodiment of the present invention applied in a driver circuit for a double phase dc brushless motor. Referring to FIGS. 2 and 7, reference numerals of the second embodiment have applied the identical numerals of the traditional double phase dc brushless motor. The driver circuit of the second embodiment has the similar configuration and same function as the traditional driver circuit and the detailed descriptions are omitted.  
         [0032]    Referring now to FIG. 7, the speed control circuit  10  in accordance with the second embodiment of the present invention is connected to the driver circuit  1  of the double phase dc brushless motor. The speed control circuit  10  includes a switch circuit  11  consisted of a transistor Q 2 , and a resistor R 4 . The resistor R 4  is an adjusting resistor Rx. When the transistor Q 2  is cut off, the Hall driver circuit IC 3  is dropped by the resistor R 4  and then grounded. Thus the motor is operated at low speed, as shown in FIG. 5. When the transistor Q 2  is saturated, the Hall driver circuit IC 3  is grounded through a collector and an emitter of the transistor Q 2 . Thus the motor is operated at high speed, as shown in FIG. 5.  
         [0033]    Referring again to FIG. 7, the speed control circuit  10  further includes a voltage-detection circuit  12  consisted of a resistor R 5 , and a resistor R 6 . The resistors R 5  and R 6  consisted in a voltage divider. The voltage divider divides the voltage of the power supply Vcc to output a high voltage level (0.6-0.7V) to saturate the transistor Q 2  when the power supply is risen above a reference voltage (xV). Consequently, the Hall driver circuit IC 3  is grounded through the transistor Q 2  and the coils L 1  and L 2  are operated at the high operating voltage. By contrast, the voltage divider outputs a low voltage level to cut off the transistor Q 2  when the power supply is fallen below the reference voltage. Consequently, the Hall driver circuit IC 3  is grounded through the resistor R 4  and the coils L 1  and L 2  are operated at the low operating voltage.  
         [0034]    Although the invention has been described in details with references to its presently preferred embodiment, it will be understood by one of ordinary skill in the art that various modifications can be made without departing from the spirit and the scope of the invention, as set forth in the appended claims.