Abstract:
A simple control circuitry is provided for controlling the rotational speed of a DC motor. The control circuitry includes a voltage reference component and a switching circuit for controlling the state of the voltage reference component. When the voltage of the DC motor and the voltage reference component is no higher than a predetermined voltage level of the voltage reference component, the switching circuit does not conduct, and the DC motor is accelerated according to a first operation mode. When the voltage of the DC motor and the voltage reference component is higher than a predetermined voltage level of the voltage reference component, the switching circuit conducts, and the DC motor is accelerated to a maximum speed according to a second operation mode.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention provides control circuitry for controlling the rotational speed of a DC motor, and more specifically, simple control circuitry that switches the operation mode of a DC motor.  
           [0003]    2. Related Art  
           [0004]    The processing and spreading of vast amounts of electronic data has facilitated the rapid exchange of information and knowledge, accelerated technological development and enriched our lives. However, when processing large numbers of data transfers, the central processing unit (CPU) of a laptop computer, for example, is prone to overheating. Therefore, it is essential that devices like laptop computers have an excellent heat dissipation device with minimal power consumption to eliminate the problem of overheating.  
           [0005]    Please refer to FIG. 1. FIG. 1 is a simple block diagram of a heat-dissipating process of a CPU  12  as performed by a prior heat dissipation device  10 . As shown in FIG. 1, the heat dissipation device  10  includes a DC motor  14 , a driving circuit  16  that is electrically connected to the DC motor  14 , and a fan  18  electrically connected to the DC motor  14 . When the heat dissipation device  10  dissipates heat from the CPU  12 , the driving circuit  16  first transmits a rotation signal, usually a current signal, to control the rotation of the DC motor  14 . Next, the fan  18  is turned on by the DC motor  14 , thereby cooling the CPU  12 . Generally, the fan  18  is directly attached to the DC motor  14 , i.e. the rotational speed of the fan  18  is that of the DC motor  14 . When heat generated by the CPU  12  increases, the current signal outputted from the driving circuit  16  increases gradually, and the rotational speed of the DC motor  14  and the fan  18  increases as well. When the heat generated by the CPU  12  is relatively small, such as when little data is being processed, the DC motor  14  is not required to turn as fast. That is, the driving circuit  16  outputs a smaller current to the DC motor  14 , thus saving power.  
           [0006]    For a common DC motor, its characteristics are set after the design stage. That is, the input voltage and the rotational speed of the motor are directly related. FIG. 2 is a graph of input voltage versus rotational speed of the common DC motor  14  according to the prior art. Suppose the relationship between the input voltage and rotational speed of the DC motor  14  is represented by a curve Ti; when the input to the DC motor  14  is 5 volts, the rotational speed of the DC motor  14  is 4000 rpm. When the input to the DC motor  14  is 2.5 volts, the rotational speed of the DC motor  14  is 200 rpm, which is too high given the circumstances (i.e. the input current is too large).  
           [0007]    The coil windings inside the DC motor  14  can be redesigned in order to reduce the motor&#39;s rotational speed at low input voltages. For example, the DC motor  14  can be designed to operate at 1500 rpm for an input voltage of 2.5 volts. The input/output graph of the DC motor  14  then follows a new characteristic curve T 2 . However, though the goal of dropping the rotational speed at lower input voltages has been achieved by following the characteristic curve T 2 , the rotational speed at higher voltages has been greatly compromised. As shown in FIG. 2, the rotational speed at an input voltage of 5 volts is only 3500 rpm.  
         OBJECT OF THE INVENTION  
         [0008]    In light of the above-mentioned issues, the object of the invention is to provide control circuitry for controlling the rotational speed of a DC motor. The invention achieves this object with a simple design, few electronic components and no modifications to the original design of the DC motor.  
           [0009]    The specific details of the contents and techniques of the invention are described with figures hereinafter: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The invention will become more fully understood from the detailed description given hereinafter. However, the drawings are for the purpose of illustration only, and thus are not limitative of the invention, wherein:  
         [0011]    [0011]FIG. 1 is a simple block diagram of a heat-dissipating process of a CPU as performed by a prior heat dissipation device;  
         [0012]    [0012]FIG. 2 is a graph of the relationship between the input voltage and rotational speed of the DC motor according to the prior art;  
         [0013]    [0013]FIG. 3 is a circuit diagram of control circuitry applied to a DC motor according to the first embodiment of the present invention;  
         [0014]    [0014]FIG. 4 is a graph of the relationship between the input voltage and rotational speed of the DC motor depicted in FIG. 3;  
         [0015]    [0015]FIG. 5 is a circuit diagram of control circuitry applied to the DC motor depicted in FIG. 3 according to the second preferred embodiment of the present invention; and  
         [0016]    [0016]FIG. 6 is a simple block diagram of control circuitry applied to the DC motor depicted in FIG. 3 according to the third preferred embodiment of the present invention; 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Please refer to FIG. 3. FIG. 3 is a circuit diagram of control circuitry  20  applied to a DC motor  30  according to the first embodiment of the present invention. As shown in FIG. 3, the control circuitry  20  comprises an input node  21 , which, along with the input node  31  of the DC motor  30 , is connected to the variable voltage source  50 , a voltage reference component  22  that is electrically connected to the input node  21 , a switching circuit  24  that is connected between the DC motor  30  and the voltage reference component  22 , and a ground node G.  
         [0018]    The voltage reference component  22  of the present invention is a Zener diode ZD, which provides a predetermined reference voltage Vzd. Between the cathode of the Zener diode and the input node  21 , there is usually a current-limiting resistor R 1 , which increases the resistance of the voltage reference component  22 , effectively limiting the current through the Zener diode ZD and thus lengthening the life of the Zener diode.  
         [0019]    The switching circuit  24  of the first embodiment of the invention is used to control the state of the voltage reference component  22  and is connected through the connector node  25  to the DC motor  30 . The switching circuit  24  consists of an npn bipolar junction transistor (BJT) Tr and a voltage drop component. The base B of the transistor Tr is connected to the voltage reference component  22 , and the collector C and the emitter E are connected to the DC motor  30  and the ground node G of the switching circuit  24  respectively. When the switching circuit  24  conducts, current generated by the variable voltage source  50  flows to the emitter E through either the DC motor  30  and collector C or the voltage reference component  22  and base B. Additionally, a resistor R 2  connected across the collector C and the emitter E functions as the voltage drop component, thus ensures a voltage difference between the collector C and the emitter E. Since the purpose of the voltage drop component is to provide a voltage difference between the collector C and the emitter E of the transistor Tr of the switching circuit  24 , the component can also be a Zener diode or any resistive component.  
         [0020]    Please refer to FIGS. 2, 3, and  4 . FIG. 4 is a graph of the relationship between the input voltage and rotational speed of the DC motor  30  of the first embodiment. The components of the control circuitry  20  described hereinafter shall assume values as described in FIG. 2 for ease of explanation of the operating principles. As shown in FIGS. 3 and 4, assume that the reference voltage Vzd of the Zener diode ZD is 3 volts and the DC motor  30  is designed to have the characteristic curve T 2 .  
         [0021]    When the variable voltage source  50  outputs a voltage of 2.5 volts to the DC motor  30 , the rotational speed of the DC motor  30  is 1500 rpm. At this point, the input voltage is still smaller than the reference voltage Vzd (3 volts) of the Zener diode ZD, so the npn BJT Tr of the switching circuit  24  does not conduct. Therefore, the current outputted by the variable current source  50  only flows to the ground node G through the DC motor  30  and resistor, and the motor operates according to the characteristic curve T 2 .  
         [0022]    When the variable voltage source  50  outputs a voltage greater than the sum of the potential difference across the current limiting resistor R 1  and the reference voltage Vzd of the Zener diode ZD, for example, 3.5 volts, the Zener diode conducts and consequently switches the npn BJT Tr on. The current generated from the variable voltage source  50  then mostly passes through the DC motor  30 , the collector C of the BJT Tr, the emitter E of the BJT Tr, and the ground node G, while only a little amount of current flows through the Zener diode ZD and resistor R 2 . As a result, the DC motor  30  switches from the characteristic curve T 2  to T 1 , and the motor reaches a rotational speed of 4000 rpm when the variable voltage source  50  provides a 5 volt input.  
         [0023]    Please refer to FIG. 5. FIG. 5 is a circuit diagram of control circuitry  60  for controlling rotational speed of the DC motor  30  according to the second preferred embodiment of the present invention. The most significant difference between the first embodiment and the second is that the switching circuit of the second embodiment of the control circuitry adopts a pnp BJT. As shown in FIG. 5, the control circuitry  60  comprises an input node  61  connected to the variable voltage source  50 , a resistor R 3  connected to the input node  61  across which a voltage difference develops due to the current generated from the variable voltage source  50 , a voltage reference component  22  electrically connected between the resistor R 3  and ground node G, and a switching circuit  64  that is electrically connected between the input node  61  and the DC motor  30 .  
         [0024]    The voltage reference component  22  of the second embodiment also utilizes a Zener diode ZD to provide a predetermined reference voltage Vzd. There is also a current-limiting resistor R 1  connected between the cathode of the Zener diode ZD and the input node  21  that boosts the resistance of the voltage reference component  22  so as to reduce the current flowing through the Zener diode ZD while it conducts, thereby lengthening the lifetime of the Zener diode ZD.  
         [0025]    The switching circuit  64  of the second embodiment of the invention is used to control the state of the voltage reference component  22  and is connected through the connector node  65  at the node between the resistor R 3  and the voltage reference component  22 . The switching circuit  64  consists of a pnp bipolar junction transistor Tr and a voltage drop component. The base B of the transistor Tr is connected to the connector node  65 , and the collector C and the emitter E are connected to the DC motor  30  and the input node  61  of the control circuitry  60  respectively. When the switching circuit  64  conducts, the current generated by the variable voltage source  50  flows to the DC motor  30  through either the collector C or emitter E of the pnp transistor Tr. Additionally, a resistor R 2  connected across the collector C and the emitter E functions as a voltage drop component, thus ensures a voltage difference between the collector C and the emitter E. Since the purpose of the voltage drop component is to provide a voltage difference between the collector C and the emitter E of the transistor Tr of the switching circuit  24 , the component can also be a Zener diode or any other component with resistive characteristics.  
         [0026]    A driving circuit  70  drives the DC motor  30 . The control circuitry  20  and  60  is used to modulate and control the rotational speed of the DC motor  30 . The function and principles behind the control circuitry  20  and  60  have been thoroughly described in the first and second preferred embodiments of the invention, so no further explanations will be provided here.  
         [0027]    The switching circuits  24  and  64  of the invention have only been embodied with npn and pnp BJTs Tr only. However, it is also possible to use a PMOS transistor, NMOS transistor, or a relay as a switch, all of which fall within the spirit of the invention, and no further details will be provided at this point. In addition, with reference to FIGS.  3  to  5 , when the switching circuits  24  and  64  are not conducting, the characteristics of the DC motor  30  are affected, aside from its original design, by only the resistor R 2 . That is, when a user needs to lower the rotational speed of the DC motor  30  for a given voltage, a larger resistor R 2  can be used to achieve this goal.  
         [0028]    In contrast to the prior art, the most significant characteristic of the present invention is that the control circuitry  20  and  60  requires only the simplest electronic components to achieve the goal of modulating and controlling the rotational speed of a DC motor; no modifications to the internal windings of the motor or additional complex circuitry is required.  
         [0029]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.