Abstract:
A motor speed control device. The motor speed control device applied to a direct current (DC) fan includes a driving element constituted by a driving IC and Hall IC, a thermal sensor and a control element electrically connected between the driving element and the thermal sensor. The present invention utilizes a thermal sensor and a simple control element to effectively and stably control the variable speed of the fan within different temperature ranges.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a motor speed control device, and in particular to a motor speed control device applied to a direct current (DC) fan.  
         [0003]     2. Description of the Related Art  
         [0004]     Traditionally, when electronic devices function under heavy load, cooling fans operate at full speed. However, under light loading, fans generally continue to operate at full speed, wasting power, generating unnecessary noise, and reducing fan life. Accordingly, a method to control the rotation speed of the fan has been developed. As shown in  FIG. 1 , when an electronic device functions under light loading, its inner temperature remains low. A thermistor RTH detects the temperature variation, adjusts its resistance accordingly, adjusts voltage and current from the power source, and outputs a signal to a driving circuit IC, which outputs a pulse width modulation (PWM) to a transistor TR, the switch frequency of which varies with duty cycle of the PWM signal, adjusting average current to the motor of the fan. Controlled rotation speed of the fan motor is thus achieved. The control theory is shown in  FIG. 2  by way of explanation, in which supply voltage Vcc is 12V. The thermistor RTH detects temperature and accordingly generates voltage VTH. Reference voltage V 0  drives the fan at low speed. The duty cycle with the lowest driving voltage is determined by comparing oscillation voltage of the PWM signal and the reference voltage V 0 . The duty cycle modulation is controlled by comparing the oscillation voltage of the PWM signal and the voltage VTH from low speed from full speed. The fan functions at full speed if temperature exceeds a specific value. When the inner temperature increases, thermistor RTH decreases resistance, and the current increases to increase rotation speed, providing suitable heat dissipation. When the temperature decreases again, the thermistor RTH again increases resistance, thus decreasing the rotation speed of the fan.  
         [0005]     However, as shown in  FIG. 1 , a voltage drop occurs at V CE  terminal of the transistor TR in the work area. The transistor consumes much power and generates heat accordingly. Also, when power consumption is too high or input voltage from the power source is too low, the thermistor RTH cannot function normally, thereby generating excess heat and increasing the inner temperature of the computer system.  
       SUMMARY OF THE INVENTION  
       [0006]     An object of the present invention is to provide a motor speed control device applied to a fan for controlling its rotation speed in different temperature ranges by a thermistor and a simple external circuit, easily controlling turning points of temperature when the fan functions at a relatively low speed.  
         [0007]     Accordingly, the motor speed control device of the present invention includes a thermal sensor detecting an environmental temperature of the fan, a driving element driving the fan to a specific rotation speed according to the detected temperature, and a control element connected electrically between the driving element and the thermal sensor for adjusting the first voltage of the thermal sensor to change the rotation speed and temperature range of the fan, wherein the thermal sensor is preferably a thermistor, and the driving element includes a Hall sensor and a driver IC.  
         [0008]     Preferably, the control element is a switch circuit including a comparator, a transistor, and two resistors, wherein one resistor of the switch circuit is electrically connected in parallel with the thermal sensor such that the first voltage rapidly decreases to be less than the reference voltage of the driving element to turn on the transistor and reduce the temperature range of the fan to the full speed.  
         [0009]     Alternatively, the control element includes a resistor electrically connected in serial with the thermal sensor and controlling the temperature range of the fan to the full speed by adjusting the resistance of the resistor and reducing the variation of the first voltage.  
         [0010]     The control element can be a subtraction circuit including a comparator and at least four resistors, wherein three resistors of the subtraction circuit form a second voltage to adjust a third voltage output to the driving element to reduce the temperature range of the fan to the full speed.  
         [0011]     Alternatively, the control element can be constituted by a division circuit, a comparator, and an output circuit, wherein when the first voltage exceeds the reference voltage of the driving element, the output circuit outputs a voltage equal to the reference voltage to be input to the driving element to keep the fan rotate at a low speed, and when the first voltage is smaller than the reference voltage of the driving element, the voltage input to the driving element is divided by N through the division circuit to quickly drive the fan to a full speed.  
         [0012]     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:  
         [0014]      FIG. 1  is a schematic diagram of the control circuit of the conventional fan.  
         [0015]      FIG. 2  is a plot of control theory concerning the control circuit of the conventional fan.  
         [0016]      FIG. 3A  is a schematic diagram of the first embodiment of the motor speed control device of the present invention.  
         [0017]      FIG. 3B  plots variation between the temperature and rotation speed in the first embodiment of the motor speed control device of the present invention.  
         [0018]      FIG. 4A  is a schematic diagram of the second embodiment of the motor speed control device of the present invention.  
         [0019]      FIG. 4B  plots variation between the temperature and rotation speed in the second embodiment of the motor speed control device of the present invention.  
         [0020]      FIG. 5A  is a schematic diagram of the third embodiment of the motor speed control device of the present invention.  
         [0021]      FIG. 5B  plots variation between the temperature and rotation speed in the third embodiment of the motor speed control device of the present invention.  
         [0022]      FIG. 6  is a schematic diagram of the fourth embodiment of the motor speed control device of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [heading-0023]     First Embodiment  
         [0024]      FIG. 3A  is a schematic diagram of the first embodiment of the motor speed control device of the present invention. As shown in  FIG. 3 , a power source supplies voltage to start fan rotation by inter-induction between winding coils and magnetic rings of the motor. A Hall induction integration circuit IC 2  detects electric waves induced by magnetic field variation between winding coils and magnetic rings of the fan. After, the Hall induction IC IC 2  outputs two positive and negative voltages to a driving integration circuit IC 1 . Thus, the circuit IC 1  and the circuit IC 2  constitute a driving element to drive the fan and send a feedback periodic pulse signal.  
         [0025]     As well, the driving element is connected to a thermal sensor (or a thermistor) RTH and a switch circuit, wherein the switch circuit  31  includes a comparator, a transistor TR 1 , and two resistors R 0  and R 5  (as indicated by the dotted line in  FIG. 3A ). The thermal sensor RTH has various resistances at different temperatures, whereby first voltage V 1  from thermal sensor RTH and the resistor R 3  varies with temperature. Second voltage (or reference voltage) V 2  is formed by the resistors R 1  and R 2 . A comparator compares the first voltage V 1  and the second voltage V 2 , and accordingly adjusts the third voltage V 3  output therefrom. Therefore, the current varies when the transistor TR 1  is turned on, and the rotation speed of the fan varies accordingly, thus achieving the goal of speed control by temperature.  
         [0026]      FIG. 3B  plots variation between the temperature and rotation speed in the first embodiment of the motor speed control device of the present invention.  FIG. 3B  shows variations in the slope between temperature and rotation speed of the fan before and after the circuit IC 1  is connected with the switch circuit. Without the switch circuit, the slope from temperature T 1  to T 2  is A. With the switch circuit, the resistor R 5  and the thermal sensor RTH are connected in parallel, the first voltage V 1  drops rapidly such that the reference voltage V 2  exceeds the first voltage V 1 , and the transistor TR 1  is turned on, thus reducing temperature range of speed variation (from T 1  to T 3 ). The slope B from temperature T 1  to T 3  exceeds the slope A without the switch circuit, so rotation speed of the fan is raised from low S 1  to high S 2  rapidly and sharply. Temperature range of speed variation is thus reduced by controlling the first voltage V 1 .  
         [heading-0027]     Second Embodiment  
         [0028]      FIG. 4A  is a schematic diagram of the second embodiment of the motor speed control device of the present invention. As shown in  FIG. 4A , the detailed circuit and control theory are similar to those in the first embodiment. The difference between these two embodiments lies in a resistor R 4  electrically connected with the thermal sensor RTH in series in this embodiment, unlike the switch circuit of the first embodiment.  
         [0029]      FIG. 4B  plots variation between the temperature and rotation speed in the second embodiment of the motor speed control device of the present invention.  FIG. 4B  shows variations in the slope between temperature and rotation speed of the fan before and after the resistor R 4  is connected with the thermal sensor RTH in series. Without the resistor R 4 , the slope from temperature T 1  to T 2  is A. After the resistor R 4  is connected with the thermal sensor RTH in series, variation of the first voltage V 1  decreases. Temperature range from T 2  to T 3 , controlled by the resistance of the resistor R 4 , presents a smaller slope C.  
         [heading-0030]     Third Embodiment  
         [0031]      FIG. 5A  is a schematic diagram of the third embodiment of the motor speed control device of the present invention. As shown in  FIG. 5A , the detailed circuit and control theory are similar to those in the first embodiment. The difference between these two embodiments lies in a subtraction circuit  51  of this embodiment replacing the switch circuit of the first embodiment. The subtraction circuit  51  includes a comparator and six resistors R 6 , R 7 , R 8 , R 9 , R 10 , and R 11 , as indicated by the dotted line in  FIG. 5A .  
         [0032]      FIG. 5B  plots variation between the temperature and rotation speed in the third embodiment of the motor speed control device of the present invention. As shown in  FIG. 5B , when resistances of the resistors R 6 , R 7 , R 8 , and R 11  are equal, voltage V 5  equals voltage of voltage V 4  taken away from voltage V 1 . Temperature range of the fan at full speed is thus reduced by adjusting fourth voltage V 4 , whereby the slope changes from A to a larger value D.  
         [heading-0033]     Fourth Embodiment  
         [0034]      FIG. 6  is a schematic diagram of the fourth embodiment of the motor speed control device of the present invention. As shown in  FIG. 6 , the detailed circuit and control theory are similar to those in the first embodiment. The difference between these two embodiments lies in the switch circuit of the first embodiment being replaced with a division circuit  61 , a comparison circuit  62 , and an output circuit  63 .  
         [0035]     When the second voltage (or the reference voltage) V 2  is smaller than the first voltage V 1 , the output circuit  63  outputs a voltage equal to the second voltage V 2  to the circuit IC 1  so as to keep the fan at a low speed. When the second voltage V 2  exceeds the first voltage V 1 , the voltage input to the circuit IC 1  divided by N (N is a natural number) through the division circuit  61 . Therefore, the desired voltage (Vcc×16%) is rapidly achieved for stably controlling the rotation speed when the fan functions at a low speed.  
         [0036]     In conclusion, the motor speed control device is applied to a DC fan for effectively and stably controlling different speeds (from low to full) and the rotation speed in different temperature ranges.  
         [0037]     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.