Abstract:
A driving control circuit of a hood motor, including: a hood motor section rotatably driven for ventilating the inner portion of a system; a rectifier circuit section for supplying direct current power to the hood motor by rectifying alternating current power, a controlling section for generating a control signal for controlling the operation of the hood motor; and a driving circuit section for controlling the operation of the hood motor according to the control signal from the controlling section.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from applications for ROTATION SPEED CONTROL CIRCUIT OF A HOOD DC MOTOR FOR A MICROWAVE OVEN OVER THE RANGE earlier filed in the Korean Industrial Property Office on the 5 th  of October 1999 and there duly assigned Ser. Nos. 42864/1999 and 42865/1999. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a driving control circuit of a hood motor, and more particularly to a driving control circuit of a hood motor for controlling rotational speeds of the hood motor. 
     2. Description of the Related Art 
     Generally, a hood motor ventilates heat generated under a microwave oven installed over the range (hereinafter called OTR microwave oven), and the smell of foods cooked by the range. The OTR microwave oven having the hood motor is usually installed over a gas range, performing not only a basic function thereof, i.e., the cooking function using microwaves, but also a ventilating function for venting smoke generated during the cooking operation of the gas range. 
     FIG. 1 is a view for showing a conventional OTR microwave oven installed, and FIG. 2 is a view for schematically showing the inner portion of the OTR microwave oven. 
     As shown in FIG. 1, the OTR microwave oven includes a body  10  having a cooking chamber  11  therein. On a lower side of the cooking chamber  11 , a hood lamp  12  is disposed, while vent ducts  14  are disposed on the left and right sides of the cooking chamber  11 . 
     Further, a hood motor M is installed at the middle rear portion of the cooking chamber  11 , and vent blowers  18  are disposed on the left and right sides of the hood motor M. Upper portions of the vent blowers  18  are connected with a connective tube  20 , and a vent passage  22 . 
     Accordingly, vapor or smoke generated during the cooking operation of the gas range are drawn into the vent ducts  14  by the rotation of the vent blowers  18 , and are exhausted outside through the connective tube  20  and the vent passage. 
     FIG. 3 is a view for showing the rotational velocity control circuit of the hood motor shown in FIG.  2 . 
     As shown in FIG. 3, the conventional rotational velocity control circuit of the hood motor includes a power on/off switch  330 , a rotational velocity selecting switch  332 , a temperature sensor  334 , and a hood motor M. 
     The rotational velocity selecting switch  332  selects the rotational velocity of the hood motor M, while being selectively switched on to a low-velocity contact L for selecting the low velocity mode, or to a high-velocity contact H for selecting the high velocity mode. 
     The hood motor M includes an alternating current (hereinafter called AC) motor selectively rotated at a low or a high velocity in accordance with the selected mode of the low/high velocity contacts L or H. 
     The temperature sensor  334  senses the temperature of a driving coil in the hood motor M, and has increasing resistance value corresponding to the rise in temperature. 
     Meanwhile, the presence of foreign substances in the vent blowers  18  of the hood motor M causes a constraint on the rotation of the hood motor M, and accordingly, the temperature of the driving coil is excessively increased. As a result, the resistance value of the temperature sensor  334  is significantly increased, cutting off the application of AC power to the hood motor M. 
     The conventional control circuit for controlling the rotational velocity of the hood motor M, however, has shortcomings of high manufacturing cost and a low productivity due to its expensive AC motor and temperature sensor. 
     Further, in the conventional hood motor control circuit, since the low/high velocity contacts of the rotational velocity selecting switch are formed to be mechanically switched on/off to control the rotational velocity of the hood motor M, there is a high possibility of having poor contacts due to frequent switching, and the danger of fire due to spark at the contacts. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed to overcome the above-described problems of the related art, and accordingly, it is an object of the present invention to provide a driving control circuit of a hood motor having a direct current motor as a hood motor, which is capable of not only controlling the rotational velocity of the direct current motor, but also capable of protecting circuit components from possible abnormalities of the motor. 
     The above object is accomplished by a driving control circuit of a hood motor according to the present invention, including: a hood motor rotatably driven for ventilating the inner portion of a system; rectifier circuit means for rectifying an alternating current voltage into a direct voltage, and for supplying direct current voltage to the hood motor; controlling means for generating a control signal for controlling the operation of the hood motor; and driving circuit means for controlling the operation of the hood motor according to the control signal from the controlling means. 
     As described above, according to the present invention, the manufacturing cost of the microwave oven can be reduced, while the productivity is improved, and the stable operation of the motor can be guaranteed since the abnormalities thereof such as a poor contact of switch contacts, etc. are prevented. Further, when there is an abnormality in the operation of the hood motor, the circuit is automatically cut off, preventing a possible overload at the motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and other advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings, in which: 
     FIG. 1 is a view of a conventional over-the-range microwave oven; 
     FIG. 2 is a view for schematically showing the inner portion of the over-the-range microwave oven shown in FIG. 1; 
     FIG. 3 is a view for showing a rotational velocity control circuit of the hood motor shown in FIG. 2; 
     FIG. 4 is a view for showing a hood motor driving control circuit according to a first preferred embodiment of the present invention; 
     FIGS. 5 and 6 are waveforms for explaining the operation of the control circuit of FIG. 4, when the hood motor is normally operated; 
     FIGS. 7 and 8 are waveforms for explaining the operation of the control circuit of FIG. 4, when the hood motor is abnormally operated; 
     FIG. 9 is a view for showing a driving control circuit of a hood motor according to a second preferred embodiment of the present invention; 
     FIGS. 10 and 11 are waveforms for explaining the operation of the control circuit of FIG. 9, when the hood motor is normally operated; 
     FIGS. 12 and 13 are waveforms for explaining the operation of the control circuit of FIG. 9, when the hood motor is abnormally operated; 
     FIG. 14 is a view for showing a driving control circuit of a hood motor according to a third preferred embodiment of the present invention; and 
     FIGS. 15 and 16 are waveforms for explaining the operation of the control circuit of FIG. 14, when the hood motor is abnormally operated. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, the preferred embodiment of the present invention will be described in greater detail with reference to the accompanied drawings. 
     FIG. 4 is a view for showing a hood motor driving control circuit according to a first preferred embodiment of the present invention. 
     As shown in FIG. 4, the driving control circuit of a hood motor according to the first preferred embodiment of the present invention includes a rectifier circuit section  100 , a controlling section  104 , a driving circuit section  106 , an overload prevention circuit  108 , and a direct current hood motor M (hereinafter called DC hood motor). 
     The rectifier circuit section  100  includes a fuse  101  for cutting off commonly used alternating current (hereinafter called AC) when there is overvoltage of AC, a bridge diode  102  for full-wave rectifying the AC, and a smoothing capacitor C 1  for smoothing the full-wave rectified DC. 
     The controlling section  104  includes a microcomputer  105 , and a first transistor TR 1 . The microcomputer  105  includes a control signal input port for inputting on/off commands and rotational velocity control command from a user, a control signal output port PWM for generating a control signal, and a ground port GND. 
     The first transistor TR 1  includes a base connected with the control signal output port PWM, and a collector connected with a voltage source Vcc through a resistor R 5 . Between the collector and the resistor R 5  of the first transistor TR 1 , the driving circuit section  106  is connected. Between the first transistor TR 1  and the driving circuit section  106 , a capacitor C 3  and a resistor R 4  are parallel connected, respectively. 
     Here, the microcomputer  105  generates a pulse width modulation (hereinafter called PWM) control signal of a certain frequency having varied duty cycle according tothe high or low velocity rotation of the DC hood motor M, in response to the on/off command and the rotational velocity control command from the user. 
     The first transistor TR 1  is on/off driven according to the duty cycle of the PWM control signal received at the base thereof from the microcomputer  105 , and outputs a phase-inverted pulse signal through the collector thereof The pulse signal generated from the collector of the first transistor TR 1  is outputted to the driving circuit section  106  in the form of DC power by the charging operation of the capacitor C 3 . 
     The driving circuit section  106  includes a switching regulator  107 , a second transistor TR 2 , and a resistor R 1 . 
     The switching regulator  107  includes a signal input port DTC connected with the collector of the first transistor TR 1 , and a signal output port OUT for outputting a certain driving pulse signal. The second transistor TR 2  includes a base connected with the signal output port OUT of the switching regulator  107 , a collector connected with one end of the DC hood motor M, and an emitter connected with the rectifier circuit section  100  through the resistor R 1 . 
     The switching regulator  107  receives the DC voltage generated by the capacitor C 3  from the collector of the first transistor TR 1  through the signal input port DTC, and outputs the driving pulse signal of a certain frequency having the duty cycle determined by the voltage level of the DC power. 
     Here, the switching regulator  107  is normally driven by the DC voltage of a certain voltage range, such as 0.7V-3V, inputted through the signal input port DTC. 
     The switching regulator  107  generates the driving pulse signal having a first duty cycle with a short period of on-time and a long period of off-time of second transistor TR 2  when the DC voltage inputted to the signal input port DTC is high voltage such as 3V. While, when the low voltage such as 1V is inputted, the driving pulse signal having a second duty cycle with a long period of on-time and a short period of off-time of second transistor TR 2  is generated. 
     The second transistor TR 2  is on/off driven according to the duty cycle of the driving pulse signal received at the base thereof from the switching regulator  107 . The resistor R 1  is a voltage reducing element for forming potential difference for driving the second transistor TR 2 . 
     The DC hood motor M is rotated at high or low velocity, by the electric current received from the rectifier circuit section  100  in accordance with the on/off driving of the second transistor TR 2 . 
     Further, the overload prevention circuit  108  includes a third transistor TR 3 , voltage dividing resistors R 2  and R 3 , and a capacitor C 2 . 
     The third transistor TR 3  includes a base connected between the second transistor TR 2  of the driving circuit section  106  and one end of the resistor R 1  through the voltage dividing resistors R 2  and R 3 , and an emitter connected with the other end of the resistor R 1 . Further, the third transistor TR 3  includes a collector connected between the collector of the first transistor TR 1  and the signal input port DTC of the switching regulator  07 . 
     The third transistor TR 3  is on/off driven by the voltage at both ends of the resistor R 1  of the driving circuit section  106 , to control the DC voltage applied to the signal input port DTC of the switching regulator  107 . 
     The voltage dividing resistors R 2  and R 3  divide the voltage at both ends of the resistor R 1 , and the third transistor TR 3  is on/off driven by the voltage at the resistor R 3 . The capacitor C 2  is for protecting the third transistor TR 3 . 
     Next, the first preferred embodiment of the present invention constructed as above will be described in greater detail with reference to the waveforms of FIGS. 5,  6 ,  7 , and  8 . 
     First, when the user selects the command for controlling the low velocity while giving the hood motor-on command, as shown in FIG. 5, the microcomputer  105  outputs the PWM control signal a 1  of a certain frequency, such as the frequency of 4 KHz having the first duty cycle with a short period of on-time and a long period of off-time of first transistor TR 1  through the control signal output port PWM. 
     The first transistor TR 1  is on/off driven according to the first duty cycle of the PWM control signal generated from the microcomputer  105 , to generate the phase-inverted pulse signal through the collector thereof Accordingly, the pulse signal generated at the collector of the first transistor TR 1  has the duty cycle which is increased as the first duty cycle thereof is inverted. 
     Meanwhile, the capacitor C 3 , which is connected with the collector of the first transistor TR 1 , generates a certain high voltage b 1  such as the DC voltage of 3V by charging/discharging the pulse signal having the increased duty cycle. 
     When the switching regulator  107  of the driving circuit section  106  receives the DC voltage b 1  of the high voltage discharged by the capacitor C 3  such as the voltage of 3V through the signal output port OUT, the switching regulator  107  outputs a certain high frequency having the first duty cycle corresponding to the high voltage, such as a driving pulse signal c 1  of frequency of 20 KHz. 
     Accordingly, the driving-on time of the second transistor TR 2  is shortened to be shorter than the driving-offtime thereof by the first duty cycle of the driving pulse signal c 1  generated from the switching regulator  107 , so that the DC hood motor M is rotated at low velocity. 
     In such a situation, as there is low voltage d 1  such as the voltage of 1V at both ends of the resistor R 1 , the third transistor TR 3  of the overload prevention circuit  108  is not operated. 
     Meanwhile, when the user inputs his/her command for velocity control of the high rotation of the DC hood motor M, as shown in FIG. 6, the microcomputer  105  outputs a certain PWM control signal a 2  of a high frequency having a second duty cycle with a long period of on-time and a short period of off-time of first transistor TR 1 , such as a PWM control signal of frequency of 4 KHz. 
     At the collector of the first transistor TR 1 , the second duty cycle of the PWM control signal is inverted, generating the pulse signal of the first duty cycle. The capacitor C 3  generates a certain low voltage b 2  such as the DC voltage of 1V, by charging the pulse signal of the first duty cycle. 
     The switching regulator  107  generates a certain driving pulse signal c 2  of high frequency having the second duty cycle such as the driving pulse signal of frequency of 20 KHz, by the low DC voltage d 2  discharged from the capacitor C 3 . 
     Accordingly, due to the driving pulse signal c 2  having the second duty cycle, the driving-on time of the second transistor TR 2  is lengthened to be longer than the driving-offtime thereof, and the DC hood motor M is rotated at high velocity. 
     In such a situation, the third transistor TR 3  of te overload prevention circuit  108  is not operated since the voltage at both ends of the resistor R 1  does not have voltage value enough for the turning-on condition thereof. 
     FIGS. 7 and 8 are waveforms for explaining the operation of the control circuit when the hood motor is abnormally operated. 
     As shown in FIG. 7, while the DC hood motor M is rotated at low velocity, the overvoltage d 10  is generated at both ends of the resistor R 1  when there is a foreign substance in the vent blowers of the DC hood motor M, which causes the constrains on the rotation of the DC hood motor M, and the electric current increase at the resistor R 1 . 
     When there occurs the overvoltage d 10  at both ends of the resistor R 1 , the third transistor TR 3  of the overload prevention circuit  108  is turned on to bypass the DC voltage inputted through the signal input port DTC of the switching regulator  107  toward the rectifier circuit section  100 . 
     Accordingly, as the low voltage e 1  is applied to the signal input port DTC, the switching regulator  107  generates the driving pulse signal f 1  having the second duty cycle through the signal output port OUT. 
     Since the driving pulse signal f 1  has the second duty cycle, the driving-on time of the second transistor TR 2  is lengthened to be longer than the driving-offtime thereof, so that the electric current flows in excess through the rectifier circuit section  100  than the fuse  101  of the rectifier circuit section  100  allows. Accordingly, the fuse  101  is opened. 
     By the opening of the fuse  101 , the operation of the DC hood motor M is stopped, and the possible harm by the overcurrent is prevented. 
     Meanwhile, as shown in FIG. 8, while the DC hood motor M is rotated at high velocity by the user&#39;s selection, the overvoltage d 20  is generated at both ends of the resistor R 1  and the third transistor TR 3  of the overload prevention circuit  108  is turned on if there is a foreign substance constraining the rotation of the DC hood motor M. 
     Accordingly, by the turning-on of the third transistor TR 3 , the low voltage e 2  is applied to the signal input port DTC of the switching regulator  107 , while the driving pulse signal f 2  of the second duty cycle is outputted through the signal output port OUT. 
     Accordingly, since the second duty cycle of the driving pulse signal f 2  has a long period of on-time and a short period of off-time, the driving-on time of the second transistor TR 2  is lengthened to be longer than the driving-offtime thereof, and the fuse  101  is opened since there is overcurrent than the fuse  101  allows. 
     Next, the second preferred embodiment of the present invention will be described in greater detail with reference to the accompanying drawings. 
     FIG. 9 is a view of the driving control circuit of a hood motor according to the second preferred embodiment of the present invention. 
     As shown in FIG. 9, the driving control circuit of the hood motor according to the second preferred embodiment of the present invention includes a rectifier circuit section  200 , a controlling section  204 , a driving circuit section  206 , an overload prevention circuit  208 , and a DC hood motor M. 
     The rectifier circuit section  200  includes a fuse  201 , a bridge diode  202 , and a smoothing capacitor C 4 . 
     The controlling section  204  includes a microcomputer  205 , and a fourth transistor TR 4 . 
     The microcomputer  205  includes an input port for inputting on/off commands and a velocity control command from a user. Further, the microcomputer  205  includes a control signal output port PWM for generating a PWM control signal of a certain frequency having the duty cycle varied according to the high/low velocity rotation of the DC hood motor M in response to the on/off command and the velocity control command from the user, and a ground port GND. 
     Here, the microcomputer  205  generates the PWM control signal of a certain frequency having the second duty cycle such as the frequency of 4 KHz when the low velocity rotation is selected by the user, while the microcomputer  205  generates the PWM control signal of the first duty cycle when the high velocity rotation is selected. 
     The fourth transistor TR 4  includes a base connected with the control signal output port PWM of the microcomputer  205 , and a collector connected with the voltage source Vcc through the resistor R 10 . Further, an emitter of the fourth transistor TR 4  is connected with the driving circuit section  206  through the resistor R 11 . Between the emitter of the fourth transistor TR 4  and the driving circuit section  206 , a capacitor C 6  and a resistor R 9  are parallel connected, respectively. 
     The fourth transistor TR 4  is on/off driven according to the duty cycle of the PWM control signal received at the base thereof from the microcomputer  205 , and outputs the pulse signal of the same phase as the PWM control signal through the emitter. The pulse signal generated at the emitter of the fourth transistor TR 4  is outputted to the driving circuit section  206  in the form of DC voltage by the charging/discharging of the capacitor C 6 . 
     The driving circuit section  206  includes a switching regulator  207 , a fifth transistor TR 5 , and a resistor R 6 . 
     The switching regulator  207  outputs a certain driving pulse signal of acertain frequency having the duty cycle varied according to the level of DC voltage generated by the charging/discharging of the capacitor C 6  through the signal input port DTC, such as the driving pulse signal of frequency of 20 KHz, through the signal output port OUT. 
     Here, if the high DC voltage such as the voltage of 3V is inputted through the signal input port DTC of the switching regulator  207 , the switching regulator  207  generates the driving pulse signal having the first duty cycle, while the switching regulator  207  generates the driving pulse signal having the second duty cycle when the low voltage such as the voltage of 1V is inputted. 
     The fifth transistor TR 5  is on/off driven according to the duty cycle of the driving pulse signal received at the base from the switching regulator  207 . 
     The overload prevention circuit  208  includes a sixth transistor TR 6 , voltage dividing resistors R 7  and R 8 , and a capacitor C 5 . 
     The sixth transistor TR 6  includes a base connected between the fifth transistor TR 5  of the driving circuit section  206  and one end of the resistor R 6  through the voltage dividing resistors R 7  and R 8 , and a collector connected to the other end of the resistor R 6 . An emitter of the sixth transistor TR 6  is connected with the signal output port OUT of the switching regulator  207 . 
     The sixth transistor TR 6  is on/off driven by the voltage at both ends of the resistor R 6  of the driving circuit section  206  to control the driving pulse signal outputted from the signal output port OUT of the switching regulator  207 . 
     Next, the operation of the second preferred embodiment of the present invention will be described in greater detail with reference to FIGS. 10,  11 ,  12 , and  13 . 
     FIGS. 10 and 11 are waveforms for explaining the operation of the control circuit of the hood motor when the hood motor is normally operated. 
     As the user selects the low-velocity rotation while giving the DC hood motor-on command, as shown in FIG. 10, the microcomputer  205  outputs a PWM control signal a 3  of the second duty cycle through the control signal output port PWM. 
     The fourth transistor TR 4  is on/off driven according to the second duty cycle of the PWM control signal a 3  generated from the microcomputer  205 , and generates the pulse signal having the same phase as the PWM control signal a 3  through the emitter thereof Meanwhile, the capacitor C 6  generates a certain high DC voltage b 3  such as the voltage of 3V by charging/discharging the pulse signal having the second duty cycle. 
     The switching regulator  207  of the driving circuit section receives the high DC voltage b 3  discharged by the capacitor C 6  through the signal input port DTC, and outputs a certain driving pulse signal c 3  of high frequency having the first duty cycle, such as the frequency of 20 KHz, through the signal output port OUT. 
     Accordingly, the driving-on time of the fifth transistor TR 5  is lengthened to be longer than the driving-offtime thereof according to the first duty cycle of the driving pulse signal c 3  generated from the switching regulator  207 , so the DC hood motor M is rotated at low velocity. 
     Here, since the voltage d 3  at both ends of the resistor R 6  is low voltage, such as the voltage of 1V, the sixth transistor TR 6  of the overload prevention circuit  208  is not operated. 
     Meanwhile, when the user selects the high velocity rotation, as shown in FIG. 6, the microcomputer  205  outputs a PWM control signal a 4  having the first duty cycle. 
     Accordingly, the fourth transistor TR 4  is on/off driven by the PWM control signal a 4  having the first duty cycle, and the pulse signal having the first duty cycle is generated through the emitter. The capacitor C 6  generates the low DC voltage c 4 , such as the voltage of 1V by charging the pulse signal having the first duty cycle. 
     The switching regulator  207  generates a certain driving pulse signal of high frequency having the second duty cycle by the low DC voltage c 4  discharged from the capacitor C 6 , such as the pulse signal of the frequency of 20 KHz. 
     Accordingly, the driving-on time of the fifth transistor TR 5  is lengthened to be longer than the driving-offtime thereof according to the driving pulse signal d 4  of the second duty cycle, so the DC hood motor M is rotated at high velocity. 
     In such a situation, since the voltage at both ends of the resistor R 6  is not enough for the turning-on condition, the sixth transistor TR 6  of the overload prevention circuit  208  is not operated. 
     FIGS. 12 and 13 are waveforms for explaining the operation of the control circuit of the hood motor when the hood motor is abnormally operated. 
     As shown in FIG. 12, while the DC hood motor M is rotated at low velocity, the presence of a foreign substance causes the constraint on the rotation of the DC hood motor M, the electric current increase at the resistor R 6 , and the overvoltage d 30  at both ends of the resistor R 6 . 
     When there occurs overvoltage at both ends of the resistor R 6 , the sixth transistor TR 6  of the overload prevention circuit  208  is turned on, and the high voltage at both ends of the resistor R 6  is applied to the fifth transistor TR 5 . 
     Accordingly, the fifth transistor TR 5  maintains on-status by a high voltage e 3  at the base thereof, and the large amount of electric current flows through the rectifier circuit section  200 , and the fuse  202  is opened. Here, the high voltage e 3  is the combination of the high voltage from the sixth transistor TR 6  and the driving pulse signal c 3  of the switching regulator  207 . 
     By the opening of the fuse  201 , the DC hood motor M stops operation, and possible harm caused by the overcurrent is prevented. 
     Meanwhile, as shown in FIG. 13, when the user selects high velocity rotation of the DC hood motor M, the sixth transistor TR 6  is turned on by overvoltage d 40  at both ends of the resistor R 1  if there is presence of a foreign substance at the DC hood motor M. 
     By the turning-on of the sixth transistor TR 6 , the fifth transistor TR 5  receives a voltage e 4 , which is the combination of a driving pulse signal c 4  from the switching regulator  207  and a high voltage from the sixth transistor TR 6 . 
     Accordingly, the fifth transistor TR 5  maintains on-status, and the fuse  201  is open since there is overcurrent flowing therethrough. 
     Next, the third preferred embodiment of the present invention will be described in greater detail with reference to the accompanying drawings. 
     FIG. 14 is a view for showing the driving control circuit of a hood motor according to the third preferred embodiment of the present invention. 
     As shown in FIG. 14, the driving control circuit of the hood motor according to the third preferred embodiment of the present invention includes a rectifier circuit section  300 , a controlling section  304 , a driving circuit section  306 , an overload prevention circuit  308 , and a DC hood motor M. 
     The rectifier circuit section  300  includes a bridge diode  301 , and a smoothing capacitor C 7 . Unlike the first and second preferred embodiments, the fuses  101  and  202  are omitted in the third preferred embodiment. 
     The controlling section  304  includes a microcomputer  305 , and a seventh transistor TR 7 . 
     The microcomputer  305  includes an input port for inputting on/off commands and a velocity control command from a user. Further, the microcomputer  305  includes a control signal output port PWM for generating a PWM control signal of a certain frequency having the duty cycle varied according to the high/low velocity rotation of the DC hood motor M in response to the on/off commands and the velocity control command from the user, and a ground port GND. 
     Here, the microcomputer  305  generates the PWM control signal of a certain frequency having the second duty cycle such as the frequency of 4 KHz when the low velocity rotation is selected by the user, while the microcomputer  305  generates the PWM control signal of the first duty cycle when the high velocity rotation is selected. 
     The seventh transistor TR 7  includes a base connected with the control signal output port PWM of the microcomputer  305 , and a collector connected with the voltage source Vcc through the resistor R 18 . The collector of the seventh transistor TR 7  and a resistor R 18  are connected with the driving circuit section  306  through a resistor R 19 . Here, the resistors R 18  and R 19  are voltage dividing resistors for dividing the voltage from the voltage source Vcc. 
     Further, between the seventh transistor TR 7  and the driving circuit section  306 , a capacitor C 9 , and a resistor R 17  are parallel connected, respectively. 
     The seventh transistor TR 7  is on/off driven according to the duty cycle of the PWM control signal received from the microcomputer  205 , and outputs the phase-inverted pulse signal of the PWM control signal through the collector thereof. The pulse signal generated at the collector of the seventh transistor TR 7  is divided by the voltage dividing resistors R 18  and R 19 , charged at the capacitor C 9 , and is outputted to the driving circuit section  306  in the form of DC voltage. 
     The driving circuit section  306  includes a switching regulator  307 , an eighth transistor TR 8 , and a resistor R 12 . 
     The switching regulator  307  outputs a certain driving pulse signal of a certain frequency having the duty cycle varied according to the level of DC voltage generated by the charging/discharging of the capacitor C 9  through the signal input port DTC, such as the driving pulse signal of frequency of 20 KHz, through the signal output port OUT. 
     Here, if the high DC voltage such as the voltage of 3V is inputted through the signal input port DTC of the switching regulator  307 , the switching regulator  307  generates the driving pulse signal having the first duty cycle,while the switching regulator  307  generates the driving pulse signal having the second duty cycle when the low voltage such as the voltage of 1V is inputted. 
     The eighth transistor TR 8  is on/off driven according to the duty cycle of the driving pulse signal received at the base from the switching regulator  307 . 
     The overload prevention circuit  308  includes a ninth transistor TR 9 , voltage dividing resistors R 13 , R 14 , and R 15 , R 16 , and a capacitor C 8 . 
     The ninth transistor TR 9  includes a base connected between the eighth transistor TR 8  of the driving circuit section  306  and one end of the resistor R 12  through the voltage dividing resistors R 13  and R 14 , and a collector connected with the other end of the resistor R 12 . Further, an emitter of the ninth transistor TR 9  is connected with the signal output port OUT of the switching regulator  307  through the voltage dividing resistors R 15  and R 16 . 
     The ninth transistor TR 9  is turned on when there is overvoltage at both ends of the resistor R 12  of the driving circuit section  306 , and disables the operation of the switching regulator  307  by supplying overvoltage at the signal input port DTC of the switching regulator  307 , such as the voltage of 3V. 
     Next, the operation of the third preferred embodiment of the present invention will be described in greater detail with reference to FIGS. 15 and 16. 
     FIGS. 15 and 16 are waveforms for explaining the operation of the control circuit of the hood motor of FIG. 14 when the hood motor is normally operated. 
     First, as the user selects the low-velocity rotation while giving the DC hood motor-on command, as shown in FIG. 15, the microcomputer  205  outputs a PWM control signal as of the first duty cycle. 
     The seventh transistor TR 7  is on/off driven according to the duty cycle of the PWM control signal a 5 . The pulse signal, which is divided from the voltage source Vcc by the voltage dividing resistors R 18  and R 19 , is phase-inverted from the PWM control signal a 5 , and is generated through the collector of the seventh transistor TR 7 . Accordingly, by the inverted first duty cycle of the PWM control signal, the duty cycle of the pulse signal generated at the collector of the seventh transistor TR 7  is increased. 
     Meanwhile, the pulse signal divided by the voltage dividing resistors R 18  and R 19  generates a high DC voltage b 5  such as the voltage of 3V, by the charging/discharging of the capacitor C 9 . 
     When the switching regulator  307  receives the high DC voltage b 5 , the switching regulator  307  outputs a certain driving pulse signal c 5  of high frequency having the first duty cycle, such as the frequency of 20 KHz, corresponding to the high DC voltage b 5 . 
     Accordingly, the driving-on time of the eighth transistor TR 8  is lengthened to be longer than the driving-offtime thereof according to the duty cycle of the driving pulse signal c 5  generated from the switching regulator  307 , the DC hood motor M is rotated at low velocity. 
     Here, when there is a foreign substance in the vent blowers of the DC hood motor M constraining the rotation of the hood motor M, the electric current flowing through the resistor R 12  is increased, and there occurs overvoltage d 50  at both ends of the resistor R 12 . 
     When there occurs overvoltage at both ends of the resistor R 12 , the ninth transistor TR 9  of the overload prevention circuit  308  is turned on, and the high voltage, which is divided from the overvoltage d 50  at both ends of the resistor R 12 , is applied to the signal input port DTC of the switching regulator  307 . 
     Accordingly, supervoltage e 5 , which is the combination of the DC voltage b 5  by the capacitor C 9  and the high voltage divided by the voltage dividing resistors R 15  and R 16 , is applied to the switching regulator  307 . By the supervoltage e 5 , such as 3V higher than the allowed value for the operation of the switching regulator  307 , the operation of the switching regulator  307  is disabled. 
     Accordingly, the driving pulse signal f 5  generated through the signal output port OUT of the switching regulator  307  maintains low level, while the eighth transistor TR 8  maintains off-status. As a result, the operation of the DC hood motor M is stopped. 
     Meanwhile, as shown in FIG. 16, while the DC hood motor M is rotated at high velocity by the selection of the user, overvoltage d 60  occurs at both ends of the resistor R 12  when there is a foreign substance in the vent blowers of the DC hood motor M constraining the rotation of the DC hood motor M. 
     The ninth transistor TR 9  is turned on by the overvoltage d 60  at both ends of the resistor R 12 , while the high voltage, which is divided from the overvoltage d 60  at both ends of the resistor R 12  by the voltage dividing resistors R 15  and R 16 , is applied to the signal input port DTC of the switching regulator  307 . 
     Accordingly, the operation of the switching regulator  307  is disabled by the supervoltage e 6  such as 3V, which is the combination of the DC voltage b 6  of the low voltage by the charging of the capacitor C 9  and the high voltage divided by the voltage dividing resistors R 15  and R 16 . Here, the driving pulse signal f 6  outputted from the signal output port OUT of the switching regulator  307  maintains the low-level, while the eighth transistor TR 8  is turned off to stop the operation of the DC hood motor M. 
     As described above, in the driving control circuit of the hood motor according to the preferred embodiments of the present invention, by the inexpensive control circuit being substituted for the conventional expensive switching circuit, the manufacturing cost of the microwave oven is considerably reduced, while the productivity thereof is increased. Further, by solving the problem of having abnormalities such as the poor contact of the switches, the stable controlling of the motor can be guaranteed. Further, by the overload prevention circuit, the fuse is opened or the operation of the driving circuit is stopped to protect the DC hood motor and the circuit components when there occurs abnormalities in the DC hood motor. 
     While the driving control circuit of the hood motor of the present invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be effected therein without departing from the spirit and scope of the invention as defined by the appended claims.