Patent Publication Number: US-6218652-B1

Title: Apparatus for eliminating inrush current of a microwave oven

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 an application for APPARATUS FOR ELIMINATING INRUSH CURRENT OF A MICROWAVE OVEN earlier filed in the Korean Industrial Property Office on the 29th of May 1999 and there duly assigned Ser. Nos. 19599/1999 and 19600/1999. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microwave oven, and more particularly, to an apparatus for eliminating inrush current in an application of initial voltage in a microwave oven. 
     2. Description of the Prior Art 
     As well known, a microwave oven is a device for radiating microwave generated from a magnetron to a foodstuff safely placed in a cooking chamber and cooking the foodstuff by fictional heat generated by an inner molecular movement. This microwave oven is generally classified into two types; an electronic type using a microcomputer and a mechanical type using a timer. 
     FIG. 1 is a disassembled perspective view of a conventional electronic type microwave oven. Referring to FIG. 1, in the aspect of its structure, the conventional microwave oven  10  is roughly composed of a controlling part  20 , a high voltage driving part  30  and a heating part  40 . 
     The controlling part  20  has a plurality of buttons  21  for determining the cooking time and the cooking power in the microwave oven, and the plurality of buttons  21  are connected to a printed circuit board (PCB)  6  on the rear side of which a microcomputer  9  and various components are built-in. The microcomputer  9  of the PCB  6  performs the cooking mode by turning on/off the driving status of the microwave oven according to the time and power determined by the respective buttons  21 . Further, the controlling part  20  includes a display window  24  for displaying various kinds of cooking times and cooking statuses, and a door lever  25 , which will be described later, for opening and closing a door  43 . 
     The high voltage driving part  30  consists of a magnetron MGT for generating microwave, a waveguide (not shown) for guiding the microwave generated from the magnetron MGT to a cooking chamber  41 , a high voltage transformer HVT for generating high voltage and pre-heating voltage needed for the driving of the magnetron MGT, a cooling fan  3  for cooling the high voltage transformer HVT, a high voltage capacitor HVC, a high voltage diode HVD, and the magnetron MGT, and a lamp  2  for lighting the cooking chamber  41 , etc. 
     The heating part  40  consists of the cooking chamber  41  as a closed space, a rotary tray  42  on which foodstuffs may be securely placed in the cooking chamber  41 , and a door  43  for opening and closing the cooking chamber  41 . 
     FIG. 2 shows the circuit diagram of FIG.  1 . As shown in FIG. 2, first and second power lines L 1 , L 2  for receiving general alternating current, AC 220V, are connected with both ends of a primary coil  7   a  of the high voltage transformer HVT via a noise filter  1 . This noise filter  1  is composed of a main fuse FUSE  1 , capacitors C 1  to C 3 , an inductor L, a resistor R 1 , etc. A detailed description about operations of the noise filter is herein omitted since it is the same as a general noise filter for cutting off high frequency (HF) transferred to the outside through the power lines L 1  and L 2 . 
     The first power line L 1  is connected in series with a magnetron temperature sensor TCO 1  which is turned on/off according to the temperature of the magnetron MGT to prevent overheating of the magnetron, a first door switch SW 1  which is turned on/off according to the opening or closing status of the door  43 , and a monitor switch SW MT  for monitoring the opening or the closing state of the door  43 . 
     The second power line L 2  is connected in series with a cooking chamber temperature sensor TCO 2  which is turned on/off according to the temperature of the cooking chamber  41  to prevent the overheating of the cooking chamber  41 , and a power relay RY 3  for turning on/off main power. The power relay RY 3  is also connected in parallel with an inrush relay RY 2  for eliminating inrush current. The inrush switch SW IR  is connected to a resistor R 2  for restricting the current, wherein the resistor R 2  for current restriction is generally a cement resistor having a high resistance capability. 
     The power relay RY 3  and the inrush relay RY 2  constitute respective relay elements, together with individual coils, respectively. The power relay RY 3  and the inrush relay RY 2  are turned on/off by the respective relay coil which is disposed on the PCB  6  and controlled by the microcomputer  9 . 
     At this time, the inrush relay RY 2  is firstly turned on by a control of the microcomputer  9 , and after a given time period, e.g., 20-40 ms, the power relay RY 3  is operated. Therefore, after an application of initial power, the inrush current of high reverse-electromotive force generated by the high voltage transformer HVT is consumed by the current restriction resistor R 2  and the inrush relay RY 2 , and then, the power relay RY 3  is turned on, to thereby prevent an overload of each driving circuit and damage to the microwave oven. 
     For the operation of the microwave, the microcomputer  9  turns on the main relay RY 4 , the inrush relay RY 2  and the power relay RY 3  through the respective coils arranged on the PCB  6 . Accordingly, the main relay RY 4  is turned on, and the lamp  2 , the fan motor  3  and the driving motor  4  of the microwave oven are operated. Further, as the inrush relay RY 2  and the power relay RY 3  are sequentially turned on, voltage is generated by the high voltage driving part  30 , and the magnetron MGT of the high voltage driving part  30  generates the microwave for cooking foodstuff placed in the cooking chamber  41 . 
     Meanwhile, the heater is operated by the same operation of the above components, except that the microcomputer  9  turns off the inrush relay RY 2  and the power relay RY 3 , while turning on the heater relay RY 1 , to operate the heater  8  instead of the magnetron MGT so that the grill cooking is performed. 
     FIG. 3 is a disassembled perspective view of a conventional mechanical type microwave oven. In FIG. 3, in its structural aspect, a conventional microwave oven  10  may be roughly divided into a controlling part  20 ′, a high voltage driving part  30 , and a heating part  40 , which is the same as in the above-mentioned electronic type. The difference of the mechanical type microwave oven from the electronics type is that the controlling part  20 ′ is composed of respective time control knob  21  and power control knob  22  for determining each of the cooking time and cooking power of the microwave oven. Each of knobs  21  and  22  is connected to a timer  23  placed on its rear side. Due to the timer  23  which turns on or off the driving status of the microwave oven according to the time and the power determined by each of the knobs  21  and  22 , the microwave oven performs a cooking operation properly. Further, the controlling part  20 ′ also includes a display window  24  for displaying various cooking times and statuses, and a door lever  25  for opening or closing the door  43 , which will be described later. 
     FIG. 4 is the circuit diagram of FIG.  3 . As shown in FIG. 4, the first and second power lines L 1  and L 2  for receiving the general alternating current, AC 220V, are connected with both ends of the primary coil  7   a  of the high voltage transformer HVT via the noise filter  1 . The noise filter  1  is composed of the main fuse FUSE  1 , the capacitors C 1  to C 3 , the inductor L, the resistor R 1 , etc. The detailed description about operations of the noise filter will be omitted since it is the same as a general noise filter used for cutting off the high frequency (HF) transferred to the outside through the power lines L 1  and L 2 . 
     The first power line L 1  is connected in series with the magnetron temperature sensor TCO 1  which is turned on/off according to the temperature of the magnetron MGT to prevent overheating of the magnetron, the first door switch SW 1  which is turned on/off according to the opening or closing status of the door, a power controlling switch SW VP  for controlling the power of the microwave, and the monitor switch SW MT  for monitoring the opening or the closing state of the door  43 . 
     The second power line L 2  is connected in series with the cooking chamber temperature sensor TCO 2  which is turned on/off according to the temperature of the cooking chamber  41  to prevent the overheating of the cooking chamber  41 , the second door switch SW 2  which is turned on/off according to the opening or closing status of the door, a time switch SW TM  for controlling the cooking time of cooking, and the inrush switch SW IR  for removing the inrush current. 
     The other side of the monitor switch SW MT , one side of which is connected to the primary coil  7   a  of the high voltage transformer HVT, is connected to the second door switch SW 2  through the monitor fuse FUSE 2 . The monitor switch SW MT  is also connected in parallel with the inrush coil  6   a  for turning on/off the inrush switch SW IR . The inrush switch SW IR  and the inrush coil  6   a  are constructed in one relay  6 , and it does not operate immediately upon application of outside voltage, but operates after a lapse of a time period, e.g., 20-40 ms, based on its operating characteristic. The inrush switch SW IR  is also connected in parallel with the resistor R 2  for a current restriction, wherein the resistor R 2  for current restriction is generally a cement resistor having high resistance capability. 
     Thus, when power is applied to the inrush coil  6   b  in an on-state of the monitor switch SW MT , the relay  6  turns on the inrush switch SW IR  after a lapse of the time period based on its operational characteristic described above. Accordingly, immediately after an application of the initial power, the inrush current of a high reverse-electromotive force generated by the high voltage transformer HVT is consumed by the current restriction resistor R 2 , and then, the inrush switch SW IR  is turned on, to thereby prevent overloading each driving circuit and any damage to the microwave oven. 
     The lamp  2 , the fan motor  3 , the driving motor  4  and a timer motor  5  are connected between the first power line L 1  and the second power line L 2 . The lamp  2 , the fan motor  3  and the timer motor  5  are driven generally by the power source voltage, AC 220V, and the driving motor  4  is driven by the low voltage, about AC 21V, supplied from one coil of the fan motor  3 . 
     The time switch SW TM , the power controlling switch SW VP  and timer motor  5  are housed in one body inside the timer  23 . 
     As mentioned above, conventional microwave ovens of the electronic and mechanical types are very useful instruments, which accurately drive the high voltage driving part  30  in the time a user wants for respective foodstuff to be cooked and eliminates the inrush current upon an application of the initial voltage. 
     However, as mentioned above, the conventional electronic type microwave oven requires a plurality of expensive relays to eliminate any inrush current, and also requires some complicated microcomputer control technique to connect the individual relays in due sequence. Further, the overall manufacturing cost for the electronic type microwave oven and the mechanical type microwave oven increases due to an expensive relay which is required for an effective operation of the timer, and due to the very complicated relay installation and wiring processes during the manufacturing process. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above problems, and it is an object of the present invention to provide an apparatus for eliminating the inrush current of a microwave oven, in which the inrush current may be effectively eliminated with a simple construction to thereby reduce manufacturing costs of the microwave oven and simplify its manufacturing process. 
     To achieve these and other advantages, and in accordance with the purpose of the present invention as embodied and broadly described, in a microwave oven including a high voltage driving part which has a high voltage transformer, an apparatus for eliminating inrush current of the microwave oven includes a switching unit for applying power source voltage to the high voltage driving part in which, when switched on, an operational contact is firstly connected to a first contact coupled with a PTC thermistor at one end of the first contact, and, after a given time period, is secondly connected to a second contact, and a driving unit for turning on/off the switching unit by generating a driving force. 
     The switching unit consists of a first fixation plate one end of which is fixed and connected to a first terminal via a PTC thermistor and at the other end of which the first contact is formed; a second fixation plate one end of which is fixed and connected to the fist terminal and at the other end of which the second contact is formed at a given distance from the first contact; and an operating plate one end of which is fixed and has a second terminal, and at the other end of which an operating contact is formed to be sequentially connected to the first and the second contacts by the driving force of the driving unit. 
     The driving unit is a relay operating part for operating the switching unit by a magnetic force generated by a control of the microcomputer. 
     The relay operating part includes a relay coil for generating the magnetic force due to electric current turned on by the microcomputer; and a push member for pushing the operating plate by being rotated with respect to a hinge by the magnetic force of the relay coil. 
     The driving unit is a cam for operating the switching unit by the driving force of the timer motor. 
     The cam includes a rotary plate having a groove at one side thereof, which is rotated by the driving force of the timer motor. The operating plate includes a projection part inserted into the groove at an initial position of the timer motor wherein the projection part is pushed by the circumferential side of the rotary plate when the rotary plate is rotated out of the initial position. 
     The distance between the operational contact and the second contact is provided as such that the operational contact is firstly in contact with the first contact and secondly with the second contact after about 20-40 ms. 
     It is preferable that the distance between the operational contact and the second contact is 2-4 mm farther than the distance between the operational contact and the first contact. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and other advantages of the invention will be realized and attained by the structure as illustrated in the written description and claims hereof, as well as the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 shows a disassembled perspective view of a conventional electronic type microwave oven; 
     FIG. 2 is a circuit diagram of FIG. 1; 
     FIG. 3 is a disassembled perspective view of a conventional mechanical type microwave oven; 
     FIG. 4 is a circuit diagram of FIG. 3; 
     FIG. 5 is a circuit diagram of an apparatus for eliminating inrush current of a microwave oven according to an embodiment of the present invention; 
     FIGS. 6 to  8  are views for showing detailed operational status of a switching unit and a driving unit shown in FIG. 5; 
     FIG. 9 is a circuit diagram of an apparatus for eliminating the inrush current of a mechanical type microwave oven according to an embodiment of the present invention; and 
     FIGS. 10 to  12  are views for showing detailed operational status of a switching unit and a driving unit shown in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanied drawings. 
     In the invention, the same reference numerals are applied to the same components throughout the description. 
     FIG. 5 is a circuit diagram for showing an apparatus for eliminating the inrush current of an electronic type microwave oven according to an embodiment of the present invention. 
     As shown in FIG. 5, first and second power lines L 1  and L 2  for receiving general alternating current voltage of AC 220V, are connected with both ends of a primary coil  7   a  of the high voltage transformer HVT via a noise filter. The first power line L 1  is connected in series with a magnetron temperature sensor TCO 1  which is turned on/off according to the temperature of the magnetron MGT to prevent any overheating of the magnetron MGT, a first door switch SW 1  which is turned on/off according to an opening or a closing state of the door  43 , and a monitor switch SW MT  for monitoring the opening or the closing state of the door  43 . 
     The second power line L 2  is connected to a cooking chamber temperature sensor TCO 2 , which is turned on/off according to the temperature of the cooking chamber  41 , to prevent the overheating of the cooking chamber  41 . 
     A heater  8  is connected between the first power line L 1  and the second power line L 2  before the noise filter  1 , and is also connected in series with a heater relay RY 1  for turning on/off the heater  8  and a heater temperature sensor TCO 3  for preventing the overheating of the heater  8 . The lamp  2 , a fan motor  3 , a driving motor  4 , and a low voltage transformer LVT are connected between the first power line L 1  and the second power line L 2  and placed on the rear stage of the noise filter  1 , and a main relay RY 4  for ting on/off the lamp  2 , the fan motor  3  and the driving motor  4 , is also connected therebetween. The low voltage transformer LVT lowers the power source voltage, and then supplies the direct current (DC) voltage to the PCB  6  through a rectification unit (not shown). 
     Various circuit elements driven by the DC voltage are disposed together with the microcomputer  9  shown in FIG. 1, on the PCB  6 . The heater relay RY 1  and the main relay RY 4  constitute individual relay elements with individual coils, and are turned on/off by each relay coil disposed on the PCB  6  and controlled by the microcomputer  9  (See FIG.  1 ). 
     The other side of the monitor switch SW MT , one side of which is connected to the primary coil  7   a  of the high voltage transformer HVT, is connected to the cooking chamber temperature sensor TCO 2  via one side of the low voltage transformer LVT and a monitor fuse FUSE 2 . 
     The monitor fuse FUSE 2  is placed on the PCB  6 , on which a second door switch SW 2  is also arranged to be turned on/off according to an opening or a closing status of the door  43 . The second door switch SW 2  transfers the opening or the closing status of the door  43  to the microcomputer  9 . 
     Since above construction is the same as that explained by the prior art of FIGS. 1 and 2, its detailed description will be omitted. 
     Now, the main principal characteristic of the present invention will be described. A switching unit  100  for controlling the application status of power voltage and also for eliminating the inrush current, is placed between the power line L 2  connected to the cooking chamber temperature sensor TCO 2  and the primary coil  7   c  of the high voltage transformer HVT. 
     The switching unit  100  is firstly connected, when switched on, to the first contact c at one end of which the operational contacts are connected to the PTC thermistor PTC, and is secondly contacted with the second contact after a given time period, to thereby apply the power source voltage to the high voltage driving part  30 . Further, the switching unit  100  is turned on/off by a driving unit  120  which will be described later. 
     FIGS. 6 to  8  show the operation statuses for the driving unit and the switching unit according to an embodiment of the present invention in detail, including such a PTC thermistor. 
     The positive temperature coefficient thermistor (PTC thermistor) means a positive characteristic thermistor remarkably increasing its resistance value according to the rise in temperature, different from that of a general negative characteristic thermistor. 
     As shown in the FIGS. 6 to  8 , the switching unit  100  is composed of a first fixation plate  113  one end of which is fixed and connected to a first terminal A via the PTC thermistor PTC and at the other end of which the first contact c is formed; a second fixation plate  114  one end of which is fixed and connected to the first terminal A and at the other end of which the second contact d is formed in a given distance from the first contact c; and an operating plate  115  one end of which is fixed, at the one end of which the second terminal B is formed, and at the other end of which the operational contacts a and b are formed which are sequentially in contact with the first contact c and the second contact d by the driving force of the driving unit  120 . 
     The driving unit  120  is a relay operating part for operating the switching unit  100  by a magnetic force generated by a control of the microcomputer  9 . The relay operating part includes a relay coil  123  for generating the magnetic force by electric current under the control of the microcomputer  9 , and a push member  121  which is rotated with respect to a hinge part  122  by the magnetic force of the relay coil  123  to press the operating plate  115 . 
     The operation of the electronic microwave oven with such construction according to an embodiment of the present invention is described in detail as follows: 
     Foodstuff is safely placed in a cooking chamber  41  and a door  43  is closed. First and second door switches SW 1  and SW 2  and a monitor switch SW MT  are turned on, so that the microwave oven becomes ready for operation. 
     Under the above state, when a user presses an operational button  21  for the sake of the execution of a cooking function of the microwave oven, the microcomputer  9  turns on the main relay RY 4  through each coil arranged on the PCB  6 , and also turns on the switching unit  100  by operating the driving unit  120 . 
     Therefore, a contact of the main relay RY 4  is turned on, and the lamp  2 , the fan motor  3  and the driving motor  4  of the microwave oven are operated. Further, immediately after an application of initial electric power, the inrush current due to a high reverse-electromotive force generated by the primary coil  7   a  of the high voltage transformer HVT is consumed by the PTC thermistor PTC, and then the microwave oven operates normally. 
     In greater detail, as shown in FIGS. 6 to  8 , the second terminal B of the operating plate  115  in the switching unit  100  is connected to an input side of the power line L 2 . The first terminal A of the first and second fixation plates  113  and  114  is connected to the power line L 2  of one side of the primary coil  7   a  of the high voltage transformer HVT. Further, the first contact c of the first fixation plate  113  and the second contact d of the second fixation plate  114  are kept at a different distance from the operational contacts a and b of the operating plate  115 . More specifically, the second contact d is placed about 2˜4 mm farther from the operational contacts a and b than the first contact c. 
     In this state, as electric current flows into the relay coil  123  of the driving unit  120  by the microcomputer  9 , the magnetic force is generated around the relay coil  123 . The push member  122  is rotated with respect to the hinge part  121  by the magnetic force. Accordingly, the operating plate  115  moves toward the first and second fixation plates  113  and  114 , and the operational contacts a and b come in contact with the first contact c and with the second contact d, sequentially. 
     Accordingly, immediately after an application of the initial electric power, the inrush current of a high reverse-electromotive force generated in the primary coil  7   a  of the high voltage transformer HVT is eliminated through the PTC thermistor PTC positioned on one side of the first contact c. Then the second contact d comes in contact-state after the lapse of a given time period, e.g., 20-40 ms due to difference of distance of 2-4 mm between the respective contacts, to thereby prevent any damage to the microwave oven and an overload of each driving circuit. 
     Then, normal driving power is provided to the microwave oven by the above operations, and the lamp  2 , the fan motor  3  and the driving motor  4  are operated as described above. The high voltage transformer HVT also generates high voltage, and the high voltage is supplied to the magnetron MGT through each of the high voltage capacitor HVC and the high voltage diode HVD. Accordingly,the magnetron MGT is driven to generate the microwave to cook any foodstuff placed inside the cooking chamber  41 . 
     FIG. 9 is a circuit for showing an apparatus for eliminating the inrush current of the mechanical type microwave oven according to an embodiment of the present invention. 
     As shown in FIG. 9, the first and second electric power lines L 1  and L 2  for receiving general alternating current of AC 220V, are connected with both ends of a primary coil  7   a  of the high voltage transformer HVT via a noise filter  1 . The first electric power line L 1  is connected in series with a magnetron temperature sensor TCO 1  which is turned on/off according to a temperature of the magnetron MGT to prevent any overheating of the magnetron MGT, a first door switch SW 1  which is turned on/off according to the opening or closing status of the door  43 , a power controlling switch SW VP  for controlling a power of the microwave, and a monitor switch SW MT . Further, the second electric power line L 2  is connected to a cooking chamber temperature sensor TCO 2  which is turned on/off according to the temperature of the cooking chamber  41  to prevent the overheating of the cooking chamber  41 , and a second door switch SW 2  which is turned on/off according to the opening or closing status of the door. The lamp  2 , a fan motor  3 , a driving motor  4  and a timer motor  5  are connected between the first electric power line L 1  and the second electric power line L 2 . 
     The above construction is the same as that explained by the prior art in FIGS. 3 and 4, thus its detailed description will be omitted. 
     Now, an inventive principal characteristic of the present invention will be described. The switching unit  100  is disposed to control an application state of power source voltage and to eliminate the inrush current, between the electric power line L 2  connected to the cooking chamber temperature sensor TCO 2  and the primary coil  7   c  of the high voltage transformer HVT. When the switching unit  100  is switched on, the operational contacts a and b firstly come in contact with the first contact c to which the PTC thermistor PTC is connected, and then after a given time period, in contact with the second contact d, to thereby apply the electric power voltage to the high voltage driving part  30 . Further, the switching unit  100  is turned on/off by the driving unit  120  which will be described later. 
     FIGS. 10 to  12  show in detail the structure and its operations for the time switch according to an embodiment of the present invention, including such PTC thermistor. 
     As shown in FIGS. 10 to  12 , the switching unit  100  is composed of a first fixation plate  113  one end of which is fixed and connected to a first terminal A via the PTC thermistor PTC and at the other end of which the first contact c is formed; a second fixation plate  114  one end of which is fixed and connected to the first terminal A and at the other end of which the second contact d is formed at a given distance from the first contact c; and an operating plate  115  one end of which is fixed, at the one end of which the second terminal B is formed, and at the other end of which the operational contacts a and b are formed which are sequentially in contact with the first contact c and the second contact d by the driving force of the driving unit  120 . 
     Further, the driving unit  120  is the cam for operating the switching unit  100  by the driving force of the timer motor  5 . The cam includes a rotary plate  124  having a groove  125  formed on one side thereof, which is rotated by the driving force of the timer motor  5 . The operating plate  115  includes a projection part  116  inserted into the groove  125  of the rotary plate  124  on an initial position of the timer motor  5  wherein the projection part  116  is pushed by the circumferential side of the rotary plate  124  when the rotary plate  124  is rotated out of the initial position. 
     The distance between the operational contacts a and b and the second contact d is provided as such that the operational contacts a and b are in contact with the first contact c and then with the second contact d after about 20-40 ms. 
     It is preferable that the respective distances from the operational contacts a and b to the second contact d are 2-4 mm farther than the respective distances from the operational contacts a and b to the first contact c. 
     In one embodiment of the present invention, the operation of the mechanical microwave oven with such a construction is described in detail as follows: Foodstuff is securely placed in a cooking chamber  41 , then a door  43  is closed. First and second door switches SW 1  and SW 2 , and a monitor switch SW MT  are turned on, thus the microwave oven becomes ready for operation. In this state, as the user manipulates a time control knob  21  and a power control knob  22 , the timer motor  5  is driven by its own rotating force. The timer motor  5  is automatically driven by the user&#39;s manipulation of the time control knob  21  and the power control knob  22 , to thereby operate the switching unit  100  and the power controlling switch SW VP  for a given period of time. 
     That is, in the switching unit  100 , immediately after an application of the initial electric power, the inrush current of a high reverse-electromotive force generated in the primary coil  7   a  of the high voltage transformer HVT is eliminated through the PTC thermistor PTC, so that the microwave oven operates normally. 
     Describing in greater detail, as shown in the drawings, the second terminal B of the operating plate  115  in the switching unit  100  is connected to the electric power line L 2  of the high voltage driving part  30  through the second door switch SW 2 . Further, the first terminal A of the first and second fixation plates  113  and  114  is connected to the electric power line L 2  at an input side of the power voltage, through the cooking chamber temperature sensor TCO 2 . The first contact c of the first fixation plate  113  and the second contact d of the second fixation plate  114  are kept at a different distance from the operational contacts a and b of the operating plate  115 . More specifically, the second contact d is placed 2-4 mm farther away from the operational contacts a and b than that of the first contact c. 
     Under this state, the rotary plate  124  of the driving unit  120  is rotated by the timer motor  5 , and the groove  125  of the rotary plate  124  moves in a rotating direction by a rotating force. On its initial position, the projection part  116 , which is inserted into the groove  125 , moves along a slant face of the groove  125  projection part, and then comes in contact with the outer circumference face of the rotary plate  124 . That is, the operating plate  115  moves to the side of the first and second fixation plates  113  and  114 , and the operational contacts a and b sequentially come in contact with the first contact c and the second contact d. 
     Accordingly, immediately after an application of the initial electric power, the inrush current of a high reverse-electromotive force generated in the primary coil  7   a  of the high voltage transformer HVT is eliminated through the PTC thermistor PTC positioned on one side of the first contact c. Then the second contact d comes in contact-state after the lapse of a given time period, e.g., 20-40 ms due to difference of distance of 2-4 mm between the respective contacts, to thereby prevent any damage to the microwave oven and an overload of each driving circuit. 
     Further, the normal driving electric power is provided to the microwave oven by the above operations, and the lamp  2 , the fan motor  3  and the driving motor  4 , which are connected between the first electric power line L 1  and the second electric power line L 2 , are operated. The high voltage transformer HVT also generates high voltage, and this high voltage is supplied to the magnetron MGT through the high voltage capacitor HVC and the high voltage diode HVD. The magnetron MGT is driven to generate a microwave so as to cook the foodstuff placed inside the cooking chamber  41 . 
     Accordingly, with the apparatus for eliminating the inrush current of the microwave oven according to an embodiment of the present invention, there is no need to install a plurality of cost ineffective relays for the electronic type microwave oven, and there is also no need to install cost ineffective relay for eliminating the inrush current for the mechanical type microwave oven. That is, since the switching unit and driving unit according to the present invention may be simply installed to effectively eliminate the inrush current, the assembly and work efficiencies are improved according to a simpler wiring process. Also, the total manufacturing cost for the microwave oven is reduced. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the inrush current eliminating) apparatus of the microwave oven of the present invention without deviating from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.