Relay driving apparatus for microwave oven

Abstract: In a relay driving apparatus and method for a microwave oven, a high relay driving voltage is applied at an initial relay driving time to reduce the operating time deviation between relay parts and the high relay driving voltage is lowered to a normal relay driving voltage through a damp resistance if contact points of a relay switch are coupled, thereby implementing a fix crossing. Therefore, rush current is minimized during relay operation, thereby preventing the fixation of the contact points of a relay switch and the generation of noises due to vibration.

Description: Background of the Invention The present invention relates to a relay driving of a microwave oven, and more particularly, to a relay driving apparatus and method for a microwave oven which can prevent the generation of rush current during relay operation, by implementing a fix crossing by minimizing the operating time deviation between relay parts. As shown in FIG. 1, the relay driving circuit for a conventional microwave oven includes a power supply 100 for supplying power to a system, a microcomputer 200 for receiving the power from power supply 100 and executing an overall control operation of the system, a key input portion 300 for selecting a function of the microwave oven through a user's key manipulation, a display 400 for displaying the selected function and the operation state of microcomputer 200 depending on the selected function, a relay driver 500 for driving a magnetron and a heater in accordance with a control signal of microcomputer 200, a door detector 600 for detecting a door state and outputting the detection result to microcomputer 200 and a peripheral circuit 700 having a buzzer circuit for generating a buzzing sound in accordance with a control signal of microcomputer 200 and an oscillator for supplying a clock signal to microcomputer 200. Relay driver 500 is constituted by a PNP transistor 13 whose base is connected to an output port A2 of microcomputer 200 via resistance 12, whose emitter is connected to a power supply port V2 via door detector 600 and a door switch 10 and is connected to base via a bias resistance 11, for being operated in accordance with the control signal output from microcomputer 200, and a relay 20 whose first side is grounded and whose second side is connected to collector of PNP transistor 13, for being driven in accordance with the operation state of PNP transistor 13. Relay 20 having a relay coil 15 with reverse voltage preventive diodes 14 connected in parallel and a relay switch 16 driven by relay coil 15 switches AC power applied to a magnetron. The operation of the relay driver having the aforementioned configuration will now be described with reference to accompanying drawings. First, as shown in FIG. 1, in a state where door switch 10 is shut, if a user selects a predetermined cooking function of a microwave oven through key input portion 300, microcomputer 200 recognizes the selection to display the information corresponding to the selected cooking function to display 400 and outputs a control signal of a low level to the output port A2 for a cooking function set time to turn PNP transistor 13 on. Thus, the voltage input to power supply port V2 flows toward relay 20 via door switch 10 and PNP transistor 13 so that a voltage V2 is applied to relay coil 15. Contact points of relay switch 16 are coupled by the voltage V2 so that AC power flows, thereby oscillating magnetron to perform the selected cooking function. Thereafter, if a completion time of the set cooking function has been reached, microcomputer 200 outputs a control signal of a high level through the output port A2 to turn PNP transistor 13 off, thereby stopping the operation of relay 20 and the oscillation of magnetron to terminate the cooking function. However, if PNP transistor 13 is turned on by the control signal output from microcomputer 200, as shown in FIG. 2A, contact points of relay switch 16 are coupled by the voltage V2 flowing along relay coil 15 and AC voltage ν and current i flow through relay switch 16, as shown in FIG. 2B. At this time, if the contact points of relay switch 16 are coupled at a point a , the voltage ν flowing therethrough becomes maximum and the current i (rush current) becomes minimum. If the contact points of relay switch 16 are coupled at a point b by the deviation of relay operating time, the voltage ν becomes minimum and the current i (rush current) becomes maximum. If the rush current is maximum, the contact points of the relay switch 16 stick together by the rush current Also, during the relay operation, the vibration due to the rush current results in making noises. GB-A-2150778A discloses an electromagnetic relay arrangement for preventing semi-actuation, wherein said relay requires more electromagnetic power for its initial actuation than to maintain it in the actuated state. Problems caused by overloading a microprocessor which is the only power source, are solved. The overload state is detected by voltage sensing means (L1, L2). Summary of the Invention Therefore, it is an object of the present invention to provide a circuit and method for a microwave oven which can minimize the operating time deviation between relay parts by applying a high relay driving voltage at an initial relay driving time and can prevent the generation of rush current by lowering the voltage through a damper resistance to a normal relay driving voltage. To accomplish the above objects, there is provided a circuit for a microwave oven comprising the features of claim 1 as well as a method for driving a relay of a microwave oven comprising the steps of claim 3. Brief Description of the Drawings The above objects and 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 schematic diagram of a relay driving circuit for a conventional microwave oven; FIG. 2 are waveform diagrams of various parts for driving a relay in FIG. 1; FIG. 3 is a schematic diagram of a relay driving circuit for a microwave oven according to the present invention; FIG. 4 are waveform diagrams of various parts for driving a relay in FIG. 3; FIG. 5 illustrates operating time deviation between relay parts at a normal voltage and a high voltage; FIG. 6 is a flowchart for driving the relay in FIG. 3; FIG. 7 is a relay driving circuit for a microwave oven according to a first embodiment of the present invention; and FIG. 8 is a flowchart for driving the relay in FIG. 7. Detailed Description of the Invention As shown in FIG. 3, the relay driving circuit for a microwave oven according to the present invention includes a power supply 100 for supplying power to the system, a microprocessor 200 for receiving the power from power supply 100 to execute an overall controlling operation of the system, a key input portion 300 for selecting a function of the microwave oven through a user's key manipulation, a display 400 for displaying the selected function and the operation state of microcomputer 200 in accordance with the function, a door detector 600 for detecting a door state and outputting the detection result to microcomputer 200, a peripheral circuit 700 having a buzzer circuit for generating a buzzing sound in accordance with a control signal of microcomputer 200 and an oscillator for supplying a clock signal to microcomputer 200, and a relay driver 800 being operated by the control of microcomputer 200 for driving a magnetron (not shown) and a heater. Relay driver 800 is constituted by a PNP transistor 13 whose base is connected to an output port A2 of microcomputer 200 via resistance 12, whose emitter is connected to a power supply port V2 via door detector 600 and a door switch 10 and is connected to base via a bias resistance 11, for being operated in accordance with the control signal output from microcomputer 200, a NPN transistor 17 whose base is connected to an output port A3 of microcomputer 200 via resistance 19, whose emitter is connected to base via ground port and bias resistance 18 for being operated by the control signal output from microcomputer 200, a relay 20 connected to collectors of PNP transistor 13 and NPN transistor 17 for being driven in accordance with the operation state of PNP transistor 13 and NPN transistor 17, and a damper resistance 21 connected to emitter and collector of PNP transistor 13 for lowering a high voltage flowing in relay 20 to a normal voltage. Relay 20 having a relay coil 15 with reverse voltage preventive diodes 14 connected in parallel and a relay switch 16 driven by relay coil 15 switches AC power applied to magnetron. Here, those parts which are the same as those corresponding parts in the conventional system are designated by the same reference numerals. The operation of the relay driver for a microwave oven according to the present invention having the aforementioned configuration will now be described with reference to FIGS. 3 through 6. First, if a user places food in the microwave oven and shuts the door, the contact points of a door switch 10 are coupled so that a high voltage output from power supply source V2 flows through door switch 10, which is detected by door detector 600 and is output to an input port A1 of microcomputer 200. Thereafter, if the user selects a predetermined cooking function of the microwave oven through key input portion 300, as shown in FIG. 3, microcomputer 200 recognizes the user's selection, displays the information corresponding to the selected function to display 400 and outputs a control signal via output ports A2 and A3 to control relay driver 800, which allows the magnetron to be oscillated to execute the cooking function. In other words, if the user selects a predetermined cooking function through key input portion 300 and manipulates keys, microcomputer 200 recognizes a key input and determines whether or not the input key is a start key (steps 31 and 32). At this time, if the input key is not a start key, the operation corresponding to the key (step 33). If the input key is a start key, a control signal of a low level is output to PNP transistor 13 via output port A2, a control signal of a high level is output to NPN transistor 17 via output port A3, as shown in FIG. 4A, and then time is counted (steps 34 and 35). Subsequently, PNP transistor 13 and NPN transistor 17 are both turned on so that the voltage input to power supply port V2 flows toward relay 20 via PNP transistor 13 and then the high voltage V2 is applied to relay coil 15. (At this time, the high voltage V2 ranges from 15V to 20V.) Therefore, the contact points of relay switch 16 are coupled by the high voltage V2 flowing along relay coil 15 and AC power flows through relay switch 16, thereby oscillating the magnetron to execute the selected predetermined cooking function. At this time, the operating time deviation between relay parts is greater than 2 milliseconds (msec) if a normal relay driving voltage, e.g., 12V, is applied to relay coil 15, and is less than 1 msec if a high relay driving voltage is applied thereto, as shown in FIG. 5. Therefore, when the high relay driving voltage V2 flows along relay coil 15, the operating time deviation between relay parts falls within X±1 msec, as shown in FIG. 4B. Thus, when the contact points of relay switch 16 are coupled, the rush current flowing through relay switch 16 becomes minimum. Thereafter, microcomputer 200 counts the time. If more than 20 msec elapses, the control signal of a high level is output to output ports A2 and A3 to turn PNP transistor 13 off but turns NPN transistor 17 on (step 36). Therefore, the high relay driving voltage (15V~20V) applied to relay coil 15 is lowered by a damper resistance 21 connected between emitter and collector of PNP transistor 13 to maintain a normal relay driving voltage 12V so that the contact points of relay switch 16 are kept to be coupled, thereby proceeding the cooking function for a predetermined time. Then, microcomputer 200 checks whether the cooking termination time is reached (step 38). If not reached, it is checked whether there is an input of a stop key or not (step 39). If there is no input of a stop key, step 37 is repeatedly performed to continuously executing the cooking function. On the other hand, if the cooking termination time is reached or there is an input of a stop key, the control signal of a high or low level is output via output port A2 or A3, respectively to turn PNP transistor 13 and NPN transistor 17 off, thereby turning relay switch 16 off and stopping the oscillation of the magnetron to finally terminate the cooking function (steps 39 and 40). FIG. 7 illustrates the relay driver for a microwave oven according to an embodiment of the present invention, in which a relay driver 900 commonly connects bases of PNP transistor and NPN transistor to the output port A3 through a resistance 19 by removing the output port A2 from relay driver 800 shown in FIG. 3. The collector of PNP transistor 13 is grounded through resistance 22 and condenser 23. The contact points of resistance 22 and condenser 23 are connected to relay 20 and a damper resistance 21. Those parts which are the same as those corresponding parts in the conventional system are designated by the same reference numerals. The operation of the first embodiment of the present invention will now be described with reference to FIGS. 7 and 8. First, as shown in FIG. 7, if the user selects a predetermined cooking function through key input portion 300 and manipulates keys, microcomputer 200 recognizes a key input and determines whether or not the input key is a start key (steps 41 and 42). At this time, if the input key is not a start key, the operation corresponding to the key (step 43). If the input key is a start key, a control signal of a low level is output via output port A3, and then time is counted (steps 44 and 45). Therefore, PNP transistor 13 is turned on and NPN transistor 17 is turned off so that the high voltage (15V∼20V) input to power supply port V2 is charged in condenser 23 via PNP transistor 13 and resistance 22. Thereafter, microcomputer 200 counts the time. If 20 msec elapses, the control signal of a high level is output to output port A3 to turn PNP transistor 13 off but turns NPN transistor 17 on (steps 46 and 47). Therefore, the high relay driving voltage (15V~20V) charged in condenser 23 is discharged in relay coil 15 so that the contact points of relay switch 16 are coupled, thereby oscillating the magnetron to execute the cooking function. Then, the high relay driving voltage (15V~20V) discharged in condenser 23 is lowered by damper resistance 21 to maintain a normal relay driving voltage 12V so that the contact points of relay switch 16 are kept to be coupled. In other words, the high relay driving voltage (15V~20V) is initially applied to relay 20 to reduce the operating time deviation between relay parts. If the contact points of relay switch 16 are coupled, the voltage is damped through damp resistance 21 to maintain the normal relay driving voltage (12V), thereby implementing a fix crossing. Then, microcomputer 200 checks whether the cooking termination time is reached (step 48). If not reached, it is checked whether there is an input of a stop key or not (step 49). If there is no input of a stop key, step 47 is repeatedly performed to continuously executing the cooking function. On the other hand, if the cooking termination time is reached or there is an input of a stop key, the control signal of a low level is output via output port A3 to stop the operation of relay 20, thereby stopping the oscillation of the magnetron to finally terminate the cooking function (step 50). As described above, according to the present invention, a high relay driving voltage is applied at an initial relay driving time to reduce the operating time deviation between relay parts and the high relay driving voltage is lowered to a normal relay driving voltage through a damp resistance if contact points of a relay switch are coupled, thereby implementing a fix crossing. Therefore, rush current is minimized during relay operation, thereby preventing the fixation of the contact points of a relay switch and the generation of noises due to vibration.

Claims: Circuit for a microwave oven, comprising: a power supply (100) for supplying power, a microprocessor (200) for receiving the power from the power supply to execute an overall controlling operation of the circuit, a door detector (600) for detecting a door state, a key input portion (300) for selecting a function of the microwave oven through a user's key manipulation, a display (400) for displaying the selected function and the operation state of the microcomputer in accordance with the function; a magnetron driving relay (20) for driving a magnetron of the microwave oven, the relay (20) including a coil (15); and a relay driving portion (800,900) for reducing the operating time deviation between relay parts of the magnetron driving relay (20), the relay driving portion including a component that applies a high relay driving voltage for a predetermined time at an initial relay driving time in response to a control signal of the microcomputer (200) and the component lowers the high relay driving voltage to a normal relay driving voltage after the initial relay driving time, wherein the relay driving portion includes a PNP transistor (13) and a NPN transistor (17) whose bases are connected to output ports (A3) of the microcomputer (200) via resistances (12,19) for being operated in accordance with the control signal outputted from the microcomputer respectively, the coil (15) of the magnetron driving relay (20) being connected directly or via resistances (22) to collectors of the PNP transistor (13) and NPN transistor (17) for being driven in accordance with the operation state of the PNP transistor and NPN transistor, and a resistance (21) being connected directly or via resistances (22) in parallel with the PNP transistor for lowering a high voltage applied to the relay coil to a normal voltage. Circuit as claimed in claim 1, wherein the component is a condenser (23) which is connected directly or via resistances (22) to the collector of the PNP transistor (17) and to the coil (15) of the magnetron driving relay (20) for charging the high relay driving voltage prior to the initial driving time of the magnetron driving relay and for discharging the high relay driving voltage to the coil of the magnetron driving relay at the initial driving time of the magnetron driving relay. Method for driving a relay (20) of a microwave oven, the method comprising the steps: providing a relay switch (16) which is coupled to a PNP transistor (13) and to an NPN transistor (17) and which has contact points; connecting the contact points of the relay switch (16) by controlling the PNP transistor (13) and the NPN transistor (17) to apply a high driving voltage to the relay 20; controlling the PNP transistor (13) and the NPN transistor (17) in a manner which dampens the high driving voltage applied to the relay if a predetermined time elapses, and maintaining a normal relay driving voltage to execute a cooking function; and operating the PNP transistor (13) and the NPN transistor (17) in a manner which turns off the relay (20) when a cooking termination time has been reached, to terminate the cooking function.

Inventors: KIM TAE WOO, KIM, TAE WOO
Applicants: LG ELECTRONICS INC, LG ELECTRONICS INC.
Application year: 1995
Main classification: H05B6
IPC codes: H05B6