Microwave oven providing a security to the high-voltage-transformer

Abstract: A microwave oven has a power supply that provides protection against an initial overcurrent condition. The overcurrent protection is provided by a relay (21) in parallel with a resistor (23). The relay (21) is initially open so that the current supplied to the oven's high-voltage transformer (9) is initially limited by the resistor (23). The relay (21) closes when the current in its coil (17) has increased sufficiently. The mains applied to the oven is reduced, rectified and smoothed before being applied to the coil (17).

Description: The present invention relates to a microwave oven including a high-voltage transformer and overcurrent protection means for protecting the high-voltage transformer from an initial overcurrent condition, the overcurrent protection means comprising a resistor in series between a power in terminal and a primary winding of the high-voltage transformer and a relay in parallel with the resistor. Figure 2 is a circuit diagram of a known power control circuit of a microwave oven. As illustrated, the microwave oven has a power supply part 101, a high-voltage transformer 109 for stepping up the normal voltage from the power supply part 101 to a high level and a magnetron 110 for generating microwaves which is powered from the output of high-voltage transformer 101. In the power supply control circuit, a thermal switch 103 responsive to the temperature in the cooking chamber (not shown), a door switch 105 responsive to opening and closing of the oven door (not shown) and a monitor switch 107 for controlling supply of power to said high-voltage transformer 109 on the basis of the states of the thermal and door switches 103, 105 are connected between the power supply part 101 and the high-voltage transformer 109. The thermal switch 103 is disposed between the power supply part 101 and the monitor switch 107, and the door switch 105 is disposed between the power supply part 101 and the high-voltage transformer 109. An overcurrent protection relay 121 connected in series between the door switch 105 and the high-voltage transformer 109. An overcurrent protection resistance unit 123 is connected in parallel with the overcurrent protection relay 121. The coil 115 of the relay 121 is connected in parallel with the monitor switch 107. The monitor switch 107 has a bypass connecting node 131 and a branch connecting node 133. The bypass connecting node 131 is a point on a bypass line 130 disposed between the relay coil 115 and the high-voltage transformer 109, for supplying power from said power supply part 101 to the bypass line 130. The branch connecting node 133 is a point on the line connected to the high-voltage transformer 109 for supplying power to the relay coil 115 and the high-voltage transformer 109. With this configuration, during normal operation, the thermal and door switches 103, 105 are closed. The moving contact of the monitor switch 107 contacts the branch connecting node 133 and the power supply part 101 is connected in series with the overcurrent protection resistance unit 123 and the high voltage transformer 109. Therefore, the initial current supplied from said power supply part 101 is reduced by the overcurrent protection resistance unit 123. In addition, the relay coil 115 is energised and the overcurrent protection relay 121 is closed. A similar arrangement is known from US-A-4556773. As described above, in the known power control device for a microwave oven, an overcurrent protection relay is operated when AC power is applied. However, the AC power changes regularly and periodically. Therefore, the known microwave oven has a shortcoming in that the operation of the overcurrent protection relay becomes unstable. A microwave oven according to the present invention is characterised by rectifying means for rectifying current input to the oven for energising the coil of the relay. Preferably, an oven according to the present invention includes smoothing means for smoothing the output of the rectifying circuit. More preferably, the rectifying means comprises a bridge rectifier and the smoothing means comprises a capacitor in parallel with said coil. Preferably, resistor means is included for reducing the voltage of the current to be rectified by the rectifying means. Preferably, there is included a monitor switch having a first state in which a power supply line is disconnected from the high-voltage transformer and connected to a by-pass line which by-passes the rectifying means and a second state in which said power supply line is connected to the high-voltage transformer and disconnected from said by-pass line An embodiment of the present invention will now be described, by way of example, with reference to Figure 1 of the accompanying drawings, in which:- Figure 1 is a circuit diagram of a power control device for a microwave oven according to the present invention; and Figure 2 is a circuit diagram of a known power control device for a microwave oven. Referring to Figure 1, a microwave oven has a power supply part 1, a magnetron 10 for generating microwaves and a high-voltage transformer 9 for stepping up mains voltage to a high level. In the power supply control circuit, a thermal switch 3 responsive to the temperature in the cooking chamber (not shown) and a door switch 5 responsive to opening and closing of the oven door (not shown) and a monitor switch 7 for controlling the supply of power to the high-voltage transformer 9 on the basis of the states of the thermal and door switches 3, 5 are connected between said power supply part 1 and the high-voltage transformer 9. The thermal and door switches 3, 5 are connected in series between the power supply part 1 and the high-voltage transformer 9. The power control device further comprises an overcurrent protection relay 21 disposed in series between the thermal and door switches 3, 5, an overcurrent protection resistance unit 23 connected in parallel with the overcurrent protection relay 21, and a rectifying part 8 for rectifying the current from the power supply part 1. The rectifying part 8 has at least one voltage-controlled resistor 13, a bridge diode 11 connected to said voltage-controlled resistor 13, a capacitor 17 connected in parallel with the bridge diode 11, and the coil 15 of the overcurrent protection relay 21. The monitor switch 7 has a bypass connecting node 31 and a branch connecting node 33. The bypass connecting node 31 is a point on a bypass line 30 disposed between the relay coil 15 and the high-voltage transformer 9. The branch connecting node 3 is a point on the line connected to the high-voltage transformer 9 for supplying power to the relay coil 15 and the high-voltage transformer 9. With this configuration, during normal operation, the thermal and door switches 3, 5 are closed. The moving contact of the monitor switch 7 contacts the branch connecting node 33. Because the current supplied from the power supply part 1 passes through the overcurrent protection resistance unit 23, the initial current is reduced and at the same time, the current branched to said rectifying part 8 is rectified so that stable current is supplied to the relay coil 15. This causes the overcurrent protection relay 21 to close shortly after current begins to flow through the primary winding of the high-voltage transformer 9. Accordingly, the high-voltage transformer is protected from an initial overcurrent condition and the overcurrent protection relay is controlled by induction energy generated from the relay coil by use of a stable current. As described above, the inrush relay is controlled by a stable current, and thus, the reliability is improved.

Claims: A microwave oven including a high-voltage transformer (9) and overcurrent protection means (8, 21, 23) for protecting the high-voltage transformer from an initial overcurrent condition, the overcurrent protection means comprising a resistor (23) in series between a power in terminal and a primary winding of the high-voltage transformer (9) and a relay (21) in parallel with the resistor (23), characterised by rectifying means (11) for rectifying current input to the oven for energising the coil (15) of the relay (21). An oven according to claim 1, including smoothing means (17) for smoothing the output of the rectifying circuit (11). An oven according to claim 2, wherein the rectifying means (11) comprises a bridge rectifier (11) and the smoothing means (17) comprises a capacitor (17) in parallel with said coil. An oven according to claim 1, 2 or 3, including resistor means (13) for reducing the voltage of the current to be rectified by the rectifying means (8). An oven according to any preceding claim, including a monitor switch (7) having a first state in which a power supply line is disconnected from the high-voltage transformer (9) and connected to a by-pass line (30) which by-passes the rectifying means (11) and a second state in which said power supply line is connected to the high-voltage transformer (9) and disconnected from said by-pass line (30).

Inventors: KO YOUNG CHEOL, KO, YOUNG CHEOL
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1999
Main classification: H05B6
IPC codes: H05B6