GB1442181A - Electric supply circuits - Google Patents

Abstract

1442181 Supply circuits for microwave ovens HUSQVARNA VAPENFABRIKS AB 24 Sept 1973 [27 Sept 1972] 55653/73 Heading H2H A supply circuit for a microwave oven comprises a transformer 17, a relay contact 18 which connects a main supply to the primary winding of the transformer and a resistor 11 which is short circuited by the contact when the relay winding 15 is energized. When a main switch 1 is closed, a transformer 20 supplies heating current to the cathode 6 of a magnetron 5. At the end of the heating period a contact 9 in a delay circuit 4 is closed to supply current via a closed contact 10, the resistor 11 and an oven door switch 16 to the transformer 17, the secondary winding of which provides the high voltage for the anode of the magnetron. The relay winding 15 is also energized via a switch 12 responsive to the oven temperature, a switch 13 and the closed contact 14 of a timer 19. The contact 18 is therefore changed over to short circuit the resistor 11, which may be a thermistor, and connect the transformer 17 directly to the mains supply. The magnetron is cooled by a fan 21, the supply switch 22 of which is controlled by a thermostat 23 adapted to sense the magnetron temperature. A light emitting diode 26 monitors the anode current and glow lamps 24, 25 indicate that the supply voltage is applied to each terminal of the switch 10. If the output of the transformer 17 is short circuited, the increased voltage drop across the thermistor 11 prevents the energization of the relay winding 15 so that the relay does not switch excessive fault currents.