GB1350919A - Microwave heating apparatus - Google Patents

Abstract

1350919 Supply systems for microwave ovens C E M TIBBS and O A VILE 15 Oct 1971 [26 Oct 1970 5 Jan 1971] 50814/70 and 499/71 Heading H2H A microwave oven powered by a magnetron has a gas-filled tube, or a probe connected to an electrode of the tube, projecting into it, which tube ionizes and cuts off the magnetron power supply if the field intensity becomes excessive, e.g. due to absence of load in the cavity, activation of the tube being prevented during an initial period after energizing of the oven. As shown, a multielectrode tube 20 has anode 1 and cathode 4 supplied with a D.C. potential via diode 15, and a primer electrode 23 connected at the junction of resistor 18 and capacitor 19 and supplied with the D.C. voltage via resistor 18. A probe 24 is connected to a trigger electrode 8 which causes ionization of the tube at large field strengths which in turn activates a relay coil 11 to open contacts (9), (9<SP>1</SP>), Fig. 1 (not shown), in the magnetron power supply and the contact 9<SP>11</SP> in the control circuit. The threshold may be adjusted by altering the amount of the probe projecting into the cavity. The tube 20 and associated components are shielded in a can 29 with the probe projecting therefrom through the roof of the oven. In a modification, Fig. 5 (not shown), the tube 20 itself is arranged to project at an adjustable distance from the roof of the oven. To prevent energization of the tube owing to surge effects immediately after activation of the oven the potential on the primer electrode 6 is caused to rise slowly by the R, C circuit 18, 19 so that the tube may not be ignited during, e.g. the first ¢ [second of energization of the magnetron. In a modification, Fig. 6 (not shown), a 2 electrode tube is used, the probe being coupled with one electrode and the activation of the tube being initially prevented either by a delay in switching on of the control circuit after the magnetron is energized, or by a thermistor circuit adapted to increase the electrode voltage slowly as the temperature rises. The tube may also form one arm of a Wheatstone Bridge circuit.