Patent GB-1186779-A (GB)
Title: Improvements in Ultra-High Frequency Control and Power Supply System
Inventor: STAATS JAMES EDWARD
IPC: H05B6/68
Filed: 1967-06-27
Abstract: 1,186,779. Delaying power supply connection to magnetrons. GENERAL ELECTRIC CO. 27 June, 1967 [27 July, 1966], No. 29700/67. Heading H3W. [Also in Division H2] A control system for an ultra-high frequency cooking oven 200 energized by way of a crossed field device 210, such as a magnetron, is provided with means whereby the magnetron 210 is initially provided with a low anode voltage in order to allow the cathode temperature to stabilize without damage thereto. A cooking operation is initiated by moving a time setting control 132 to a desired cooking time and a power selector switch 110 to high or low power setting, e.g. to a H1 setting when the pairs of switch blocks 115-120 are closed. Movement of the control 132 rotates cams 157, 179, 180 to engage clutch 146; to place a wedge 165 in the magnetic circuit of a buzzer 162 and to close contacts 182, 183 respectively. The system is thus set in the starting condition in which a supply line L1, at a potential of 118 volts, is connected to a terminal 231 of a voltage doubler 230 by way of a switch 105 associated with the door of the oven 200, closed contacts 117, 118; closed contacts 182, 183 and line 186. The terminal 232 of the voltage doubler is connected to earth N of the supply by way of closed contacts 274, 275 of relay 270 and resistor 262 and thermistor 260 in parallel. A transformer 253 for energizing heater 214 of the crossed field device 210 is also connected to the supply line L1 by way of line 129; closed contacts 271, 272 and line 186. Heater 214 thus begins to warm up, but the potential applied to the anode 212 of the device 210 from the voltage doubler 230 is low due to the high cold resistance of the thermistor 260. As the thermistor heats up, the voltage at the anode 212 increases and when the cathode 213 reaches full emission the device 210 begins to oscillate. The flow of current through the field coils 220a, 220b of the device 210 then causes the contacts of a magnetic switch 285 to close, thus energizing relay 270 to change the positions of the switch contacts 271-278. Opening of contacts 271, 272 places resistors 244, 245 in series with the supply to transformer 253 to reduce the energization of the heater 214 of the device 210; closure of contacts 273, 275 removes, inter alia, the thermistor 260 and connects the terminal 232 of the voltage doubler 230 to the supply line L2 by way of line 128, thus supplying the circuit with 236 volts to increase the output voltage of the voltage doubler to the device 210 and closure of contacts 276, 278 energizes the timer motor 140 by way of lines 186, 190. The timer motor thus rotates to drive the cams 157, 179, 180 and the control 132 backwards from the chosen cooking time to zero. When this position has been reached the clutch 146 is disengaged to stop the drive from the motor 140; the wedge 165 is retracted, whereupon the buzzer 162 sounds to announce the end of the cooking period and contacts 182, 184 close to remove the supply L1 from line 186 to terminate the supply to the device 210, thereby to end the cooking operation in the oven 200. Closure of contacts 182, 184 maintains the supply to the motor 140 so that the buzzer 162 continues to function until the control 132 is moved to its OFF position. Complete shut down of the equipment is effected when the power selector 110 is returned to its OFF position. During the changeover from 118 volts to 236 volts supply, oscillation of the device 210 is maintained by the by-pass capacitor 290. If low power operation is required, the power selector 110 is moved to its " LO " position to close the contact pairs 121-126. The operation then proceeds as before until the relay 270 operates. Operation of this relay now only inserts resistor 244 in series with the supply to the transformer 253 by virtue of the closure of contacts 121, 122 to interconnect lines 104, 129 and the supply line L2 is connected to terminal 232 of the voltage doubler 230 by way of a voltage dropping resistor 137 to the line 128. Thus, the oven 200 is operated at low power. In both types of operation a blower motor 138 is energized to cool the device 210 and a smoke eliminator is operative. Safety devices are provided by the door latch switch 105 and a thermal switch 280 responding to the temperature of the device 210.