GB932314A - Improvements in or relating to control systems for electrical ovens comprising high frequency heating means - Google Patents

Abstract

932,314. Control systems for high-frequency heating ovens. HUSQVARNA VAPENFABRIKS A.B. May 3, 1961 [May 9, 1960], No. 16095/61. Class 38 (4). The operating period of a high-frequencyheating oven is controlled by a capacitor which is charged through an adjustable resistor to cause an electronic switch to switch off the oven after a predetermined time, by ionisation of an inert gas in the switch thereby operating a relay to break the oven supply. In operation actuation of a switch 43 closes a main switch 32 to supply the following circuit components: the heating means of the cathode 4 of the oven magnetron 2 ; a lamp 26; resistor 24, rectifier 17, operating coil of relay 7 and resistor 20; high resistance 27 to capacitor 10 through push-button switches 15 . . . 240; filter capacitor 18 and a fuse 12. The current is insufficient to operate the relay 7, but the potential of capacitor 10 increases until, after a time equivalent to that required to raise the cathode 4 to its operating temperature, the gas in a tube 9 is ionised to establish a circuit through the operating coils of relays 7 and 8 and a switch 37, thereby actuating the relays and lighting a lamp 25. Upon introduction of an article into the oven 1, a suitable working period is selected by opening one or more of the switches 15 . . . 240 which operation closes the switch 43 to supply the capacitor 10 through the resistor(s) 11 corresponding to the opened switch(es). The working period is commenced by opening switch 37 which recloses upon release, this opening de-energizing the relay 8 to energize a relay 6 through a micro-switch 14 (assuming the oven door 13 to which it is mechanically connected has been shut) and the contact of relay 7 which remains energized through a holding circuit formed by resistors 20, 22. Hence the opening of switch 37 energizes the oven transformer 5 and terminates the discharge of the tube 9, which allows the potential of capacitor 10 to rise at a rate dependent on its capacitance, the resistance of resistor(s) 11 and the voltage from the divider formed by resistances 20, 22. After a time determined by the value of resistor(s) 11, the capacitor 10 ionises the tube 9 to operate the relay 8, de-energize relay 6 and break the supply to transformer 5. The working period is made independent of interruptions caused by opening the oven door (after which the oven has to be restarted by opening the switch 37) by ensuring the current consumption of the lamp 25 and resistor 24 to be such that upon these interruptions the direct voltage from the rectifier 17 is reduced to alter the potential on capacitor 10 only very slowly. If desired, the relay 6 may be a step relay which successively inserts different functions of the oven, e.g. power stages, and also successively interconnects different resistors 11 in the charging circuit of capacitor 10 for controlling a whole working cycle with varying length of time of the individual operations.