GB598290A - Improvements in and relating to electric valve circuits for controlling the supply of alternating current to a load circuit - Google Patents

Abstract

598,290. Control systems for consumption apparatus. BRITISH THOMSONHOUSTON CO., Ltd. Sept. 28, 1943, No. 15876. Convention date, Sept. 30, 1942. [Class 38 (iv)] Intermittent energization of a load 10, such as a resistance welding transformer, from an A.C. source 11 for a predetermined number of half cycles is effected over a grid controlled valve 14, the conductivity of which is controlled by an R.C. circuit 20, 21 of low time-constant in combination with a timing circuit 32 which controls the charging of the condenser 20. Normally the condenser is charged each cycle when a tube 33 of the timing circuit conducts and the condenser voltage, in series with a hold-off voltage provided by winding 22 of a former 23 and an opposed peaked voltage from a transformer 31, is applied to the grid of valve 14 to maintain the valve non-conducting. The timing circuit allows conduction of tube 33 to be prevented for the predetermined number of cycles so that valve 14 conducts-and energizes the load for a corresponding number of half cycles due to the quick discharge of condenser 20 by resistor 21. Preferably, a valve 15 trails valve 14 and energizes the load during the alternate half cycles. the peaking transformer 31 is fed from an adjustable phasing circuit 29, 30 whereby the points in the A.C. cycle at which the valves conduct may be controlled. Cyclic conduction of tube 33 is provided by a grid excitation circuit comprising a winding 46 of transformer 23 and an R.C. circuit 42, 43 connected by a normally closed weld-initiating switch 45, the arrangement resulting in grid rectification. A shield grid 37, energized from winding 46 through a phase shifting circuit 40, 41, delays conduction of the tube until the 90-degree point of the anode voltage cycle. Upon operation of the initiating switch the A.C. voltage input to grid 36 is reduced to that appearing at the adjustable tapping 48 of a potentiometer 49 shunting part of the winding 46 ; the tube consequently ceases to conduct until the charge on condenser 42 falls to a lower value by leakage through the adjustable resistor 43 and the load circuit is energized during alternate half cycles of this period by valve 14. A transformer 60 in parallel with the load circuit nullifies, when valve 14 conducts, a hold-off bias provided by a further winding 64 of transformer 23 on a tube 59 which fixes and reduces the hold-off bias on the trailing value 15 derived from winding 54. The valve subsequently conducts under the control of winding 53 of the peaking transformer. For half cycle-working a switch 66, ganged with the control member of the timing resistor 43, is closed to shunt a resistor 67 across the transformer 60 to prevent conduction of tube 59.