GB621466A - Improvements in and relating to electric timing circuits, particularly for resistance welding - Google Patents

Abstract

621,466. Control systems for resistance welding and the like. BRITISH THOMSON HOUSTON CO., Ltd. Nov. 1, 1944, No. 21366. Convention date, Nov. 1, 1943. [Class 38 (iv)] An electric circuit interconnecting an A.C. supply 12 and a load circuit 13, e.g. a welding transformer, includes two valves 10, 11, each having a control member 16, which is energized with a periodic peaked voltage to render the valves conductive, and means responsive to the load voltage for delaying the application of the peaked voltage until the anode voltage of the valve which is to conduct next becomes positive. The valves 10, 11 are reversely connected in parallel and are rendered non-conducting by the resultant of the peaked secondary voltage of transformer 23 and the secondary voltage of transformer 21, but are rendered conducting- when transformer 19 whose secondary is in phase opposition to that of 21 is energized by timing circuit 25. The transformer 23 is supplied from a phaseshifting circuit 31 including valves 32, 33 which conduct during alternate half cycles. The voltage impressed on the control members 40 of these valves is the resultant of a D.C. output 48, 49 from a voltage regulating circuit, and a phase-shifting bridge 43, 44, 45. The delaying. means include a transformer 58 energized from welding transformer 13, secondaries 59, 61 of transformer 38 energized from the mains 12, and a rectifier 54 having two anodes 55, 56. Assuming valve 10 to be conducting, the transformers 13, 58 are energized, the voltage in the secondaries 571, 60 of the latter being greater than and of reverse phase to those in 59, 61 so that the valve 54 is conducting and supplying negative grid bias to overcome the +ve voltage impressed on grids 40 of valves 32, 33, thereby rendering them non-conductive. When valve 10 stops conducting, transformers 13, 58 are de-energized, thereby making rectifier 54 non-conducting, and valves 32 and 33 conducting to energize valve 11 through transformer 23.