Patent GB-790257-A (GB)
Title: Improvements in or relating to electric control circuits
IPC: B23K11/26
Filed: 1955-09-15
Abstract: 790,257. Control systems. for resistance welders. SQUARE D CO. Sept. 15, 1955 [Sept. 16, 1954], No. 26387/55. Class 38 (4). Spot welding transformers 42-45 are energized in sequence, upon closure of an initiating switch 16, under the control of discharge tubes individual to each transformer and each associated with a timing circuit which determines the period of ener. gization of the transformer, the arrangement being such that conduction of one tube causes energization of the corresponding transformer until the tube becomes cutoff by its timing circuit and also renders non-conducting the tube controlling the next transformer of the sequence, whilst preparing for operation the timing circuit for that tube. Cut-off of the first tube allows the second tube to conduct to effect energization of its transformer, the second tube then cutting-off under control of its timing circuit and initiating conduction of the next tube in the sequence. In the system shown, prior to closure of a weldinitiating switch 16, tube 12 is non-conducting, as its cathode circuit is open; tube 25 is conducting to charge condenser 30 from the D.C. lines 7, 8 over resistors 14, 28; and tubes 76, 102, 104 are held non-conducting by D.C. grid bias and by negative screen bias obtained over rectifier 56 from a control transformer 31. On closure of the initiating switch 16, tube 12 fires, with a delay imposed by condenser 19 and at a definite point in the A.C. cycle on account of an A.C. feed to its screen, and remains conducting so long as the switch is closed. Consequential drop of cathode potential of tube 59 causes condenser 30 to apply positive bias to that tube, which now conducts on positive half-cycles of secondary voltage supplied over line 34 from transformer 31. Relay 52 consequently operates to fire the ignitions 37 associated with welding transformer 42, the relay holding in on the negative half-cycles by a holding winding 53 energized from a tapping on transformer 31. Condenser 30 commences to discharge over tube 12 and resistors 69, 28, its eventual discharge cutting off tube 59 to terminate flow of welding current in transformer 42. During this period a condenser 74, charged negatively by grid rectification at tube 76 of a voltage provided by a transformer energized by tube 59, holds tube 76 non-conducting. A condenser 86 for timing the second weld also charges over an auxiliary contact 85 of relay 52 from D.C. lines through resistor 106, tube 12 and the secondary winding of transformer 31. When tube 59 cuts off, condenser 74 quickly discharges and recharges with opposite polarity from the D.C. source to provide positive bias for tube 76, which then conducts on positive half-cycle of voltage of transformer 31 to energize relay 82 and initiate flow of current in welding transformer 43. Condenser 86 starts to discharge through resistors 87, 89 and the secondary winding of transformer 31, eventually causing cut-off of tube 76 and termination of the flow of welding current in transformer 43. Energization of welding transformers 44, 45 follows in sequence, controlled by tubes 102, 104 and associated timing condensers 96, 98 operating similarly to the circuit associated with tube 76.