Patent GB-670584-A (GB)
Title: Improvements in and relating to electric valve control circuits
IPC: B23K11/24
Filed: 1948-04-19
Abstract: 670,584. Control of power supply by space-discharge apparatus. BRITISH THOMSONHOUSTON CO., Ltd. April 19, 1948 [April 19, 1947], No. 10768/48. Class 38 (iv). Firing valves 13, 14 for back-to-back welding ignitrons 8, 9 are controlled by keying valve 107 and timing valves 98, 100 through a pair of relay valves 40, 54 which are normally held-off (as described in Specification 659,996), by whichever is the more negative of two A.C. control voltages-one of fixed phase obtained from transformer 39-and applied over rectifier 68 and the other obtained from transformer 51 and adjustable phase-shifter 90-92 and applied over transformer 76. Valves 98, 107 have inductive anode loads 94, 112 and are energized in phase opposition from the transformer 39, each grid circuit being energized (when switch 144 is set for repetitive welding, viz. into the position other than that shown) by the voltage across the other valve, so that the two valves trail each other in conduction, once conduction is initiated in one of them. Windings 69 coupled to the inductive load 94 (which together with a further winding 95 and capacitor 96 form an oscillatory circuit) pass triggering potentials to the grids of the relay valves 40, 54 when valve 98 is conducting and so cause the flow of welding current. Conduction of valve 107 is initiated by a pilot valve 143 in parallel with 98 and which becomes conducting when a timing circuit 160, 162 (determining the period between welds) has timed out. Simultaneously with initial conduction of valve 107, a condenser 120, which has previously been charged from the anode of 107 over rectifier 122, commences to discharge over an adjustable resistance network which determines the period of each weld, the falling voltage of the condenser being applied to grid of valve 100 which eventually conducts. Its anode circuit includes the winding 95 which now opposes winding 94, so that the triggering potentials in windings 69 disappear, condenser 96 of the oscillatory circuit is effectivelv shortcircuited and the anode circuit of valve 98 becomes non-inductive. Valve 107 becomes non-conducting in consequence and the absence of pulses in a winding 165 coupled to inductor 112 causes cut-off (by the volt drop across a resistor 159) of a controlled rectifier 161 through which the " off " timing condenser 160 is charged. The increasing anode potential of the rectifier, as 160 discharges, is applied to the grid of pilot valve 143, which eventually reconducts, to initiate another cycle, under the control of a phase-shifted voltage applied to its shield 167 from transformer 39, capacitor 169 and resistor 170. For single cycle operation siwtch 144 is moved to the position shown. Closure of push-button 140 then removes hold-off bias from valve 107, which conducts so long as the button is pressed. Valve 98 trails 107 and welding current flows until such time as valve 100 eventually conducts. The secondary winding 127 of the heater transformer for rectifier 122 is arranged in the charging circuit of the timing condenser 120 to counteract the " selfgenerating" voltage of the rectifier. The loading of the condenser 120 by the grid circuit of valve 100 may be removed by the insertion between the two of a hard valve cathode follower stage. 45, 173 and 21 are safety switches which must be closed before the system will operate. The action of the relay valves 40, 54 upon conduction is to reverse the phase of the voltage applied to the primary windings of the transformers 26, 31 which normally hold-off the firing valves 13, 14.