Patent GB-570591-A (GB)
Title: Improvements in and relating to electric valve circuits
IPC: H02M1/02
Filed: 1943-06-25
Abstract: 570,591. Supply systems for consumption apparatus. BRITISH THOMSONHOUSTON CO., Ltd. June 25, 1943, No. 10267. Convention date, June 29, 1942. [Class 38 (iv)] Switching control.-An A.C. load supplied through electric valves, such as a resistance welding machine, is energized for a predetermined time, or intermittently energized, by a control circuit permitting the period of energization to be readily adjusted as well as the interval between successive energizations, the control circuit comprising means for impressing on the grids of valves controlling those in the load circuit (1) an intermittent or periodic voltage, and (2) a variable hold-off voltage. The latter voltage is obtained from a D.C. supply through a pair of parallel circuits, each comprising a resistance in series with a gridcontrolled discharge device, the anodes of the two devices being connected by a commutating condenser and the grids being supplied with negative bias upon which is superimposed peaked alternating voltages of a value sufficient to overcome the negative bias, the phase of the peaked voltages being adjustable independently of that of the intermittent or periodic voltage. As shown in Fig. 1, a welding transformer 3 is supplied from A.C. mains 2 through back-to-back immersion ignitrons 45 controlled by gas or vapour-filled relays 10, 11 which are normally maintained non-conducting by anti-phased grid bias voltages provided by windings 19, 20 of a transformer 16 having a centre tapped primary winding 17 supplied from mains 15 or 2. Negative grid bias may also be applied by grid resistors and condensers 22, 23. Striking of relays 10, 11 and consequently firing of the ignitrons, is effected by transformers 24, 25 which inject voltage impulses in the grid circuits of the relays when further relays 26, 27 pass current, this occurring when a periodic voltage supplied to the grids of relays 26, 27 by a peaking transformer 30 overcomes a variable hold-off D.C. bias applied via conductors 29. The primary winding of transformer 30 is supplied through an adjustable phase shifting network 33-38, whereby the magnitude of the welding current may be controlled. Timing of the welding period, and of the interval between successive welds in the case of recurrent welding operations, is effected by control' of the hold-off bias which is derived from D.C. mains 39, 40, the control being effected by grid-controlled relays 51, 53 connected together multi -vibrator fashion across sections 42, 43 of a potential divider 41-45. A two-path rectifier 69 provides negative bias in the grid circuits upon which is superimposed alternating triggering potentials derived from the windings 61, 62 of a peaking transformer 63 fed from an adjustable phase shifting circuit 33-35, 64-66. This circuit determines the point in the A.C. cycle of mains 15 or 2 at which the hold-off bias for relays 26, 27 (the voltage drop across resistances 41, 55 and 52) is reduced to a value which can be overcome by transformer 30. A condenser 59 connected between the anodes of relays 51, 53 provides for alternate conduction of the relays. Adjustable discharge resistors 76, 77 determine the respective periods of conduction of the two relays and thus control the welding period and the interval between welds. When a single weld is to be effected, switch 83 is moved from contact 85 (which provides for continuous operation of relays 51, 53) to contact 84, whereby a negative hold-off bias is impressed on relay 51 through resistor 86 when the electromagnetic welding relay 78 is energized and recurrent conduction of relay 51 prevented.