416,374. Control systems for electric welding ; converting. BRITISH THOMSONHOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Lord H. W.; 1103, Palmer Avenue, Schenectady, New York, U.S.A.) Jan. 9, 1933, No. 729. Convention date, Jan. 7, 1932. [Class 38 (iv).] The supply of alternating current to a resistance welding apparatus is controlled by a pair of reversely connected electric discharge valves rendered conductive in response to conductivity changes in a pair of grid controlled valves the conductivity of which is determined by an auxiliary discharge valve to the grid of which is applied an alternating voltage from the main source and a control voltage derived from a direct current source arranged in such a manner that the conductivity of the auxiliary valve and of the dependent valves is initiated at a predetermined part of the cycle of the A.C. source. The control voltage is obtained from a circuit including a capacitor charged from one of D.C. source through a resistance and discharged through the auxiliary valve and a potentiometer in parallel with the charging circuit has a point connected to the grid of the auxiliary valve. The welding circuit is supplied from the secondary of a transformer 7, Fig. 1, which is in series with an impedance such as a winding of a transformer 1 and with a switch 8 used to prevent arcing at the welding electrodes. Welding current is caused to flow by substantially eliminating the impedance in the primary circuit by short circuiting the secondary winding of the transformer 1 by valves 2. The conductivity of the valves 2 is controlled by shifting the phase of their grid voltage which is derived from the supply circuit 10. The phase shifting circuit contains an impedance 26 which is substantially eliminated to reverse the phase of one grid potential by short circuiting an inductively related winding 30 by valves 31. The control circuit governs the grid voltage of valves 31 in such a way that they are rendered conducting periodically each period extending over a predetermined number of cycles of the supply, commencing at an adjustable point in a cycle. A direct current source 34 feeds a valve 32 and a condenser 33 in series with an impedance 36. The grid 38 of the valve 32 is biassed by a tapping 41 on a potentiometer 42 and an alternating potential of adjustable phase derived from the supply 10. Its potential with reference to the point 44 is represented in Fig. 2 by the trace a. The potential of the point 58, or of the cathode of the valve 32 represented by the trace c, falls as the condenser 33 charges during periods of nonconductivity of the valve 32 until at a point d the grid 38 is at the same potential as the cathode and the valve conducts. This discharges the condenser 33, the cathode potential rises, and the valve ceases to conduct and the cycle repeats itself. The potential changes at the point 58 are applied to the valves 31 the grids of which are biassed by a tapping 57 on the potentiometer 42, and preferably also by an alternating potential from the supply 10, as shown by the trace g in Fig. 2 so that they conduct for periods represented by h to l or h to k depending on the character of valve used. Corresponding changes therefore occur in the current supply to the welding electrodes. The switch 50 is placed in the lower positions when the " off " periods are short and " on " periods long and in the upper position when the reverse condition applies. Adjustment of operation is effected by the sliders 41, 57 which change the average grid potentials, by shifting the phase of the A.C. component of the trace a and by changing the value of the impedance 36 or of the condenser 33. In a modification for obtaining single surges of welding current the connection 44 is made to the mid-point of the potentiometer 42, Fig. 3, and the average bias of the grid 38 of the valve 32 is changed when the valve 32 conducts to prevent it from conducting again. This is effected by a valve 75 connected between high and low bias tappings 72, 41 on the potentiometer and normally in a conducting condition so as to impose high bias and prevent the valve 32 conducting. A condenser 83 is connected to the anode of the valve 75 and when the control switch 82 is closed after the welding electrodes have been engaged this condenser is connected across the valve 75 to bring its anode potential below that of the cathode and interrupt the high bias circuit for the grid. The valve 32 thereupon conducts at about the maximum of the A.C. component and discharges the condenser 33. The valve 75 which is meanwhile held nonconducting by negative grid bias is started by the discharge current of condenser 33 acting through a transformer 80 coupling its grid circuit with the discharge circuit of this condenser and further operation is prevented by restoration of high bias to the grid 38.