GB1199429A - A Resistance Welding Control System. - Google Patents

Abstract

1,199,429. Resistance welding systems. WELTRONIC CO. 7 Dec., 1967, No. 55688/67. Heading H2H. In a resistance welding circuit, means are provided for sensing the instant at which each half cycle of the load current passes through zero which means initiate through a further circuit providing a variable delay the connection of a load to a source. A circuit is also provided to allow the ignition firing pulses to a pair of ignitrons only when they are blocked. When the load current through ignitrons 1Ig, 2Ig ceases, the voltage across them increases which is detected by a transformer 4T causing a transistor 4Q to block, hence the potential on a conductor 122 rises and passes a current to a heat adjusting potentiometer P2 and a resistor P5 to charge a capacitor C27 which determines the firing delay. At a determined voltage, unijunction, transistor UJT3 fires rendering a thyristor SCR7 conductive which through a diode RE46 grounds the base of a transistor 3Q, blocking it. This allows a voltage, also from transformer 4T, through a diode RE39 and 4 layer diode RE52, to fire a thyristor SCR6 discharging a capacitor C24 through a transformer 3T creating a pulse to fire SCR2 to fire 2IG. Capacitor C24 is charged in the opposite half cycle. An identical circuit 102 is provided for igniting 1IG, also supplying a firing pulse only if both ignitrons are quiescent. The circuit may incorporate a known arrangement (not shown) whereby the first cycle has its firing pulses delayed with respect to the remainder (sec for example Specification 1,067,893). Thyristors SCR2 and (SCR1) (not shown) may be employed directly in the load current circuit.