GB1037852A - Improvements relating to apparatus for protecting welding equipment - Google Patents

Abstract

1,037,852. Protection circuits for welding equipment; converting. SOUTH LONDON ELECTRICAL EQUIPMENT CO. Ltd. May 3, 1965 [Feb. 17, 1964], No. 6568/64. Headings H2F, H2H and H2K. A protection circuit for spot welding equipment (not shown) comprises a relay whose operating coil RLA is energized to cause termination of a welding operation should a preset welding time be exceeded or should the welding current depart from a desired A.C. waveform, e.g. full-wave. The former protective feature is provided by the right hand part of the circuit of Fig. 1 in which, prior to the commencement of a welding operation, a capacitor C2 is maintained charged through a transistor TR1 thus causing transistors TR2, TR3 to be cut off. Initiation of welding cuts off transistor TR1 to allow capacitor C2 to discharge through resistors R5, VR1, the transistors TR2, TR3 remaining cut off until the voltage across C2 has fallen to a predetermined value in a time slightly greater than the welding time set by the equipment control gear. Thus if this welding time is exceeded, transistors TR2, TR3 conduct to cause disconnection of the main circuit breaker and/or the ignitron circuit, feedback through members R10, SD1 locking the protection circuit in the relay energized condition until reset by operation of the switch PB. The left-hand part of Fig. 1 comprises two circuits respectively supplying D.C. voltages X and Y to a transistor TR4, the voltage X being approximately constant and the voltage Y being dependent on the A.C. waveform of the welding current. When this is full wave, voltage Y is greater than X and transistor TR4 is cut off. Should the waveform depart from normal, e.g. positive half-cycles only, Fig. 2 (not shown), the voltage Y will fall below X to render transistor TR4 conducting, thus producing conduction of transistor TR3, energization of the relay RLA and the disconnection of the equipment previously described. The arrangement of neon tube N1, capacitor C5 and Zener diodes ZD2, ZD3 produces a square waveform of constant amplitude and pulse width, provided that an integral number of welding cycles occurs, and this waveform is fed through a differentiating capacitor C6 to the bridge BR2. The resulting D.C. voltage Y applied to the transistor TR4 is of sawtooth form which degenerates to a short triangularshaped pulse when the welding current waveform departs from normal as mentioned above. The normally-open contacts A1 of relay RLA close to discharge and reset the capacitor C5 when a fault occurs.