A sensor near the welding zone picks up acoustic emission (e) and feed a correpd. signal (ea) into an electronic circuit in which the signal is integrated and compared with an average signal (eref). When the difference between the momentary actual value (ea) and (eref) reaches or exceeds a threshold value, the circuit provides a control signal (d) feed via a coupling circuit to a controller which switches off the welding current. The electronic circuit pref. includes a logic gate circuit, and a barrier circuit including a counter for the cycles of a.c. synchronised signals; a comparator; and a memory store. Used esp. for spot welding, e.g. on motor car assembly lines. Automatic control substantially reduces electricity consumption, esp. at expensive peak load times, and also increases electrode life.
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