GB1508485A - Electric stud welding apparatus - Google Patents

Abstract

1508485 Protective circuits for stud welding apparatus HILTI AG 3 Sept 1975 [13 Sept 1974] 36192/75 Heading H2H Stud welding apparatus comprises a condenser battery 23, a thyristor welding switch 32 for closing the welding circuit to initiate a welding operation and a protective circuit which prevents the closing of the welding switch when the magnitude of the resistance between the welding terminals 43, 44 exceeds a predetermined value. The value of the effective resistance 46 between the terminals at which the protective circuit responds is selected to be between 100 and 700 ohms. The condenser battery 23 is charged from a D.C. source 2 via the closed contacts 6/1, 6/2 of a de-energized relay 6. When the condenser battery voltage reaches a value determined by the setting of a potentiometer 22, a transistor 7 conducts to energize the relay 6 via a D.C. source 3 thereby opening the contacts 6/1, 6/2. When the welding operation is commenced, provided that the effective resistance 46 between the terminals 43, 44 is less than the predetermined value, the increased current flowing through a resistor 37 reduces the potential at the cathode of a Zener diode 38 below the breakdown value so that a transistor 29 is non-conductive. On closing a push button switch 45, a relay 42 is energized from a D.C. source 41 to close contacts 42/4, 42/5 so that a relay 11 is energized via a conductive transistor 10. The contacts 11/4, 11/5 of the relay 11 close to supply trigger current to a thyristor 34 via a resistor 16. The thyristor 34 triggers the thyristor 32 to connect the condenser battery across the welding terminals. If the resistance 46 exceeds the predetermined value, the conduction of the transistor 29 prevents the triggering of the thyristors 34, 32. On completion of the weld and removal of the gun from the welded stud, the value of the resistance 46 becomes infinite so that the relay 42 is de-energized and current flows through contacts 42/5, 42/6, resistance 15, contacts 11/2, 11/1, diode 40 and a resistance 35, the voltage drop across which triggers a thyristor 25. The conduction of the thyristor 25 completes a discharge circuit for the condenser battery 23 and enables the transistors 7, 10 to be switched off. The relays 11, 6 are therefore de-energized to allow re-charging of the condenser battery.