The invention relates to a welding device for joining at least two objects (9, 10), comprising at least two electrodes (6, 7) for contacting said objects (11, 12), and a transformer (5) for supplying a current to the electrodes (6, 7). The invention is characterized in that it comprises a rectifier (1) which is connected to an inverter (2), which in turn is connected to the transformer (5), and a frequency control device (3) for controlling the frequency of the inverter (2). The invention provides an improved welding device for welding of sheet-like material, particularly roofing panels.