Resistive welder having a controlled output voltage unaffected by secondary circuit disconnection

摘要

A resistance welder is provided having an electronic contactor, a welding transformer coupled in series with the electronic contactor, and a secondary circuit with a welding load connected to the welding transformer. The series circuit of the electronic contactor and the welding transformer is adapted for receiving power from an AC power source. The resistance welder also includes a control device having a voltage detector coupled to detect a voltage across the electronic contactor for producing a first signal according to the detected voltage, a delay circuit coupled to receive a primary voltage across the welding transformer for delaying the primary voltage by a value determined by a first time constant thereof to produce a voltage signal, a switch coupled to receive the first signal for resetting the delay circuit according to the first signal, and a gain control circuit coupled to receive the voltage signal for controlling a transmission gain of the voltage signal to produce a gain controlled signal. The control device further includes a processor coupled to receive the gain controlled signal for comparing the gain controlled signal with a predetermined reference value and for producing a gate control signal based on a comparison result. The electronic contactor is also coupled to receive the gate control signal for controlling the primary voltage so that said primary voltage becomes equal to a predetermined value corresponding to the predetermined reference value.