Apparatus for generating and regulating welding currents

Apparatus for generating and regulating high intensity welding currents which are developed in a welding apparatus having electrodes which are brought in contact with a workpiece comprising:

• a. a transformer having a primary winding and a secondary winding, the welding current which is to be regulated being taken from across the secondary winding, said secondary winding being coupled to the electrodes;
• b. a choke coupled in series with the primary winding across an a.c. power source;
• first and second thyristors coupled in parallel with each other and in parallel with the choke and thyristors arranged so as to have opposite polarity;
• d. variable phase triggering means for developing firing pulses for said thyristors to control the point in the positive and negative half cycles of the a.c. power at which they are fired, said triggering means including a first RC timing circuit having at least a resistor and capacitor in series;
• and
• e. means associated with said variable phase triggering means for causing the firing pulse outputs thereof to be shifted in a direction to reduce the effective currents through the thyristors both during turn on of the apparatus and in the case of a short circuit of the secondary of the transformer, said means comprising a semiconductor switch coupled in parallel with said resistor in said first RC timing circuit, said switch having a control electrode, means coupled to the secondary of said transformers for generating a control signal when the voltage at said output is greater than a predetermined value, and a second RC circuit coupling said control signal to the control terminal of said semiconductor switch.