Thyristor controlled welding power supply

In a three-phase welding power supply including a positive output electrode and a negative output electrode for providing welding current to an arc welder and a three-phase input transformer with a wye connected secondary including a neutral terminal,

• a plurality of current controlling thyristors arranged so that at least one of said thyristors is connected to each phase of said secondary and to said positive output electrode and at least one of said thyristors is connected to each phase of said secondary and said negative electrode;
• control circuit means connected to said secondary and said plurality of thyristors for providing firing pulses to the gates of said thyristors;
• and
• a holding impedance connecting said positive output electrode and said negative output electrode to said neutral terminal for drawing a holding current through one of said plurality of thyristors subsequent to provision of one of said firing pulses.

In a welding power supply including an input port, characterized by a first terminal and a second terminal, for alternating current input voltage and a plurality of control thyristors for controlling the output current of said power supply, an improved control circuit for providing firing pulses to a thyristor gate to vary the conduction angle of said thyristors comprising in combination:

• a rectifier connected to said input port for providing a pulsed output voltage across said first terminal and said second terminal;
• a variable first resistance leg connected between said first terminal and one end of a firing control capactitor at a first junction point, the other end of said firing control capacitor being connected to said second terminal;
• a second resistance leg connected between said first terminal and a second junction point;
• a reactive impedance connected between said second junction point and said second terminal;
• said reactive impedance having a lagging phase angle at the frequency of said alternating current input voltage of less than 90.degree.;
• a breakdown device connected between said first junction point and said second junction point for conducting current when a predetermined voltage exists between said first and second junction points;
• and
• capacitive coupling means for connecting said thyristor gate and said second junction point.