Abstract: An alternating current constant potential power source suitable for use in a submerged arc welding process. An inductor (Z1) of large value is placed in the D.C. output of thyristor bridge (SCR1 to SCR4) fed from a transformer (T1). The four thyristors (SCR1 to SCR4) and the inductor (Z1) are controlled in such a manner as to provide essentially square wave currents. The square wave is adjustable in amplitude with very rapid zero crossing. The thyristor bridge (SCR1 to SCR4) is controlled in a firing sequence so as to prevent free wheeling of the square wave inductor (Z1) at polarity reversals of the voltage supply. The inductor (Z1) is capable of storing vast amounts of energy, and the thyristor firings are timed to deliver this energy into the arc. Two regulating sub-systems respectively responsive to arc voltage and arc current operate together to control the output.