Each section of the power supply has a bank of four pairs of silicon controlled rectifiers (SCR's) that are connected in inverse parallel. The four taps of the secondary of the main transformer are connected to these four pairs of SCR's, providing four different controlled voltages. The SCR's are used to control the available voltage, and thereby control the current flowing to the silicon rods. Each of the four pairs of SCR's are gated on in different amounts to provide the necessary voltage for the desired current.