Pn, where n is an integer greater than 1 and k is selectively l . . . n, supply Pk being responsive to an a.c. source and supplying a d.c. current to a common d.c. load for the power supplies; supply Pk including:(1) power output terminal means connected to supply the d.c. current to the d.c. load; (2) a controller for the amplitude of the d.c. current supplied by the power output terminal means o