A multiphase solid-state contactor for controlling a principal circuit, comprising a plurality of thyristors connected in anti-parallel relation, and an electronic control circuit connected to the thyristors including a firing control circuit, an input circuit and a surge absorbing circuit. The input circuit may include a rectifier and voltage detector for compensating for variations in the operating voltage of the principal circuit. Each of the firing control circuit, the input circuit and the surge absorbing circuit may comprise separate printed circuit boards which may be mounted in stacked relation in a casing for ease of handling and repair. Auxiliary terminals are also provided for outputting an auxiliary output signal from the device.