The power supply for the main magnets of the CERN Proton Synchrotron consists of a motor/generator set and thyristor converters. It has been equipped with a new control system incorporating high-precision digital magnetic field regulation, which is presented in this paper. The new controls are based on a standard control system for LHC thyristor converters, adapted for field instead of current control. Since magnetic field determines beam orbit, field is preferable over current regulation in the presence of magnetic saturation and hysteresis, and it is necessary for a multi-cycling machine with variable remanence. The magnetic field is regulated by a digital controller implementing an algorithm based on the RST three-polynomial form, in which the performance of the regulation and tracking can be independently optimized. It acts through an analogue proportional/integral voltage loop which drives the original firing system. The fast and high-precision field measurement is provided by an existing system, which integrates the field derivative measured by flux pick-up coils. The paper reports on system design, operation and performance.