This paper describes the circuits and algorithms used in a microprocessor-controlled AC/DC thyristor convertor to implement a wide-bandwidth closed-loop current source. The convertor consists of two three-phase bridge circuits for four-quadrant operation, with direct digital control using a M6809 microprocessor. The timing pulse generator is linked to the supply frequency via a phase locked loop, making the system insensitive to supply frequency variation. The use of proportional-plus-integral (PI) control algorithms leads to fast responses to step changes in reference input. Results from constant-gain PI controllers are compared in the paper with theoretical responses based on approximate z-transform analysis to obtain optimum parameters for various modes of operation. In addition, results are presented from a simple variable-gain adaptive controller which adjusts the controller parameters depending on the reference change and operating point.