A miniature full-wave converter to control the current of a superconducting magnet is developed and tested in our institutes. Typical design values of the device are: AC voltage of 35 V, current of 7 A, and frequency 50-60 Hz; DC voltage of ± 0.2 V, and current of ± 1 kA. An efficiency is better than 97% and a ‘cold’ volume is less than 1 litre. The rectification is achieved by means of thermally controlled superconducting switches. The high reliability of the device is ensured by means of a mechanical switch. The device provides an efficient way to a fine control of the magnet current.