Abstract: A refrigeration control apparatus especially adapted for use in conjunction with cryogenic coolers includes a brushless variable speed drive motor for powering a mechanical cooler-compressor and electronic control circuitry coupled to the motor allowing motor speed and hence cooling rate to be responsive to changes in the temperature of the volume sought to be cooled. A signal corresponding to sensed temperature is compared to a reference signal derived from motor speed. The difference therebetween used to vary the duty cycle of a high frequency pulse train. The pulse train is used to modulate the drive current applied to the motor through pulse position switching circuitry to vary the speed of the drive motor. High and low motor speed limits are provided along with sensor failure protection circuitry to insure continuous motor operation between chosen limits.