A Single Directional Thermostatic Controller can maintain a constant temperature set point within an enclosure surrounded by a relatively constant ambient temperature. The set point tolerance is defined by a hysteresis range. Once the set point is achieved, the controller shuts off the thermoelectric cooler assembly. When the temperature rises above the hysteresis range, the controller turns on power to the thermoelectric cooler assembly and starts the cooling mode process. This cycle continues until the controller is shut down. Thermostatic control is often used in refrigeration mode in an indoor environment with climate control, where a narrow temperature swing can be tolerated. Laird Thermal Systems offers two standard single directional thermostatic temperature controllers, QC-50 and QE-50.