A programmable electronic digital thermostat provides improved control of a temperature
conditioning system for varying load conditions in order to maintain the ambient temperature within a predefined range of a scheduled set temperature while minimizing ambient temperature overshoots and undershoots. The thermostat includes an improved power supply circuit for simply and effectively providing its operating power utilizing an external alternating current power source of the temperature conditioning system and utilizing conventional existing wiring. The thermostat includes a microprocessor for storing a program and for performing logical control operations in accordance with the stored program and a temperature sensing circuit that provides a pulse width modulated PWM signal representative of ambient temperature in response to an enable control signal from the microprocessor.
Among the important objects of the present invention are to provide an improved programmable electronic digital thermostat for controlling a temperature conditioning system for varying load conditions; to provide such thermostat utilizing actual heating and cooling history values for determining effective recovery times; to provide such thermostat enabling effective duty cycle control of the temperature conditioning system; to provide such thermostat including a power supply circuit enabling efficient, effective and reliable operation utilizing existing wiring for a conventional electromechanical thermostat; and to provide such thermostat having optimized power requirements.
In brief, in accordance with the above and other objects of the invention, there is provided a programmable electronic digital thermostat for controlling a temperature conditioning system to maintain a scheduled set ambient temperature. The thermostat includes at least one controlled semiconductor switch for selectively activating or deactivating an external relay connected in series with an external power source of the temperature conditioning system. The thermostat includes a power supply circuit utilizing the external alternating current power source. The power supply circuit includes a bridge rectifier connected between the controlled semiconductor switch and the external relay for rectifying the alternating current to produce a direct current voltage. A voltage regulator is coupled at its input to the bridge rectifier for providing a predetermined supply voltage. A capacitor is connected to the voltage regulator input for charging to a first predetermined potential. Control circuitry is connected between the capacitor and the controlled semiconductor switch for sensing the voltage potential of the capacitor and for deactivating the semiconductor switch in response to a sensed voltage potential below a predetermined value, thereby to enable the capacitor to recharge to the first predetermined potential without deactivating the external relay.