Importance

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Abstract

A microcomputer controlled digital thermostat is powered by supply voltage derived from a large capacitance (e.g. 3.3 farad) voltage supply capacitor. The voltage supply capacitor is normally charged by an AC voltage which is derived from the furnace and which is rectified by a full wave rectifier bridge. Time and temperature data are stored in read/write RAM storage. The microcomputer controls operation of the thermostat as a function of sensed temperature and the stored time and temperature data. A voltage sensing circuit senses the supply voltage from the voltage supply capacitor and prevents the supply voltage from being provided to the microcomputer and other portions of the thermostat control circuitry when a prolonged interruption of AC power causes the supply voltage to decrease to a predetermined value. The supply voltage continues to be supplied to the read/write RAM to retain the stored time and temperature data. When AC power is restored, and the voltage supply capacitor is recharged, the digital thermostat is ready to resume operation without requiring the user to re-enter all of the stored time and temperature data. The capacitance of the voltage supply capacitor is sufficiently large to allow the capacitor to supply the current needed to retain the stored time and temperature data in the RAM through prolonged power interruptions without the need for a backup battery.

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First Claim