Setpoint limiting for thermostat, with tamper resistant temperature comparison

摘要

A thermostat for heating or cooling, operative to maintain an indoor air temperature to a limited setpoint range, is provided with a comparison circuit to prevent tampering intended to exceed a setpoint limit. A first temperature sensor responds to ambient air temperature and can include a first thermistor. A second thermistor is partly thermally isolated from the air, and senses a reference temperature that lags temperature changes in the ambient air. The two thermistors form a voltage divider coupled to one input of a differential amplifier comparator, the other input of which is coupled to a fixed voltage reference. When the ambient and reference temperatures exceed a predetermined temperature difference, the differential amplifier operates a relay that breaks a circuit including the contacts. This occurs when locally cooling the ambient air to generate a spurious call for heat (or vice versa). Two differential comparators can be included to sense positive or negative differences between the ambient and reference temperatures, and can be set to respond to different temperature differences using jumpers. A third temperature sensor can be similarly coupled to a low temperature responsive differential comparator that inhibits operation of the tamper-resistant circuit at an absolute low safety threshold temperature (i.e., to permit a call for heat), and/or coupled to a high temperature responsive comparator, to block a call for heat redundantly, above a high temperature threshold.