Thermostat with load activation detection feature

Abstract

A thermostat for control of an AC-operated HVAC unit (or a unit providing only heating or only cooling) is provided with load activation detection sensing for increased reliability of latching relay activation. The thermostat includes a sensing transformer in series with the heating or cooling load (or both, if both are in the system), so that, when activation of the load is called for by the thermostat, current flows in a primary winding of the transformer, inducing a current in the secondary winding. A voltage derived from the current in the secondary winding is sensed by the thermostat controller and used to determine whether the heating or cooling load has been properly activated or deactivated. Sensing of the AC power source in the HVAC unit may also be provided, so that the controller can confirm that the absence of the voltage derived from the current in the secondary winding is actually due to the state of the latching relays rather than to a failure of the AC power source. When the sensed voltage derived from the current in the secondary winding does not correspond to that expected when the latching relays are in their expected states, the thermostat controller provides additional pulses, twice as long in duration as the original pulses and emitted at spaced intervals, to attempt to correct the fault by placing the relays into their correct states. Power for the thermostat controller may be derived, at appropriate times, from the current flowing in the secondary winding.

Dependencies are inferred from the claim text.