Control device with current protected solid state relays
A control device, such as a smart thermostat, employs solid state relays as switches to activate and deactivate systems controlled by the device. Current flows through at least some of the solid state relays are monitored to determine the bulk heating produced in the solid state relays, and their associated circuitry and printed circuit board traces, and this determined amount of bulk heat is added to other determined amounts of bulk heat and is used to compensate the reading provided by temperature sensors within the control device which have been affected by the bulk heat. Further, by measuring the current flow through the power buses to one or more of the solid state relays of the control device, potentially damaging over current conditions can be distinguished from permissible transient over-current conditions and the control device can deactivate any solid state relays which would be damaged while allowing solid state relays which are experiencing allowable transients to remain operating. In the case of a severe over current condition, a current monitoring device can issue a fault signal, triggering an interrupt condition which will cause a processor in the controller to shut down the affected solid state relays very quickly.
1. A control device for controlling one or more electrical subsystems in accordance with a control program, comprising:
a housing;
a memory storing an operating program;
at least one environmental sensor within the housing to determine the temperature within the housing;
at least one solid state relay operable, when activated, to connect at least one electrical subsystem to a supply of power and further operable, when deactivated, to disconnect the at least one electrical subsystem from the supply of power;
at least one current measuring device to determine the amount of current flowing from the supply of power to the at least one electrical subsystem when the at least one solid state relay is activated; and
a processor operable to execute the operating program to: determine the amount of bulk heating produced by the determined amount of current and an associated predetermined electrical resistance; sum the determined amount of bulk heating with the amount of bulk heating produced in the housing by other components to obtain a determined sum of the bulk heating in the housing; compensate the determined temperature value by an amount corresponding to the determined sum of the bulk heating in the housing to obtain a compensated determined temperature value; and to activate and deactivate the at least one solid state relay to activate and deactivate the at least one electrical subsystem as needed to maintain the compensated determined temperature value within a preselected range of desired temperature values;
wherein the control device comprises at least two solid state relays, each of said at least two solid state relays controlling current flow from the supply of power to a respective electrical subsystem and having a predetermined respective electrical resistance, the at least one current measuring device determining the total amount of current flowing from the supply of power to each respective electrical subsystem through the respective solid state relays, the processor further operating to determine the portion of the determined total amount of current flowing through each of the at least two solid state relays and to determine the amount of bulk heating produced by the determined proportion of the total current and the respective predetermined electrical resistance to obtain the determined amount of bulk heating.