Control device with bulk heating compensation
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 two solid state relays, each of the at least two solid state relays operable, when activated, to connect respective electrical subsystem to a supply of power and further operable, when deactivated, to disconnect the respective electrical subsystem from the supply of power, each of the at least two solid state relays having a predetermined respective electrical resistance;
at least one current measuring device to determine the total amount of current flowing from the supply of power to each respective electrical subsystem when the respective solid state relay is activated; and
a processor operable to execute the operating program to:
determine the proportion of the determined total amount of current flowing through each of the at least two solid state relays;
determine the amount of bulk heating produced by the determined proportion of the total current and the respective predetermined electrical resistance to obtain a determined amount of bulk heating;
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; and
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.
2. The control device of claim 1 wherein each of the respective electrical subsystems are part of an HVAC subsystem.
3. The control device of claim 2 wherein each respective solid state relay comprises: a pair of FETs; a charge pump to deliver a control voltage to the gates of the FETS from the processor; and a fast shutdown circuit, operable to connect the output of the charge pump to ground when the fast shutdown circuit detects that the control voltage is less than a predefined value.
4. The control device of claim 1 wherein the processor monitors the value of the determined total amount of current reported by the at least one current measuring device and compares that value to a predefined set of threshold values comprising a current value and a permitted time value, and the processor deactivates the at least two solid state relays if a threshold value is exceeded for more than the corresponding permitted time value.
5. A method of controlling, with a control device, a HVAC system comprising at least two electrical subsystems:
measuring, with a temperature sensor located within a housing of the control device, a measured temperature;
determining the sum of the bulk heating created by components and power flows within the housing, comprising the steps of:
measuring the total current flowing through at least two solid state relays to respective electrical subsystems, each of the respective electrical subsystems having a predetermined respective electrical resistance;
determining the proportion of the total current flowing through each of the at least two solid state relays,
determining the amount of bulk heating produced by the determined proportions of the total current and the respective predetermined electrical resistances to obtain a determined amount of bulk heating;
determining a bulk heating value corresponding to bulk heating created by other components within the housing; and
summing all determined bulk heating values to obtain a total bulk heating value; and
compensating the measured temperature to correct for the determined total bulk heating value to obtain a compensated temperature value.
6. The method of claim 5 wherein each respective solid state relay comprises: a pair of FETs; a charge pump to deliver a control voltage to the gates of the FETS from the processor; and a fast shutdown circuit, operable to connect the output of the charge pump to ground when the fast shutdown circuit detects that the control voltage is less than a predefined value.
7. The method of claim 5 wherein the processor monitors the value of the determined total amount of current reported by the at least one current measuring device and compares that value to a predefined set of threshold values comprising a current value and a permitted time value, and the processor deactivates the at least two solid state relays if a threshold value is exceeded for more than the corresponding permitted time value.