Two terminal line voltage thermostat

• US 5,456,407 A
• Filed: 03/25/1994
• Issued: 10/10/1995
• Est. Priority Date: 03/25/1994
• Status: Expired due to Term

• Current Assignee: Electric Power Research Institute Incorporated
• Original Assignee: Electric Power Research Institute Incorporated
• Inventors: Ingalls, Jim E., Hoglund, Steve R., Stalsberg, Kevin J.
• Primary Examiner(s): Wayner, William E.
• Application Number:
  US08/218,484
• Time in Patent Office: 564 Days
• Field of Search: 219/501, 236/46 R, 165/12, 307/66
• US Class Current: 236/46.00R
• CPC Class Codes: G05D 23/1902 characterised by the use of a variable reference value
• CPC Class Codes: G05D 23/24 the sensing element having a resistance varying with temperature, e.g. a thermistor

Abstract

A two terminal line voltage thermostat includes a switch which effectively connects line voltage to a heater load. The entire process is controlled by an integrated circuit microcontroller which is powered by a rectified voltage from a transformer secondary connected to a primary which is in series with the heater load. Backup battery power is provided to maintain limited functions of the microcontroller in the event of overall power loss. The microcontroller is programmed to meet the temperature sensing and control requirements specific to a two terminal electric baseboard heating installation.

Claims

1. A two terminal line voltage thermostat including a switch for connecting said line voltage across a heater load connected to one of its terminals with the other of the two terminals connected to said line voltage, said thermostat comprising:

a transformer having voltage and current primary windings and a secondary winding, said switch in a heater on condition energizing only said current winding and in a heater off condition energizing both said voltage and current windings;

blocking capacitor means connected to said voltage winding for providing a high impedance series circuit to said heater load with said switch in said heater off condition;

a relay coil for actuating said switch between said heater on and heater off conditions and connected to said current winding;

temperature responsive means receiving power from said secondary winding in both said heater on and heater off conditions;

integrated circuit controller means responsive to said temperature responsive means for actuating said relay coil and connected to and receiving power from said secondary winding;

backup battery means connected to said integrated circuit controller and energizing said controller in response to a loss of power from said secondary winding, including means for preventing any current drain from said battery when power is being received from said secondary winding;

said means for preventing any current drain including diode means connected to said secondary winding, for regulation to a common d.c. power output of a predetermined level for said integrated circuit controller and said temperature responsive means;

said battery means having a voltage less than said predetermined level and including a diode series connecting said battery means to said common d.c. output to provide an effective open switch when said predetermined level is higher than said nominal battery voltage whereby current drain is prevented;

comparator means for comparing said battery voltage to said common d.c. output level, and for providing a binary output signal when said common d.c. output level drops in value below said battery voltage.

2. A thermostat as in claim 1 where one input to said comparator includes said common d.c. output and the other is the common d.c. output dropped by a diode in series with such output, such diode also being coupled to said series connected diode to said battery means.

3. A thermostat as in claim 1 where the binary output of said comparator drives an input of said integrated circuit controller means which is responsive to said output of said comparator means to indicate a common d.c. power loss, said integrated circuit controller means having a halt mode and a hold mode, said halt mode being activated in response to said power loss to cause said integrated circuit controller to retain only its random access memory (RAM) and time-of-day function and to eliminate all other power using functions, said integrated circuit controller including timing means for providing a timeout of a predetermined number of hours to switch to a hold mode where only RAM retention occurs and said time-of-day is not supported, whereby a significant amount of energy is saved, thus extending the life of said battery.

4. A thermostat as in claim 1 where said controller means interfaces with a keypad and display for customer programming and operational status display.

5. A two terminal line voltage thermostat including a switch for connecting said line voltage across a heater load connected to one of its terminals with the other of the two terminals connected to said line voltage, said thermostat comprising:

a transformer having voltage and current primary windings and a secondary winding, said switch in heater on condition energizing only said current winding and in a heater off condition energizing both said voltage and current windings;

blocking capacitor means connected to said voltage winding for providing a high impedance series circuit to said heater load with said switch in said heater off condition;

a relay coil for actuating said switch between heater on and heater off conditions and connected said current winding;

integrated circuit controller means for actuating said relay coil;

temperature responsive means receiving power from said secondary winding in both said heater on and heater off conditions, said temperature responsive means including a thermistor spaced from a mounting board to reduce temperature sensing errors due to thermal coupling between the mounting board and thermistor leading to higher sensitivity to undesirable immediate temperature swings in the vicinity of the thermostat, said integrated circuit controller means requiring relatively less said sensitivity and thus incorporating time delay filter means for smoothing rapid changes in the temperature sensed by said thermistor and for subtracting a previously sensed temperature by the thermistor from a newly sensed temperature, this difference being multiplied by a selected constant and then added to the previous temperature to provide a filtered sensed temperature, said constant by which the difference is multiplied being selected to provide an effective time constant of minutes, said integrated circuit controller means including a temperature control routine requiring said time constant to provide integral control to eliminate droop and anticipation control to reduce swings in the temperature provided by said heater load.