Power stealing for a thermostat using a TRIAC with FET control

1. An electronic thermostat circuit with improved power stealing for controlling an AC control device, said circuit comprising:

a source of AC control power coupled to an electronic switch means having an electronic switch means control input, the electronic switch means for controlling the AC control device;

a diode bridge means, the diode bridge having an AC connection and a DC connection, the AC connection electrically coupled to the electronic switch means control input, the diode bridge for controlling the electronic switch means by a DC control signal applied to the diode bridge means DC connection;

an amplifier means having an on state and an off state, the amplifier means electrically coupled to the diode bridge means DC connection, the amplifier means having an amplifier input for controlling the state of the amplifier and an amplifier output for generating the DC control signal;

an isolated gate FET means electrically coupled to the amplifier input for controlling the state of the amplifier, the isolated gate FET means further electrically coupled to a digital input signal, the digital input signal controlling the state of the DC amplifier means, the isolated gate FET means powered by the current derived from the source of AC control power by power stealing;

wherein the digital signal controls the state of the AC control.

2. The thermostat of claim 1 wherein the electronic switch means comprises a TRIAC.

3. The thermostat of claim 1 wherein the FET means comprises an enhancement mode P channel or an enhancement mode N channel FET.

4. The thermostat of claim 1 wherein the amplifier means comprises a composite Darlington amplifier.

5. The thermostat of claim 1 wherein the digital input signal is generated by a microcontroller.

6. The thermostat of claim 1 wherein the AC control device is selected from the group consisting of a solenoid, a relay, and a solid state AC switch.

7. The thermostat of claim 1 wherein the isolated gate FET means power steals less than one milli Amp.

8. An HVAC system including an electronic thermostat circuit with improved power stealing requirements to control an AC control device, said circuit comprising:

a source of AC control power coupled to an electronic switch having an electronic switch control input to control the AC control device;

a diode bridge, the diode bridge having an AC connection and a DC connection, the AC connection electrically coupled to the electronic switch control input, the diode bridge for controlling the electronic switch by a DC control signal applied to the diode bridge means DC connection;

an amplifier having an on state and an off state, the amplifier means electrically coupled to the diode bridge DC connection, the amplifier having an amplifier input for controlling the state of the amplifier and an amplifier output for generating the DC control signal;

an isolated gate FET electrically coupled to the DC amplifier input to control the state of the amplifier, the FET further electrically coupled to a digital input signal, the digital input signal controlling the state of the DC amplifier, the digital signal controlling the state of the AC control;

wherein the FET is powered by a power stealing circuit, the power stealing circuit deriving DC current from the source of AC control power, the FET drawing current from the power stealing circuit.

9. The thermostat of claim 8 wherein the electronic switch comprises a TRIAC.

10. The thermostat of claim 8 wherein the FET comprises an enhancement mode P channel or an enhancement mode N channel FET.

11. The thermostat of claim 8 wherein the amplifier comprises a composite Darlington amplifier.

12. The thermostat of claim 8 wherein the digital input signal is generated by a microcontroller.

13. The thermostat of claim 8 wherein the AC control device is selected from the group consisting of a solenoid, a relay, and a solid state AC switch.

14. The thermostat of claim 8 wherein the FET power steals less than one milli Amp.