Low power and low emi power stealing circuit for a control device

A power stealing circuit for a control device controlling a supply of power from an AC power source to a load, the control device comprising a first switch having a first state in which the first switch connects the load to the power source for supplying an electrical current from the power source to the load and a second state in which the first switch disconnects the load from the power source, the power stealing circuit comprising:

• a second switch arranged in parallel electrical connection with the first switch for diverting the electrical current away from the power source with the first switch in the second state;
• a charge storage device coupled to the second switch and adapted to store therein an electrical charge when the electrical current is diverted away from the power source;
• and
• a linear semiconductor device arranged in parallel electrical connection with the first switch and adapted to control a rate of change of a first voltage across terminals of the first switch upon the first switch being brought to the first state.

A method for controlling a supply of power from an AC power source to a load, the method comprising:

• diverting an electrical current away from the power source;
• storing an electrical charge in a charge storage device upon the electrical current being diverted away from the power source;
• comparing a voltage across terminals of the charge storage device to a threshold value;
• if the voltage reaches the threshold value, outputting a control signal for activating a linear semiconductor device, the linear semiconductor device, when activated, controllably lowering a line voltage supplied by the power source towards zero volts;
• and
• applying, when the line voltage approaches zero volts, a first pulse on a gate terminal of a switch arranged in series electrical connection between the power source and the load to trigger the switch.

A two-wire line voltage thermostat for controlling a supply of power from an AC power source to a load, the thermostat comprising:

• a switch arranged in series electrical connection between the power source and the load, the switch having a first state in which the switch connects the load to the power source for supplying an electrical current from the power source to the load and a second state in which the switch disconnects the load from the power source;
• a power stealing circuit arranged in parallel electrical connection to the switch and adapted to divert the electrical current away from the power source with the switch in the second state, the power stealing circuit comprising a linear semiconductor device adapted to control a rate of change of a first voltage across terminals of the switch upon the switch being brought to the first state;
• and
• a communications module adapted to receive one or more input radio-frequency signals indicative of instructions to operate at least one of the load and the thermostat.