Two-wire load control device for low-power loads

A load control device for controlling the power delivered from an AC power source to an electrical load, the load control device comprising:

• a thyristor having first and second main load terminals adapted to be coupled in series electrical connection between the AC power source and the electrical load for conducting a load current from the AC power source to the electrical load when the electrical load is on, the thyristor having a gate for conducting a gate current to render the thyristor conductive, the thyristor becoming non-conductive at the end of each half-cycle;
• a gate coupling circuit comprising a control input for receiving a drive voltage, the gate coupling circuit comprising at least one MOS-gated transistor operatively coupled between the first main terminal and the gate of thyristor for conducting the gate current, the at least one MOS-gated transistor operable to conduct the gate current through the gate of the thyristor in response to the drive voltage;
• a microprocessor operable to generate an on-state control signal that is representative of whether the load should be controlled on or off;
• an energy storage capacitor operable to produce a supply voltage for powering the microprocessor;
• a power supply operatively coupled in parallel with the thyristor and operable to charge the energy storage capacitor when the thyristor is non-conductive;
• and
• a control circuit operable to receive the on-state control signal generated by the microprocessor and to generate the drive voltage received by the gate coupling circuit, the control circuit operable to maintain the thyristor non-conductive when the magnitude of the supply voltage is below a predetermined threshold.

An electronic switch for controlling the power delivered from an AC power source to an electrical load, the electronic switch comprising:

• a controllably conductive device adapted to be coupled in series electrical connection between the AC power source and the electrical load;
• a microprocessor operatively coupled to the controllably conductive device for turning the electrical load on and off;
• an energy storage capacitor operable to produce a supply voltage for powering the microprocessor;
• an on-state power supply operatively coupled in parallel with the controllably conductive device and operable to charge the energy storage capacitor when the controllably conductive device is non-conductive;
• and
• a control circuit operatively coupled to the controllably conductive device for maintaining the controllably conductive device non-conductive when the magnitude of the supply voltage is below a predetermined threshold.