Solid state relay harvesting power from load by measuring zero crossing

A solid state relay control circuit, comprising:

• a voltage detection circuit, having an input coupled to receive an output of a charging capacitor, and having an output arranged to generate a LOW voltage signal when a voltage level of the charging capacitor is below a low threshold value;
• a zero crossing circuit, arranged to couple to a first relay line and a second relay line, and having an output to generate a clock signal when a zero crossing event takes place between the first relay line and the second relay line;
• and
• a switch toggle circuit, having a plurality of inputs that are coupled to receive the clock signal and the LOW voltage signal, respectively, wherein after the LOW voltage signal is output, when the clock signal generated, the switch toggle circuit outputs a control signal to change an external switch from an ON state to an OFF state;
• wherein the switch toggle circuit further comprises:
• an inverter, coupled to the output of the voltage detection circuit;
• and
• a D-flipflop circuit, comprising:
• a set input, coupled to an output side of the inverter;
• a D input, coupled to the output of the voltage detection circuit;
• and
• a clock input, coupled to an output of the zero crossing circuit.