The protection circuit of 555 fridge(2)

See as Figure 13-7, the circuit includes step-down rectified circuit, turning on delay circuit and controllable silicon control circuit. In the circuit, the connection of C4 and R4 can help with power failure protection. Because when it starts to electrify, the voltage on C4 can not be changed suddenly, which makes 555 reset, and the 3-pin is in a low LEV, SCR is blocked and the outlet is out of power. When C4 charges the negative pole of the capacitor, i.e the 2-pin is 1/3VDD lower than the trigger LEV, 555 is set, 3-pin is in a high LEV, SCR is conducting, and the outlet gets power. The delayed time of starting is td=1.1R4C4, and the delayed time of parameters indicated in the figure is about 6 min.