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The simple power-off self-lock circuit

2017-01-06 07:48  
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The simple power-off self-lock circuit
When the grid is supplying power normally, it is like an ordinary switch. By pressing K1, the 220V AC is distributed to the dual-way SCR as the trigger voltage, which makes the SCR conducting. When the SCR is conducting, during the positive half-cycle period of the power supply, a little current is charging C through R4 and D, at the same time, after being distributed by R3 and R2, it charges the SCR; in the passive half-cycle, C discharges R3 and R2 and then triggers the SCR, so that the SCR keeps conducting, which makes sure the load work normally. Once the grid was power-off suddenly, C would be discharging through R3 and R2.