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The simple self-lock switch circuit

2017-01-08 20:28  
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The simple self-lock switch circuit
See as the figure, before AN is pressed, SCR1 is blocked, the relay or contactor is still, which is equal to OFF state. When AN is pressed, with the help of R1,AN and VD1,the 12V voltage provide for SCR1 with trigger current which makes it conducting, so K gets power and the contactor is connected with the load and gets into work. As the voltage on the two terminals of the capacitor can't mutate, so SCR2 is not conducting when AN is pressed, the current is charged to C by R3 and VD2. If AN is pressed again, the voltage on C is becoming the trigger voltage of SCR2, which makes SCR2 conducting when AN is blocked.