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555 high-low voltage protection delay circuit

2015-01-27 16:36  
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555 high-low voltage protection delay circuit
When the input voltage is higher than the set value, VT2 turns on, 555's 2 foot is low level(<1/3VDD), 555 is set, J is released, J1-1 is cut off, A, B terminals' output is cut off; when the mains voltage is lower than the set value, the voltage of 555's 2 foot is divided by D6, RP2, its voltage is lower than 1/3 VDD, then 555 is set, J is released, A, B terminals' output is cut off. At the same time, C5 is charged by R5, when it is charged to 2/3 VDD, 555 is reset, its delay time is decided by the time constant of R5, C5. The delay time td can be changed by adjusting R5C5. When it reaches the delay time, if the input voltage is still lower than the set value, then 555 is also in reset state. The needed low voltage set value can be got by adjusting RP2.