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ZERO_POINT_WITH_OVERVOLTAGE_PROTECTION

2016-09-14 16:46  
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ZERO_POINT_WITH_OVERVOLTAGE_PROTECTION
Used to protect voltage-sensitive load from excessive line voltage Switch section operates conventionally to turn on triac almost immediately after each zero crossing between half-cycles,For normal line voltages, SCR Q3 is off. When overvoltage condition is sensed during any half-cycle, SCR Q1 s turned on, discharging C2 and turning Q2 off. This allows Q3 to turn on and divert triac gate drive, removing power from load As long as overvoltage condition exists, Q1 is turned on each half-cycle and C2 is unable to charge enough to turn Q2 on,When overvoltage condition ceases, C2 charges to voltage set by D8 in about 20 ms, saturatingQ2 so Q3 turns off and Q4 turns on R2 can be set to allow line voltage variations from almost 0 to 11 v.-“Circuit Applications for the Triac,” Motorola, Phoenix, AZ, 1971, AN-466, p 14.