Zero-crossing trigger circuit design based on thyristor

　　Circuit principle: The zero-crossing synchronization pulse is a pulse at the zero-crossing moment of the 50Hz AC voltage , which can trigger the conduction of the thyristor when the AC voltage sine wave crosses zero. The zero-crossing synchronization pulse is generated by the zero- crossing trigger circuit . A more detailed circuit diagram is shown in the figure. In this figure, the voltage comparator LM311 is used to convert the 50Hz sinusoidal AC voltage into a square wave. The positive edge and negative edge of the square wave are used as the input trigger signals of the two monostable triggers respectively. After the two narrow pulses output by the monostable trigger are mixed by the diode or gate, the process corresponding to the AC 220V mains power can be obtained. Zero sync pulse. On the one hand, this pulse is added to the temperature control circuit as a trigger synchronization pulse for the thyristor , and on the other hand, it is also added to the T0 and T1 terminals of the 8031 ​​as a counting pulse.