SCR AC voltage regulating circuit

The circuit below is a thyristor AC voltage regulation circuit. The 220V mains power is converted into three sets of 12V AC voltage through transformer B. One set (U3) serves as the working power supply of the circuit and the signal power supply formed by the sawtooth wave. The other two sets (U2, U4) serves as the synchronous power supply for silicon controlled thyristors SCRl and SCR2 respectively. The sawtooth wave voltage formed by the transistors VTl, VT2, resistors R1~R4, capacitor C1 and other networks is compared with the given signal by the operational amplifier IC1. The square wave signal is output, and then drives VT3 and VT4 through the photoelectric couplers V1 and V2, and triggers the corresponding silicon controlled thyristor SCRl or SCR2 with the synchronization voltage of U2 and U4 to control the voltage of the load RL. Adjust RP to change the given voltage, and the output square wave width of the op amp IC1 will change accordingly, thereby changing the conduction angle of the thyristor. When the RP boom is lowered, the potential of the inverting input terminal of the op amp IC1 decreases, and the output square transition time of the output terminal is advanced and the width is widened. After the isolation coupling of V1 and V2, the conduction time of VT3 and VT4 is advanced, and the thyristor The conduction angles of SC Rl and SCR2 increase, and the voltage obtained on the load RL increases. When the RP boom is raised. The potential of the reverse input terminal of the op amp IC1 increases, and the time for the output terminal to output a square wave is delayed. As the width becomes smaller, the conduction time of VT3 and VT4 moves later, and the conduction angles of thyristors SCRl and SCR2 decrease; the voltage obtained on the load RL decreases, thereby achieving the purpose of voltage regulation.

Working process: During the positive half cycle of the alternating current (the A terminal is positive and the N terminal is negative), the same terminal of the synchronization voltage U2 is positive and the opposite terminal is negative. After rectification by D4 and current limiting by R9, it provides the synchronization voltage to VT3, and the trigger can Silicon controlled SCRt is turned on. During the negative half cycle of the alternating current (the A terminal is negative and the N terminal is positive), the opposite end of the synchronization voltage U4 is positive and the same end is negative. After rectification by D5, R10 provides the synchronization voltage to VT4 after current limiting, triggering SCR2 to turn on. .