SCR MF Power Digital Trigger
The digital trigger features a phase-shifting approach using a clocked pulse count. The clock pulse oscillator of the digital trigger is a voltage-controlled oscillator, the output pulse frequency is controlled by the α-phase shift control voltage Vk. When Vk is lowered, the oscillation frequency increases and the number of the counter is fixed (256). The counter pulse frequency is high, which means that the time required to record a certain number of pulses is short, that is, the delay time is short, the α angle is small, and conversely the α angle is large. The moment the counter starts counting is also controlled by the synchronous signal, which starts counting at α = 0°.
Now assume that at a Vk value, according to the relationship between the control voltage and frequency of the voltage control oscillator to determine the output oscillation frequency of 25KHZ, then the time required to count to 256 pulses (1/25000) × 256 = 10.2 (ms), equivalent to 180 ° electrical angle. The trigger’s count clearing pulse is at 30° of the synchronous voltage (line voltage), which corresponds to the β=30° position of the three-phase fully controlled bridge rectifier circuit, and from the clearing pulse, a delay of 10.2 ms produces the output pulse, which is close to the α=150° position of the three-phase bridge rectifier circuit at one phase of the thyristor. The potentiometer W4 can be adjusted slightly if an accurate α=150° trigger pulse needs to be obtained. There are three identical trigger circuits, and the voltage control oscillator and Uk control voltage are common so that six trigger pulses with a phase difference of 60° are produced in one cycle.
The advantage of digital triggers is that they are stable, especially with HTL or CMOS digital integrated circuits, which can have a strong anti-interference capability.
The IC16A and its surrounding circuitry form a voltage-to-frequency converter whose output signal period varies linearly with the output voltage VK of the regulator. Here the W4 fine-tuning potentiometer is the lowest output frequency adjustment (equivalent to analog circuit sawtooth wave amplitude adjustment).