SCR has two trigger modes: phase shift trigger and zero crossing trigger trigger. SCR as a kind of AC components, there are bidirectional and single-phase SCR, because of bidirectional SCR bidirectional, so in the positive and negative power supply can be conductive, so it is often used for AC regulating load circuit, bidirectional SCR general control mode zero crossing trigger and phase shifting trigger two kinds.

1. Zero crossing is triggered

As the name implies, zero crossing trigger, which triggers the thyristor at zero crossing. Because of the plus or minus half cycle, ac must pass through zero point to minus half cycle or from minus half cycle to minus half cycle in half cycle. The average output power of SCR can be changed by changing the conduction cycle number within a certain period of time to achieve the effect of adjusting the load power. The cycle number refers to the complete change of the alternating current, that is, the time that the sinusoidal waveform goes through called the period. This is similar to regulating the output of the motor pulse width modulation signal. The more times of conduction in a certain time, the greater the average output power.

2. Phase shift flip-flop

Phase shift triggering is changing the starting or ending position of the circumguide to adjust its output power or voltage. In fact, the thyristor conduction flux is controlled by controlling the thyristor conduction Angle. For example, the figure below shows that the 100% power output and 50% power output waveforms, and the 50% output voltage waveforms are defective sine waves.

3. Similarities and differences between zero crossing trigger and phase shift trigger

1) Both can realize stepless adjustment of power, voltage and other output loads;

2) Zero-crossing trigger principle is to change the thyristor conduction cycle number, the output waveform is still sine wave, phase shift trigger is to control the thyristor conduction Angle to control the thyristor conduction flux, output waveform cutoff;

3) Zero-crossing trigger is characterized by low frequency interference. Due to intermittent work, easy to flicker, not suitable for continuous current motors and other occasions. But the phase shift trigger fluctuation is small, and the output current and voltage are relatively smooth. But there is also a problem, because of voltage distortion, easy to produce electromagnetic interference, sometimes in EMI is not easy.