Turning ON a Thyristor
There are 4 quadrant sensitive gate type Thyristors with max voltage ratings and there are 3 quadrant types which are better for commutation and preventing back EMF from false triggering. In both cases, however dv/dt can trigger false latching and low R and medium sized plastic cap called a snubber circuit or LPF for high voltage are required to avoid that.

You can try two discrete transistors to make an SCR and examine the Absolute Maximum Ratings to fully appreciate how this latch works.


from Wiki....

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