4-layer switching elements. They are normally turned off and you can turn them on by impulse on the gate or by increasing the voltage on the thyristor electrodes above the breakdown voltage. The thyristor switches off when the current flowing through it drops below the threshold value.
It is mainly used in AC systems as a two-state actuator, e.g. in regulators (lighting, rotational speed), in converters. The regulation is based on switching on the thyristor with a delay in relation to the zero crossing of the sinusoid. The waveform of the undercut sine wave has a lower RMS value, which is reflected e.g. in the brightness of a light bulb. The thyristor turns off at the next zero crossing.
In DC circuits, it is used, for example, in security systems (as a key that short-circuits and burns fuses if the voltage on the system exceeds the permissible value.
A thyristor with a reduced breakdown voltage without a gate is called a Diac (and is used in thyristor control circuits)
Two thyristors connected in parallel (A1 with K1 and A2 with K2) placed in one housing and having a similar gate are called Triac.
