What can a thyristor do?
A thyristor is a switch that turns on when a certain threshold is exceeded on its gate circuit, and remains on until the current flowing through the switch is reduced below a threshold. The behaviour is very similar to a vacuum tube device called a thyratron. One of the most common applications is a controlled rectifier, meaning one that can be turned on rapidly during the half cycle of the AC signal, hence the name SCR for silicon controlled rectifier. Another related device is a triac, which operates with either switch current polarity. These are both used in simple lamp dimmers or drill speed controllers sometimes. A bit difficult to say that nothing else can do these things, as there are usually many ways to do the same things in engineering, and this is no exception. However if you wanted something to turn on a (DC) device to a trigger pulse, and it should stay on until switched off, this is an easy way to do it. In the past thyristors have been used for relatively large inverters
