Trisil

For general Silicon TRISIL - a voltage-controlled triac without gate - if we don't know the type.

A Trisil is trade name for a thyristor surge protection device, an electronic component designed to protect electronic circuits against overvoltage. Unlike a transient voltage suppression diodes, such as Transil, it acts as a crowbar device, switching ON when the voltage on it exceeds its breakover voltage.

A Trisil is bidirectional, behaving the same way in both directions.