The ST DB3TG is a DIAC — a bidirectional trigger diode — with a breakover voltage range of 30 to 34 V and a peak output current rating of 2 A. It is used in phase-control circuits to fire a triac gate at a precise voltage threshold, common in light dimmers, motor speed controls, and heater regulators. The DO-35 axial package suits through-hole assembly on single-sided boards or point-to-point wiring in legacy power modules.
The 30-34 V breakover window sets the firing threshold for the triac; a DIAC with a tighter or shifted range would change the conduction angle at a given RC timing constant. The 15 µA breakover current is low enough that the timing RC network does not need a high-current driver, keeping the control-side component count down.
ST officially lists the DB3TG as obsolete. It is no longer in volume production, so new designs should not target this order code.