25 A RMS, 1.2 kV off-state — the load-switching envelope
The T2550-12G-TR is an STMicroelectronics Snubberless™ alternistor triac rated for 25 A RMS continuous on-state current and 1.2 kV off-state voltage. That 25 A figure is the thermal limit in a 25 °C ambient with the D2PAK tab properly sinked — derate for enclosure temperature and airflow. The 1.2 kV blocking voltage covers 480 VAC line-to-line with margin for switching transients in motor-drive and power-converter circuits. Non-repetitive surge capability is 240 A at 50 Hz and 252 A at 60 Hz — this handles the inrush from a motor start or capacitor bank charge without exceeding the device's peak current rating.
Snubberless alternistor — inductive load without the RC network
The Snubberless™ alternistor structure means this triac commutates inductive loads — motors, solenoids, transformers — without the external snubber RC circuit that standard triacs require. The internal turn-off characteristic prevents false triggering from the dV/dt that an inductive load generates at zero-crossing. For a PCB layout engineer, this saves two components and the board area they occupy, and eliminates the snubber resistor's power dissipation in the enclosure. The D2PAK (TO-263-3) surface-mount package has a large exposed copper tab on the bottom — the tab is the main current path and the primary thermal path. Solder the tab to a copper pour on the PCB with thermal vias to inner-layer planes. The junction-to-case thermal resistance is low enough that a 25 A RMS load at 125°C junction requires the tab held below 100°C, which typically means a 1 oz copper pour of at least 10 cm² on a two-layer board.