What this Snubberless Alternistor triac is for
The STMicroelectronics T435-800B-TR is a Snubberless™ Alternistor triac rated for 800 V off-state voltage and 4 A RMS on-state current in a surface-mount DPAK package. The Snubberless construction means it commutates inductive loads — motors, solenoids, transformers — without the external RC snubber network that standard triacs require, saving board space and two passives per phase. The Alternistor subtype (also called a high-commutation triac) gives better dV/dt immunity than a standard triac, which matters when switching inductive loads off at the zero-cross. Gate trigger current is 35 mA max, hold current 35 mA max, so a standard microcontroller GPIO with a transistor driver can fire it.
4 A RMS and 800 V — sizing the load
The 4 A RMS on-state current is the continuous load limit at a given case temperature; derate from 125 °C junction downward. For a 230 VAC resistive heater or incandescent lamp, 4 A gives about 920 VA. For an inductive motor or valve, budget a 20–30 % derating for inrush and back-EMF, keeping the steady-state load under 3 A RMS. The 800 V off-state voltage covers 277 VAC line-to-neutral with a 2.5× safety margin for mains transients. The non-repetitive surge rating of 30 A at 50 Hz and 31 A at 60 Hz handles capacitor inrush or motor start events — a single-cycle surge, not a repetitive condition.
DPAK package and mounting
Housed in a TO-252-3 DPAK (DPak, SC-63) with two leads plus a tab. The tab is the main terminal (MT2) and carries the thermal path. Surface-mount assembly with the tab soldered to a copper plane on the PCB is expected; without that thermal relief, the junction temperature rises quickly above 1 A continuous.
The DPAK footprint is shared with other Snubberless triacs in the same series, so a cross to a higher-current variant (e.g., T835-800B) is a board-layout compatible option if the load grows.