What this Zener is and where it lands on the board
The STMicroelectronics SM30T33AY is a single-direction Zener diode from the TRANSIL™ family, designed to clamp voltage transients on 28 V rails. It is rated for 3 kW peak pulse power (10/1000 µs waveform) and packs into a DO-214AB (SMC) surface-mount package. The 28 V reverse standoff means it sits across a supply line without conducting until a surge pushes it into breakdown at 31.1 V minimum; the clamp holds at 45.4 V maximum while shunting 66.1 A peak pulse current.
The 3 kW peak pulse rating (10/1000 µs) is the headline number a designer uses to size the protection for a given surge energy. For a 28 V rail, this part can absorb a transient that would otherwise punch through a downstream regulator or controller. The 66.1 A peak current tells you the diode can handle a short, high-energy event without failing short — the usual failure mode for a Zener that is undersized for the surge. Because it is a single unidirectional channel, it protects one polarity. If the application needs bidirectional clamping on an AC-coupled line or a signal pair, you would use two back-to-back or a dedicated bidirectional TVS.
Package and rework — DO-214AB (SMC)
The DO-214AB (SMC) body is a standard footprint for automotive-grade TVS diodes. It is large enough to handle the 3 kW pulse without excessive die temperature rise, and the J-bend leads give a visible solder fillet after reflow. No orientation ambiguity.