1.5SMC91AT3G — what it is and where it sits
The 1.5SMC91AT3G is a unidirectional Zener-based transient voltage suppressor in the DO-214AB (SMC) package, rated for surface-mount assembly.
Surge clamping capability — what 125V clamping means for the design
The maximum clamping voltage of 125 V at a peak pulse current of 12 A (10/1000 µs) sets the let-through voltage the downstream circuit must survive when a surge arrives. At 12 A the device clamps cleanly within that ceiling; the minimum breakdown of 86.5 V defines the point at which conduction begins, so the protection window between standoff (77.8 V typ) and clamping (125 V max) is the margin the board designer evaluates against the transient threat level. For a board running near the upper end of that band, the clamping voltage spec still applies at rated peak current; derating for continuous thermal conditions is a layout question, not a spec sheet gap.
Lifecycle — what obsolete means for your BOM
No L* cross-reference, no second-source designation — the BOM line needs a sourcing plan, not a reorder confirmation. For new designs this part is not a viable first choice; for legacy boards under repair or MRO re-supply the window for last-time-buy is already closed through authorized channels. Independent distribution and surplus traders are the remaining route — availability is lot-specific and lot traceability is not guaranteed through the manufacturer. Confirm the datasheet revision and lead-free / RoHS status against the board's certification requirements before committing to any independent-source stock.