Zener TVS diode — 1.5 kW peak pulse, 77.8 V standoff
The 1.5KE91ARL4G: Clamping is specified at 125 V maximum at the 12 A peak pulse current (10/1000 µs waveform), so a 77.8 V nominal bus sees the surge energy absorbed before the voltage spike reaches the 125 V ceiling. The single unidirectional channel handles one polarity; for AC or bidirectional lines a different part is needed.
Thermal and package constraints for the BOM
Operating junction temperature spans -65 °C to 175 °C, covering military and industrial extremes. The DO-201AD axial package (through-hole) requires a lead-formed footprint on the PCB — the body diameter and lead spacing match standard 0.100-inch pitch layouts. Power line protection is not supported (the part lacks the bidirectional or low-capacitance structure needed for high-speed data lines). It is a bulk transient suppressor for power inputs, relay coils, or DC motor terminals where the 10/1000 µs waveform represents the typical surge from inductive kickback or lightning-coupled energy.
Obsolete — sourcing through independent channels
Littelfuse has marked the 1.5KE91ARL4G as obsolete. The Mosorb™ series covers a wide voltage range in the same DO-201AD footprint. A functionally similar Zener TVS with the same standoff voltage and package can be cross-referenced, but no pin-compatible direct replacement from Littelfuse is documented. A board-level qualification is advised before substituting.