1500W peak pulse, 256V standoff — the surge protection ceiling
The 1.5KE300A is a 1500W Zener TVS diode from Littelfuse's 1.5KE series, designed to clamp transients on DC rails up to 256V reverse standoff. This is a unidirectional device — one diode per package, cathode band marked. It's meant for DC bus protection where the rail stays positive relative to ground; for AC or bidirectional clamping you'd need two in series or the bidirectional variant.
DO-201 axial package and thermal range
Housed in a DO-201AA / DO-27 axial leaded package, also referred to as DO-201 supplier package. The through-hole body handles the 1500W pulse without the thermal constraints of a surface-mount package — the leads conduct heat into the PCB copper and the air around the body. Junction temperature range spans -55°C to 175°C, which covers both cold-start industrial environments and hot engine-bay or power-supply compartments. No power line protection feature — this is a standalone transient suppressor, not a filtered TVS array.
What the ratings mean for your rail
The 285V minimum breakdown is where it starts to avalanche; the 414V clamp is where the voltage is held during the surge. If your downstream circuitry can't survive 414V for the 10/1000µs pulse duration, you need a lower-voltage TVS or a secondary clamp.