What the 1500W rating means on the bench
The 1.5SMC39A is a single-direction Zener TVS diode from Littelfuse's 1.5SMC series, rated for 1500 W peak pulse power at 10/1000 µs waveform. That 1500 W figure is the maximum energy it can clamp in a single transient — for a 28.2 A peak pulse current (Ipp), the voltage clamps at 53.9 V max before the part self-destructs. The reverse standoff voltage is 33.3 V, meaning the diode stays off below that rail voltage and only starts conducting when the transient pushes past the 37.1 V minimum breakdown threshold. This makes it a fit for 24 V or 28 V DC buses where you need the clamp to engage before the downstream regulator's abs-max rating.
Package and field-swap reality
The body is large enough to hand-solder with a standard iron if you are careful — no hot-air station required for a board-level repair. Polarity is marked by a cathode band on the body; the anode is the tab side, so orientation is one-look obvious.