What this Zener does on the board
The STMicroelectronics 1.5KE440ARL is a 1500W peak pulse power Zener transient voltage suppressor from the 1.5KE TRANSIL™ series, designed to clamp overvoltage transients on AC or DC power rails. The through-hole DO-201AA axial package suits point-to-point wiring or PCB mounting with a traditional leaded footprint.
The clamping voltage hits 776V at the peak current of 13A (8/20 µs waveform), so the downstream circuitry needs to survive that clamp level. The 418V minimum breakdown voltage gives a guard band above the 376V standoff, meaning the part stays out of conduction during normal line ripple.
ST continues to manufacture the 1.5KE series, and this variant remains a standard catalog item.
Capacitance and signal considerations
The 360 pF capacitance at 1 MHz is typical for a 1500W-class Zener in this voltage range. On a 376V power rail that capacitance is negligible, but if the suppressor is placed on a data or communication line (e.g., a 376V-powered RS-485 bus), the added load can round off fast edges. For pure power-rail clamping, it is not a concern.