Automotive-rated Zener TVS for 18V rail protection
Rated for 400W peak pulse power (10/1000µs waveform), it handles 59A of surge current in the 8/20µs pulse shape.
The 20V minimum breakdown voltage means the device starts conducting at 20V, clamping any transient above that. With no power line protection (it is not designed for continuous overvoltage), this is a pure transient suppressor for signal or low-power supply rails.
Package and footprint — DO-214AC SMA
Housed in a DO-214AC (SMA) surface-mount package, the SM4T21AY is hand-solderable with a fine-tip iron or hot-air station — the two-terminal body is about 4.5 mm long, with a cathode band on one end. The SMA footprint is common across the SM4TY family, so swapping between voltage variants (e.g., SM4T20AY, SM4T22AY) is a direct PCB layout match.