What this TVS diode protects and where it goes
The STMicroelectronics SMAJ28CA-TR is a bidirectional Zener-type transient voltage suppression diode from the TRANSIL™ SMAJ series, designed to clamp voltage spikes on 28 V nominal rails. The DO-214AC (SMA) surface-mount package suits automated assembly on standard PCB footprints.
28 V rail protection — the voltage window
The reverse standoff voltage of 28 V means this diode will not conduct at the normal operating rail voltage, so it sits across a 24 V or 28 V supply without leakage issues. Breakdown starts at 31.1 V minimum, and clamping is guaranteed at 59 V maximum at the peak pulse current of 39 A (8/20 µs waveform). That 31 V to 59 V window gives the designer a clear margin: the protected circuit must survive a 59 V pulse, and the rail must never exceed 28 V steady-state.
With a single bidirectional channel, the SMAJ28CA-TR clamps positive and negative transients symmetrically. That saves a diode pair on AC signal lines or on DC rails where reverse-polarity events are possible. No power line protection is built in — this is a pure transient clamp, not a crowbar or a filter.