What this TRANSIL™ TVS does on the board
The STMicroelectronics SMAJ100A-TR is a unidirectional transient voltage suppressor from the TRANSIL™ series, built on Zener technology. It is designed to clamp voltage spikes on a 100V DC rail — the reverse standoff voltage (100V) is the maximum steady-state voltage it can sit across without conducting. When a surge hits, it breaks down at a minimum of 111V and clamps the peak to 212V at the rated pulse current (11A for an 8/20µs waveform).
The SMAJ100A-TR is officially marked obsolete. STMicroelectronics no longer manufactures this exact order code. For new designs or repair of existing boards, you will need to source from the independent surplus and broker channel.
Key ratings for the repair bench
If you are troubleshooting a board where this part is scorched or shorted, the 100V reverse standoff tells you the rail it protects — measure the DC bus voltage; if it exceeds 100V in normal operation, the TVS was underspecified. The unidirectional channel means it only clamps positive spikes relative to ground; a bidirectional version would be needed for AC or bipolar rails. The DO-214AC (SMA) footprint is common and easy to hand-replace with a hot-air station — just watch the polarity marking (cathode band) when placing the new part.