The SMCJ120A-13: That 1500W rating means it absorbs a transient surge up to 7.9A peak pulse current (Ipp) before the clamping voltage hits 193V — the voltage the protected rail sees during the event. The 120V reverse standoff voltage (working voltage) sits below the 133V minimum breakdown voltage, so the diode stays transparent during normal operation and only conducts when the transient exceeds the breakdown threshold.
DO-214AB SMC — board-fit and thermal
Housed in the DO-214AB (SMC) surface-mount package, the SMCJ120A-13 mounts on standard PCB pads with a copper area sized for the 1500W transient thermal pulse. Single unidirectional channel per device, so for bidirectional protection on an AC signal line you would pair two back-to-back or select a bidirectional variant from the SMCJ family.
The base product number SMCJ120 covers the family — the -13 suffix indicates Tape & Reel packaging for automated pick-and-place assembly.