The 1N6171AUS: It clamps a 121.6 V reverse standoff rail at 218.4 V, shunting up to 6.9 A of surge current before the downstream circuitry sees the overvoltage. The SQ-MELF (C) package is a surface-mount ceramic barrel — the same footprint as a standard MELF diode but with a square cross-section for better solder-joint inspection.
At 1500 W peak pulse power, this part absorbs a 6.9 A surge. The 218.4 V clamp voltage is the ceiling the rail sees during the event — if the protected circuit's absolute-max rating sits below that, you need a lower-voltage TVS or a secondary clamp. The 152 V minimum breakdown voltage gives margin above the 121.6 V standoff — that window keeps the diode off during normal line ripple and only fires when a real transient arrives.
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The base product number is 1N6171; the 'AUS' suffix denotes the SQ-MELF package variant.