What a 1.5 kW standoff-voltage TVS actually protects
The 1.5SMCJ78C is a unidirectional Zener-type TVS diode rated 1500 W peak pulse at a 10/1000 µs waveform — meaning it survives a 1.5 kJ transient event when the pulse fits within that time envelope. The 78 V reverse standoff is the working voltage the protected rail normally sits at; the diode stays off and invisible until a surge arrives. When the transient exceeds the 86.7 V breakdown minimum, the device clamps the line to 139 V maximum at 10.8 A peak — that 1.79× clamping ratio determines how much voltage the downstream load actually sees during the event.
Package and thermal context
Mounted in the DO-214AB (SMC) surface-mount package, the part dissipates the transient energy into the PCB copper under the device. Junction temperature rating is -50°C to 150°C — this is the industrial and automotive transient suppression band, not a commercial-interior temperature grade. Thermal impedance from junction to ambient is governed by the pad layout; the -50°C floor means the part can be specified into equipment that sees cold storage or outdoor deployment before energisation.
What the part is not — and why that matters for selection
Bidirectional protection requires the bidirectional variant (1.5SMCJ78CA — the CA suffix denotes bidirectional).