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I have an automotive application where I have this:

schematic

simulate this circuit – Schematic created using CircuitLab

Would the 20V 1W or 5W zener diode be enough together with a typical automotive fuse to protect the LCD drive in case there is a voltage spike. Fuse would be 2A as it should be enough for the normal operation of the driver and screen. The RBO40-40 clamps to 40V so there could be still some over voltage after it.

Edit: I have understood crowbar is useful for over voltage situations but in my case I am mostly worried about very short voltage spikes.

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    \$\begingroup\$ It won’t do much after the LDO and of the LDO survived the obervoltage in the first place, didn’t it clamp the output enough for you? \$\endgroup\$ – winny Feb 1 '18 at 15:31
  • \$\begingroup\$ RBO40-40 doesn't clamp enough, so there is the 40V > 25V situation. \$\endgroup\$ – Petteri Hietavirta Feb 1 '18 at 15:48
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What you propose is a lower voltage (20V) Zener in parallel with the internal Transil (40V), it will work up to 20V but common zeners aren't designed for transient protection. I use the RBO40-40 before the electronic module voltage regulator, and another TVS in the output of this regulator, matched with it's output voltage. The RBO40-40 is designed to protect electronics acording IEC/ISO standards, maybe you can use a high input voltage regulator and the TVS in it's output.

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