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My setup is two DC-DC converters (hobby BECs) supplies, one acting as a backup for the other. A relay, powered by the primary BEC will automatically switch to the secondary BEC if the output from the primary one goes dead.

Now, I want to protect both BEC's against overvoltage (in case they fail to short the main 18V power supply to my 5V devices). For this I was planning to use a combo zener and polyfuse (or even a polyzen if I can find a suitable one), but I want one that is non-resettable, i.e. when it blows it blows catastrophically. Can I achieve this by replacing the polyfuse with a normal fuse?

The output from the BEC at 5V will be no more than 5A. Diagram of my planned setup

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It sounds like you need a crowbar circuit. Essentially a circuit that will deliberately cause a short circuit should an overvoltage condition occur. The short should then blow a fuse.

But it sounds like you need high reliability in your application. With your current design, when the relay switches there will be a brief period with no output available. And the relay is a single point of failure.

Consider connecting both BECs to the 18V power supply, via individual fuses. have separate crowbar circuits on the output of each BEC. Then diodes to recombine the outputs together.

This allows the BECs to share the current during normal operation. And should one fail, there is zero transfer time for the other one to start supplying your 5V load.

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Why such brute force protection?

Why not have smart OVP shut down in each supply?

Redundant supplies should b4e more reliable by redundancy if not unstable from current sharing under low load.

Brute force {aka crowbar} OVP is usually an SCR with zener to blow a fuse fast.

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