I am using a buck converter to convert 12v to 5v and some devices run on 5 v and some in 12v and i don't want my 5v device to get damaged if the buck converter fails. Is there any way to make 5v device safe from 12v
Why do you think the buck converter might fail?
I mean, there is such a thing as a crowbar circuit, but before fixing a problem, it helps to understand if and how the problem can happen. Personally, if I were worried about it, I'd start by only using buck converters from reputable manufacturers.
That is a very broad question. Please narrow it down, post an annotated schematic showing the system and possible points of entry for the 12V. Is it just 12V or possibly more. How much current is available. How much overshoot is allowed on the 5V? Is this a steady state 5V or a pulse. Can it be less then 12V? This is most of the information needed.
I am using 2 weigand rfid card reader of 12v and nodemcu to read the data from it the 5v devices are relay ,nodemcu and level shifter .Currently i am providing power supply of 12v 1 amp adapter . I need something to break the circuit incase buck converter fails ,currently i have buck converter that is not much reliable .The schematic i have uses 7805 i changed that part with buck converter due to heat issues
Your best solution is to replace the buck converter. The next best thing is to use a crowbar circuit. Put this in the circuit in front of the Buck Converter. Note this shorts the power supply so you will need either a fuse or a current limited power source.
How do i choose the fuse that is suitable for my circuit
That should not be to hard, be sure the fuse is handling the current of the project and the power source has enough power to blow the fuse. You can look into poly fuses as a non replaceable fuse.
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