I am adding
over-current protection in my circuit. My circuit/PCB has many DC-DC converters and these converters power a processor and various other ICs. The input voltage range (that I want to allow but my circuit can withstand more than that) is
7-12V and current will be 350mA (at 12V) or 600mA approx (at 7V).
The over-voltage protection will be provided by Crow-Bar circuit and fuse. The over-current protection should be provided by fuse alone.
After reading about various PTC fuses, I got to know they have
maximum Voltage Rating. After reading more, I got to know may be PTC fuses are not good choice for over-voltage protection but I want to know what will happen if we exceed it's voltage rating?
Will it open or short? I guess if it gets open-circuit after applying voltage more than it's rating than it's still useful for my circuit as it will act as PTC re-settable fuse below 60V (rating of my fuse) and as conventional fuse for voltages more than 60V.
The acceptable voltage range to my circuit/PCB (although the circuit can withstand more voltage) is 7-12V but I have selected a PTC re-settable fuse with maximum voltage rating of 60V (
After reading document of my PTC fuse, I got to know PTC fuses has to maintain it's internal temperature, then a short-circuit event at 60V (caused by crowbar circuit) will have higher resistance (and lesser current) than a short-circuit (actual short-circuit event) at 7V. This means at 7V, there will be more current flowing through my circuit (few hundred ohms) and I am not sure if it can cause some issue? Please help.
Also, if the user applies 60V and my TRIAC gets triggered (shorting power rails), then due to very high resistance of PTC fuses (at 60V), a current less than holding current of TRIAC will be flowing which may turn-off my TRIAC. I can sense that TRIAC will keep on turning-on and off. Am I correct? If yes, then how to fix it?