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PTC resettable fuse issue

Hey guys,

I am currently working on a project where I am using PTC resettable fuses as a way of cutting of current to an output. However, the lithium battery that I am using trips before my fuse and thus cuts off power to my entire box. I am using a fuse with a 2 amp hold current and 4 amp trip current, the battery is good for 4.6 amps. Any idea where I am going wrong? Is the trip time too slow possibly? It has a trip time of 0.5 seconds. Hopefully someone can help!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
If the lithium battery has one of those protection circuits in it then it will trip *very* fast.

Possibly faster than a traditional fast blow fuse, probably faster than a slow-blow fuse, and almost certainly faster than a PTC resettable fuse.
 
Thanks for the reply, what kind of protection circuit would be used? Would you be able to point me in the right direction of what kind of circuitry I should add to address this problem? The more simple the better as I am working with a limited amount of space. Thanks.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
It all depends on what you're doing.

You might be able to use a current sense resistor that turns on a transistor when the voltage drop hits approx 0.6V. The transistor could then romm base current from a pass transistor, or gate voltage from a mosfet.

Either solution will drop some voltage (greatest if you use a bipolar transistor) and both will dissipate quite a bit of heat if the output is shorted.
 
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