PTC resettable fuse issue

[acoles1619]

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*)]

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.

 

[acoles1619]

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*)]

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.