Fuse Help

[Rajinder1268]

Hi,
I am starting a automotive project, where the electronics will be powered from the 12V car battery.

The load current of my design is 0.9A maximum. I am looking to use a resettable fuse and have selected the following from Littel fuse
nanoASMDCH050F/24.
This has a working voltage of 24V (car battery charging can be at 14.4V), I trip of 2.5A. It is meant for automotive applications.
Looking at the rerating curve it mentions that at 80C, the trip current will fall to 60%. So 60% of 2.5A would be 1.5A, hence the circuit will have
protection at this temperature range. Does my theory look ok?

I am looking to add the fuse from the 12V supply, then the fuse and possibly looking at a reverse protection diode i.e. SBR Super Barrier Rectifier

I would appreciate any help or advice.

 

[KlausST]

Hi,

It has a hold current of just 0.5A. This is too low for your application.

Mind: it has a power dissipation of 1.7W, thus you may expect a current of about 150mA when tripped.

Klaus

 

[dick_freebird]

What about automotive circuit breakers, which are with the fuses in any
day-glo auto parts chain store? I use these on troublesome branches of
my vehicles. Self-resetting, though, rather than like residential circuit
breakers,

 

[Rajinder1268]

Hi,

It has a hold current of just 0.5A. This is too low for your application.

Mind: it has a power dissipation of 1.7W, thus you may expect a current of about 150mA when tripped.

Klaus

Hi you are correct the hold current is too low for my application. I missed that. I have seen the following instead MF-LSMF185/33X.
Hold current is 1.85A, trip at 3.7A.
The only issue is that defeating at 85C the hold current is 0.85A.
Is there anything else I have missed out when selecting the fuse?

--- Updated Apr 27, 2022 ---

Hi,

It has a hold current of just 0.5A. This is too low for your application.

Mind: it has a power dissipation of 1.7W, thus you may expect a current of about 150mA when tripped.

Klaus

--- Updated Apr 27, 2022 ---

Hi,

It has a hold current of just 0.5A. This is too low for your application.

Mind: it has a power dissipation of 1.7W, thus you may expect a current of about 150mA when tripped.

Klaus

--- Updated Apr 27, 2022 ---

Hi,

It has a hold current of just 0.5A. This is too low for your application.

Mind: it has a power dissipation of 1.7W, thus you may expect a current of about 150mA when tripped.

Klaus

--- Updated Apr 27, 2022 ---

Hi you are correct the hold current is too low for my application. I missed that. I have seen the following instead MF-LSMF185/33X.
Hold current is 1.85A, trip at 3.7A.
The only issue is that defeating at 85C the hold current is 0.85A.
Is there anything else I have missed out when selecting the fuse?

--- Updated Apr 27, 2022 ---

--- Updated Apr 27, 2022 ---

--- Updated Apr 27, 2022 ---

Hi
I am a bit confused. If the power dissipation is 1.7W then at 12V, it is around 142mA. I always thought the fuse would trip at the iTrip value? Am I missing something?

 

[KlausST]

Hi,

I am a bit confused. If the power dissipation is 1.7W then at 12V, it is around 142mA. I always thought the fuse would trip at the iTrip value? Am I missing something?

A poly fuse is a PTC.
This means on overcurrent it gets hot and thus it´s resistance increases. But it does not get "very high ohmic" it needs a lot of current to keep it´s temperature high. Mind: "when tripped".
It needs about 150mA to keep it´s temperature high.


Klaus