Google for an article called fuseology.  Very good info

Gordon Williams <gwilliams@ncf.ca> wrote:

>Hi,
>
>I have put together two 12V SLA battery based power supplies for an IP
>cameras that has a 5 V and a 48 V DC converter.   For protection I have a
>500mA slow blow fuse and about 375 mA average draw.
>
>One of the two power supplies has had it's fuse blow after a short period =
of
>time (maybe 30 turn ons and 40 hours operating time).   The other is worki=
ng
>OK so far.
>
>The DC-DC converters have a electrolytic caps on their input and output.
>The is several hundred uF of capacitance.
>
>I think what is blowing the fuse is the inrush current.  I know that there
>is a noticeable spark when I touch the battery power lead to the circuit.
>
>Are there rules that I should be following to size the fuse properly?  I'm
>concerned about reliability.
>
>Thanks,
>
>Gordon Williams
>
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