I think with this type of inrush its best to power the supply with the load
connected and the n connect the battery ( we do this with UPS that run on
400v battery) the sizing of a fuse isn't likely to save the semis anyway it=
s
only really there for protection in event of a short circuit (semi already
failed)

Nail... the semis will protect the fuses anyway

Steve

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Gordon Williams
Sent: 03 March 2012 21:39
To: pic microcontroller discussion list
Subject: [EE] Slow Blow Fuse Sizing

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 o=
f
time (maybe 30 turn ons and 40 hours operating time).   The other is workin=
g
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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