Re: Appropriate fuse value for expansion port

From: Jim Brain <>
Date: Sun, 07 Mar 2010 21:15:28 -0600
Message-ID: <>
On 3/7/2010 8:16 PM, Bil Herd wrote:
> Hi Jim
> One school of thought is that the expansion port is fused against shorts by virtue of the fuse in the main power supply.
I think the Cardco cart expander and FOTIOS' FXB-3 put a fuse on the 
expansion board as a way to make it easier to replace a fuse without 
opening the unit.
> shorts, they don't do well at trying to just tell if your pulling too much current as they are too gross for that kind of discrimination.
I was considering the use of PTCs, not a regular fuse.
> I also don't recommend a fuse on the regulated side of a supply, at that point you don't want any resistive components in series with the high current spike supply rails, you will drop voltage across the fuse element.
I am OK with that, but I have noticed that other designs put the fuse 
there.  A PTC has a small resistance (.003 ohm) under normal load.
> But otherwise if you wanted to figure a ballpark, take the power supply spec in A or W and subtract the measured amount that the main board is using. The expansion port budget that is what will be left.
I'm torn.  Many of the designs include a fuse, though I agree it is 


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