RE: Appropriate fuse value for expansion port

From: Bil Herd <>
Date: Sun, 7 Mar 2010 21:16:44 -0500
Message-ID: <>
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.

Fusing the expansion port may otherwise be a little problematic:  For example if the main supply is fused or maxes in the case of current regulated at 5A,  and if the main board pulls 2A, then a 4A fuse on the expansion would not trip before the supply maxes out. Unfortunately a 3A fuse will eventually blow (most of the time) if used right at 3A.  Basically you have multiple fuses in series with the same load and they are really there to prevent 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 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.

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.


-----Original Message-----
From: [] On Behalf Of Jim Brain
Sent: Sunday, March 07, 2010 8:45 PM
To: CBM Hackers
Subject: Appropriate fuse value for expansion port

Anyone have knowledge of an appropriate value for a fuse to put on the
C64/C128 Expansion port?  I suspect the REU requires the most draw from
the port, but I can't find a useful value.


Jim Brain, Brain Innovations                                      (X)
Dabbling in WWW, Embedded Systems, Old CBM computers, and Good Times!

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Received on 2010-03-08 03:00:03

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