I have just got an old super socket 7 AT motherboard. The AT keyboard
is not longer working. I find out that the fuse for the keyboard is
open.
The rating on the fuse is 3A.
I got a lot of dead AT motherboards, but the fuses has no marking at
all.
Question1:Is the current important in replacing the fuse?
Question2: Is it safe to use any old fuse in the junk AT board and put
it to the super socket 7 motherboard?
1) Yes. The width of the copper track that runs to the fuse,
was designed for a certain level of current. If you make the
fuse a higher value, there is a small possibility of the track
getting burned. It would require a partial short at the keyboard
to do that, and isn't too likely. (Copper track width is selected
based on delta_T rise for the track. For example, I might select
a track width, such that 3 amps could flow, with the copper track
being 30C hotter than ambient. If the combination of the ambient
temp, plus the delta_T, gets to the "glass transition temperature"
for the FR-4 PCB, then the motherboard would become brittle or the
physical properties of the fiberglass would change. A good piece of
circuit board, might have a Tg of 135C or so. The PCBs come in all
different values, and more expensive material has a higher Tg.
I don't know how tight the Taiwanese are in their selection of
delta_T - narrow tracks mean they can do a denser layout of the
motherboard.)
The keyboard connector pins are likely more sensitive to an overcurrent
situation, than the copper track in the motherboard. Some of
those might be rated for 1A of current or so. (Just a guess on
my part, as I'm too lazy to spend an hour trying to look it up.
Looking up connectors is a PITA.)
2) At the level of currents involved, if you swap fuses, probably
relatively safe at the "fire insurance level". If you happened
to grab a fuse with a higher current rating, and there was a
short or partial short in the keyboard, the fuse might not blow,
and the keyboard connector or a track in the motherboard might
cook. But since all the fuses were intended for keyboard usage,
I would expect them to all be in the same ballpark. You aren't
likely to end up with a 10A fuse for example.
You should be able to find fuses on Digikey, Mouser, Newark etc.,
in fact you'll likely find too many fuses to make an easy choice.
My experience with fast blow fuses, is they tend to fatigue and
blow without provocation. We used to do stuff like place a 5A fast
blow in a 1A circuit, to prevent nuisance tripping. The fuse would
still blow plenty fast, if you dropped the ground from a oscilloscope
probe, onto the circuit, so we still had protection. A regular or
a slow blow fuse, will likely last longer, but obviously won't blow
as fast in a short or partial short. I might use something like a
regular blow fuse, especially if the end user of the equipment is
known to be smart enough not to plug in keyboard/mouse when the
system is powered
The reason for the existence of slow blow
fuses, is where you know the load draws a slug of current, every
time the circuit starts up. An example might be in a motor circuit.
with a high locked rotor current.
Currently designed motherboards all tend to use Polyfuses, which
have a material that changes states when it gets hot. Such fuses
recover when the material cools down, which means fewer occasions
to replace them. Good for motherboard makers who offer an extended
warranty. I don't know what kind of manual soldering times such
things will take. Solder profiles are carefully controlled on
circuit boards, and it is hard to match them when hand soldering.
Paul
