14 Mar
2013
14 Mar
'13
2:24 p.m.
This had me scratching my head. Then I happened to
notice that the fuse
(F1 on the 12V supply board) was glowing a dull orange and getting brighter.
Well, turns out that someone (me!) had installed the wrong fuse -- it was a
2.5A fuse where a 10A was required (F2 takes the 2.5A, btw). Apparently
when fuses start to give out they draw a lot of current -- lesson learned
:).
Not quite.. Remeember the fuse is in series iwt hthe laod and carries the
load current.
THink of how a fuse works. A fuse melts when the current through it gets
too high. The fuse wire melts, of course,becuase it gets too hot. And
that heat comes from the power -- I^2*R -- disipated in the fuse wire.
So a fuse must have some resistance and disipate power to work. In
general, lower-current fuses have higher resistanec than thigh current
ones. So your 2.5A fuse probably had a higher resistane than the correct
10A one, and thus got hot at hte notmal operating current. Having a
higher resistane, it also dropepd more voltage at that current than the
correct oen would have done.
Fuse replaced and all is happy. So, the lesson is: check your fuses
carefully.
Dre I admit that I once spent a good few minutes attemtpign t ofind out
why a coplece PSU weas giving no outputs at all only for find that some
rotten sod ahd 'borrowed' the fuse from the mains plug (UK BS1363 mains
plugs ahve an inte4rnal cartridge fuse). ARGH!
-tony