Fuses are wonderful electrical protective devices. They work till they don’t, and in the case of glass cartridge fuses, looking at the remains when they blow can give you some insight into what happened at the moment of fault (prolonged overload, dead short, slow overheating, etc).
Or in this case, uh, what
Today’s contestant: a 1.25 amp with some delay characteristic out of a switching power supply in a bookshelf stereo.
One look at this told me there was no need to fear a big nasty fault with the power supply. It went out very, very gently, in fact, STRANGELY so.
If you see the element slumped, that indicates it was running hot a while.
The element blowing up and becoming silvered to the glass indicates a high current fault. Often that’s a shorted rectifier bridge or caps when it happens on a switching supply.
What follows is an attempt to get a photo of this under the microscope.
VERY unusual. Note that the fuse wire itself looks perfectly fine and the fault looks like it occurred without any serious heat.
I really just don’t get it. My best guess is the fuse wire actually cracked instead of melting, possibly due to long term thermal cycling or vibration.
The alloy bead is a heat sinking feature to give the fuse element a time delay curve. As heat builds up on the fine wire it will be absorbed through the connections to the end caps and to this blob. Once it gets the blob hot, the delay time ends and a sustained overload will melt the element. Of course, a high current fault can always blow the element to slag in a very quick instant.
See, this all makes sense, right? Here’s something that doesn’t… a CrapTrex Freedom SW unloading undocumented fault codes like a bag of soda cans at the recycling center.