why did this fuse blow?

[willbanks]

i know how fuses work and why they blow up, but I have no idea why this particular one did.

I salvaged a voltage step down converter that turns AC into 12VDC. I plugged it into the nearest wall socket, checked its voltage range with a provided trim pot, all worked fine.

But when i brought it over to my workbench, I plugged it in once again, but this time into a 3 way wall outlet splitter. My iron was plugged into one of the inputs (it was on) and the other my scope (turned off)

i plugged it in the third slot and it exploded after a few seconds (it was so powerful i thought i saw green!)

But why did it blow? It could of been a fluctuation in the electricity but I don't think that's the case because all my other devices were fine.

[Vtile]

The problem is introducing a current/power in magnitude that is on the edge of the fuses characteristics curve and it took some time to melt (blow)? I'm shooting to the dark here.

[willbanks]

but it's been plugged in for much longer in different outlets. i thought it has something to do with the soldering iron being on at the same time.

[jeroen79]

What brand/model is the power supply?
Is just the fuse blown?
Or is anything else damaged?
Does the power supply work again when the fuse is replaced?

I don't see how connecting the power supply parallel to a soldering iron in the same mains powerstrip would make it's fuse blow.
But if you want to know for sure just test it again with and without the soldering iron.

[willbanks]

doesn't say. i replaced fuse with a lesser rated voltage but same amp. it said 250 for EU, 125 for US. works fine now.

[Shock]

Perhaps inrush current, also the core may have been in an energized state and when powered on again (at a point during the cycle) that caused the transformer to run well into saturation (or exceeded the fuses rating).

[Brumby]

Have you tried replacing the fuse (with one of the correct rating) and trying again?

If it blows in a short space of time, then you have likely had a hard fault develop - and it's going to blow fuses for the rest of its days, unless the fault is found and corrected.

If it seems to be working fine, then it may be the original fuse failure was due to some coincident circumstance of the state of the unit and the incoming power - such as Shock suggested - possibly including an overvoltage or transient event.

Also - don't assume there was no transient involved just because your other gear didn't suffer.  They may have been designed well enough to survive it - and your 12V supply not.

[Zero999]

doesn't say. i replaced fuse with a lesser rated voltage but same amp. it said 250 for EU, 125 for US. works fine now.

As long as the replacement fuse is rated to the voltage in your area and can handle the fault current, then it should be safe.

Fuses often blow because they degrade over time, especially if they're repeatedly subjected to current surges or operated close to the tripping current.

[Brumby]

doesn't say. i replaced fuse with a lesser rated voltage but same amp. it said 250 for EU, 125 for US. works fine now.

  I need to read things more carefully.

[PA0PBZ]

doesn't say. i replaced fuse with a lesser rated voltage but same amp. it said 250 for EU, 125 for US. works fine now.

If there was an EU fuse in there it was probably too light: half the voltage is double current to get to the same power.
Did you try to find if the Amp rating is different for 250/120V?