I'm curious to know if there is a sane way to determine the specifications of a blown SMD fuse without resorting to manufactures schematics / repair manuals.

In my particular case there are no obvious markings on either the board or the component (naturally).

  • \$\begingroup\$ Do you have a model number for whatever it is the fuse is part of? \$\endgroup\$ – Oli Glaser Jul 16 '12 at 6:23
  • \$\begingroup\$ It's a recharger for a battery powered electrical drill. Some brand I've never heard of (ie Chinese knockoff). I'll get more details when I get back out to the device, but I'm interested in the general case of this problem on its own merits. \$\endgroup\$ – nelsonda Jul 16 '12 at 21:20


Even for an intact fuse it's impossible to find out its rating in a non-destructive way if there's no marking. If it's blown it's even less possible.

You'll need the schematic to know the value. If you would reverse engineer the circuit to re-create the schematic you may have an idea of the value, but most often the designer has a certain range to choose from, depending on how cautious he is.

But without schematic it's not possible.

  • \$\begingroup\$ Unblown-fuse voltage drop at a given current gives some clue. \$\endgroup\$ – Russell McMahon Jul 16 '12 at 15:23
  • \$\begingroup\$ @Russell- A clue, yes, I agree, but not the right value. BTW, I was more thinking of the good old glass type fuses when I wrote that. Those hardly have a voltage drop. \$\endgroup\$ – stevenvh Jul 16 '12 at 15:28
  • \$\begingroup\$ Glass fuses and SMD fuses work by thermal fusing so require I^2R heating - so voltage drop is noticeable when fusing. I've tried to manufacture low current (sub 1A) fuses and found that the thinnest wire I had available (probably 45 gauge!) was thicker than needed. \$\endgroup\$ – Russell McMahon Jul 17 '12 at 5:26

No way I'm aware of, no.

If you have a manual and can find a current rating it may give you a clue as to what rating it was. Also, as Steven mentions, if you are able to poke around and reverse engineer the circuit a bit you should get a fair idea of what kind of fuse it would need.

If finding a schematic/service manual is impossible then you're a bit stuck. Maybe phoning the original manufacturers (if they are locatable) might be an idea - I've been successful in getting info this way on a couple of occasions when fixing stuff (it helps if it's not a current product they are very precious about) although mostly it's a dead end.


Measure the worst-case operating current and use a fuse maybe 2x this. Fusing is not a precise science, and as long as it is rated high enough to not blow in normal use, it won't be too critical.

  • \$\begingroup\$ As I understand it, because OP speaks of an SMD fuse, it's a fuse inside the circuit. I may be wrong. But in that case it may be hard to find out what worst-case is. \$\endgroup\$ – stevenvh Jul 16 '12 at 9:00
  • \$\begingroup\$ If the fuse is blown then there is a chance that something more is damaged. Measuring the current doesn't help much, since the worst case could be a shortened cap or something like that. \$\endgroup\$ – Bruno Ferreira Jul 16 '12 at 18:33
  • \$\begingroup\$ @BrunoFerreira I'm pretty sure that the fuse blew due to my own physical ineptness (ie I shorted something I should not have). I don't appear to have let the magic smoke out of anything else on the board, but I don't want to find out I'm wrong by igniting a real fire. \$\endgroup\$ – nelsonda Jul 16 '12 at 21:18

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