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did this fuse blow funny?

Discussion in 'Electronic Design' started by [email protected], Jun 27, 2007.

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  1. Guest

    i have a 1 amp , glass, fast blow, 1 1/4" fuse on the hot wire going
    into a 65W SMPS. It blew on several units being tested, but its not
    clear to me why. And the way the fuse "blew" was strange. Its seems
    more like it "broke" off at the "weld" inside one of the metal cups.
    You cant see where the fuse blew, but if you shake the fuse the
    element bounces oscillates inside like a tuning fork. Its still
    connected to one of the metal cups inside. Anyone ever see something
    like this? There is no blackening of the glass or any other indication
    the fuse blew.
    And its not clear why a 1A fuse would blow on a power supply whos max
    input current is supposed to be 72W. (60A inrush)

    Its the LPS53-M by Astec
  2. My guess is that the input side of the supply is a rectifier
    and filter capacitor, with little to limit the inrush
    current that charges the cap very fast if the power is
    applied at the peak of the line cycle. The pulse does not
    last long enough to melt the fuse element, but gives it a
    large mechanical sag-and-retighten because of the rapid
    temperature change. This repetitive bending and tensioning
    at the wire ends, eventually fatigues the metal and it lets go.

    Most such supplies require a slow blow fuse, possibly one
    with a spring mechanism inside, that absorbs the expansion
    and contraction of the element with a material with a high
    fatigue resistance.
  3. Guest

    Most such supplies require a slow blow fuse, possibly one
    I keep reading and getting told by Astec that I should use a fast blow
    fuse. Whats the deal here? "slow blow does not adequately protect a
    SMPS". Is this true? I would think that with the inrush current specd
    at 60A, that a fast blow would be too sensitive, and a slow blow more
  4. If Astec is telling you that the fuse is there to protect
    the supply, they are admitting that the supply is poorly
    designed. Fuses are normally used to protect the wiring
    from overload after the supply has failed, shorted, and to
    protect the surroundings from fire caused by supply failure.
    Evidently, Astec is having lots of trouble with this model
    failing and they are trying to improve its chances of
    survival by using a twitchy fuse, instead of adding the
    proper inrush limiting system.

    Sounds pretty futile, to me.
  5. Eeyore

    Eeyore Guest

    60A inrush and you're surprised it blew ?

    I'm sure it should be a slow blow. Who specced the fuse ? Is it mounted in / on
    the PSU itself ?

    It's not is it ? Was it your idea to use a fast blow fuse ?

  6. Eeyore

    Eeyore Guest


    The job of a fuse is to prevent fire, not protect electronics.

  7. Eeyore

    Eeyore Guest

    Idiotic in fact.

  8. Or more generously, desperate, and grasping at straws.
  9. Rich Grise

    Rich Grise Guest

    Idiotic all the way to the bank, as long as they can get away with it. ;-)

  10. mpm

    mpm Guest

    In case anybody "cares", you do know you can unsolder the outside end
    caps on those glass cartridge type fuses and remove the remains of the
    fuse element wire....

    I "invented" this fuse repair job on a site so remote, that if it
    wasn't already on the truck, you really didn't NEED it....

  11. Guest

    Just a thought. If there is a poor connection on one end
    of the fuse, it could overheat on that end and blow it.
    I have an inexpensive IR thermometer that I use all the
    time to check equipment with. I can find an overheating
    fuse, breaker or connection quickly with this little jewel.
    I paid around $20.00 for it.

    [8~{} Uncle Monster
  12. joseph2k

    joseph2k Guest

    Sounds like a good way to Astec to lose UL, CSA, TUV, etc., safety labels.
    That's why accountants aren't allowed to do engineering.
  13. legg

    legg Guest

    I see an F1 located on the artwork...........

    I'd be more interested to note markings on the fuse. (mfr - series -
    rating) . As you've stated, the fuse is for fire protection/open trace
    protection/single-fault abnormal test compliance. If it's certified
    with one type of fuse that doesn't allow reliable operation at high
    line, then the whole product series is a dog and should be avoided

    Adding external inrush limiting might cut down on returns of current
    inventory. It looks loke hey are counting on the series resistance of
    the line filter components alone to do this. Normally a commercial
    design would try to satisfy the 35Amp peak ratings for commodity line
    switch-gear. This product's spec sheet shows that this has not been
    done and it's a case of buyer beware in that case. Safety approved
    NTCs in the 10 ohm range are available for PCB mounting - but would be
    a headache to incorporate as a retrofit nto external field wiring.

    Depending on how the fuse is mounted, Astec could be damaging it in
    the lead-forming or the wave-soldering process. Does it break always
    at the same end?

    Perhaps a more reliable mfr of fuse, with similar safety
    certifications is called for if you are stuck with the model. You'd
    need accurate info from the listed safety critical components in the
    mfr's actual certification folder to determine that the part you use
    complies. Rough to have to rework this prior to application.

    I've never stuck fast-blow fuses smaller than 2A into low power
    off-line switchers, and then it may be questionable whether a
    glass-bodied low-interrupt rating part is safe from explosive
    behaviour, when carefully tested to cover single-fault abnormals.

  14. Eeyore

    Eeyore Guest

    So there is.


    That makes more sense.

  15. Eeyore

    Eeyore Guest

    There is an NTC.

    It's marked TH1. As in thermistor.

  16. legg

    legg Guest

    I note also, on the board layout, that the printed fuse rating is
    "T3.15A". This is a slow-blow 3.15A fuse. I expect that's a mistake of
    some kind or another. It certainly shouldn't produce nuisance
    tripping, if used.

    2.5A also shows up in the neighbourhood. This kind of ambiguity in a
    fuse label would make certification a pain.

    I think you really need to examine the actual product in hand and get
    a copy of the test files for the model in question.

  17. Eeyore

    Eeyore Guest

    It wouldn't get past proper certification with an ambiguity like that.

  18. neon


    Oct 21, 2006
    a fuse of 1A never blows exactly at 1A it varies with imlementation heat sink and others. a power supply of 72w will require a slow blow fuse because of inrush current durng start up. it may survive once or twice depends on when you turn on the power ac wise.
  19. joseph2k

    joseph2k Guest

    Maybe the production units are not quite the same as the the qualification
    units. If so UL takes a very dim view of such. It wouldn't be the first
    time though, especially with accountants twisting managements arm.
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