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How to calculate current to melt wire?

Discussion in 'Electronic Design' started by Michael, May 23, 2008.

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  1. Michael

    Michael Guest

    Hi there - I found an interesting failure on a PCB today. A large part
    of one of the leads of a TO-220 was simply gone. That pin was carrying
    a fairly high voltage and got shorted to ground.

    Does anybody know how much current it would take to blow a TO-220 pin
    like this? Additionally, is there a rough calculation for how to find
    out how much current would actually melt a wire in open air?


  2. Tom Bruhns

    Tom Bruhns Guest

    Of course it depends a lot on how the wire gets rid of heat; if it's
    in air and the air is still it will be considerably less current than
    if there's high velocity cool air flowing over it. OTOH, the power
    goes as the square of current--and in fact as something a bit higher
    than that because the resistance of the wire goes up with temperature,
    too--so the rated fusing current won't be too far off from what you'll
    actually get. Google "fusing current" and you'll get quickly to
    tables and papers--including one that addresses fusing current of PC
    traces. Obviously, fusing current for a thin, wide rectangular
    conductor of given cross-section will be different than for a round
    wire of the same material in an otherwise similar environment.

  3. Guest

    It's an I-squared t hing I believe.
  4. Phil Allison

    Phil Allison Guest


    ** According to you, the pin did not melt.

    What likely happened is that a DC arc formed between the pin and whatever
    had been shorting it to ground - just as that short was broken. Heat
    generated by the arc then melted through part of the pin.

    The heat production of a DC arc is given by V times I - a much larger
    value than I squared R, when R is only a fraction of a milliohm.

    ...... Phil
  5. Tim Williams

    Tim Williams Guest

    Ballpark 100-200A. I recall discussion here some time ago concerning those
    MOSFETs that claim something like 120A peak drain current.....

  6. Eeyore

    Eeyore Guest

    In short, no.

  7. Rich Grise

    Rich Grise Guest

    You're shooting birds out of a cannon? With HE for propellant?

    Wouldn't you want something more like cordite or ordinary BP, so
    that you shoot whole birds rather than bird molecules? ;-)

    (in college once, we blew up a hamster with an M-80; we found
    one piece, and the rest was just gone.)

    Would it be possible to get videos? ;-)

  8. John KD5YI

    John KD5YI Guest

    Mostly urban legend, according to:

  9. John KD5YI

    John KD5YI Guest

    Yes, Jim, I know birds are actually fired into aircraft windshields and
    engines. The part about the British being so stupid as to not thaw the bird
    first is the legend.

  10. neon


    Oct 21, 2006
    open air ? when is wet it will never blow the current will be bypassed. my guess that pin has a bad joint or a pass to somewhere else from there. what goes out of one pin must have come in from another pin a bad joint will have resistance causing IR drop causing heat at the pin.
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