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Fuses in parallel

Discussion in 'Electronic Repair' started by Lenny, Jun 17, 2004.

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  1. Lenny

    Lenny Guest

    This is probably going to sound like a stupid question but in a pinch
    is it OK for example if you need to replace a .250A MDL ( or any type
    fuse for that matter), to install two .125A MDL fuses in parallel?
    Thanks, Lenny Stein, Barlen Electronics
     
  2. john

    john Guest

    Yep...OK

    kip
     
  3. Bill Jeffrey

    Bill Jeffrey Guest

    Lenny -

    You will still be protected, meaning that no more than 1/4 amp can flow,
    but you will be subject to a lot of nuisance pops. Two fuses, even if
    they are nominally identical, don't have the same resistance. And two
    fuse holders, even if they are nominally identical, don't have the same
    resistance. So the current through the two fuses won't be identical -
    it will divide in (inverse) proportion to the resistances.

    Say for example, that one fuse+holder combination has a resistance of
    100 milliohms, and the other has a resistance of 50 milliohms. The
    current will divide, 1/3 passing through the first one and 2/3 through
    the second one. As a result, the second one will be overloaded while
    the total current is still in bounds. In this example, when the total
    current is 1/4 amp, the current in one fuse is 83 mA, and the current in
    the other is 167 mA. Pow - the second one will blow, and then the first
    will blow, even though the total current is OK.

    Bill
     
  4. Now and then

    Now and then Guest

    you are a funny guy Ha Ha Ha
     
  5. Asimov

    Asimov Guest

    "Lenny" bravely wrote to "All" (17 Jun 04 12:06:49)
    --- on the heady topic of "Fuses in parallel"

    Le> From: (Lenny)

    Le> This is probably going to sound like a stupid question but in a pinch
    Le> is it OK for example if you need to replace a .250A MDL ( or any type
    Le> fuse for that matter), to install two .125A MDL fuses in parallel?

    Be sure and put equal value swamping resistors in series with each to
    make certain they share the current evenly... ;-)

    Just kidding! Sure, parallel them... (seriously)

    A*s*i*m*o*v

    .... Bald spot? It's a solar panel for a sex machine.
     
  6. john

    john Guest

    Do you have a problem with my correct answer ?
    kip
     
  7. Not necessarily - thre nominal current is usually much lower than the
    current where the fuse blows, and the protection is OK
    (But your explanation is of course correct)
     
  8. Franc Zabkar

    Franc Zabkar Guest

    I'm not laughing, but your answer is incorrect. One should never
    parallel two fuses for the same reason that one should not parallel
    two diodes or two transistors, this reason being that there is no
    guarantee that current will be shared evenly.

    See Bill Jeffrey's answer for a better explanation.
    http://groups.google.com/groups?selm=bRmAc.182380$&output=gplain


    - Franc Zabkar
     
  9. john

    john Guest

    True! But close enough till the correct fuse can be purchased.

    kip
     
  10. Lenny

    Lenny Guest

    In my case the fuses were MDL slow blow type and selected. They
    measured 21.05 and 21.12 ohms respectivly. I paralleled them by
    soldering with half inch # 24 solid wire. One of them was then clipped
    into the fuse holder. The extra length of wire connecting the other
    fuse I suppose in theory may have increased that fuses' resistance but
    if at all minutely I'm certain. I operated the equipment under worst
    case conditions and everthing seems to be OK. It never occurred to me
    (although admitedly it should have) that a rated fuse could be
    something other its actual rating. I appreciate all the advice from
    everyone particularly about matching resistances. Lenny Stein, Barlen
    Electronics
     
  11. Also keep in mind that fuse ratings are not exactly very precise and also
    vary based on type. The specifications for one type might say something
    like: Time to blow at rated curret: 4 hours; time to blow at 1.5X rated
    current: 15 seconds. It's not that a 1 A fuse will blow instanly if
    1.000001 A is put through it!

    So, worrying about matching fuses in this case may be obsessive compulsive
    behavior. If the equipment was running so close to the fuse ratings that
    it matters, the equipment was poorly designed in the first place. Parallel
    the fuses for now but plan on installing a proper replacement.

    --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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    To contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  12. Asimov

    Asimov Guest

    "Lenny" bravely wrote to "All" (19 Jun 04 06:13:28)
    --- on the heady topic of "Re: Fuses in parallel"

    Le> From: (Lenny)

    Le> In my case the fuses were MDL slow blow type and selected. They
    Le> measured 21.05 and 21.12 ohms respectivly. I paralleled them by

    Wow, I didn't realize they had that much resistance! Well... I suppose
    they must have *some* to act as fuses. This means I'd have to be extra
    careful measuring the resistance of such low current fuses with an
    analog ohmmeter for fear of blowing them with the internal current.

    A*s*i*m*o*v

    .... That was a fascinating period of time for electronics
     
  13. Franc Zabkar

    Franc Zabkar Guest

    I was taught that the current rating of a fuse specified the maximum
    continuous current it could handle without rupturing. Theoretically, a
    10A fuse protecting a 9.5A continuous load should never fail.

    Have a look at the "Fuse Time-Current Characteristics" in Table 1 of
    this interesting primer:

    http://www.wickmannusa.com/download/fuseology.pdf

    Depending on the fuse type, a fuse can last forever when operated at
    100% of its rated current. At 135% it survives for < 1hr, at 200% for
    < 2min, and at 1000% for < 20msec.


    - Franc Zabkar
     
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