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fusible resistor substitute

Discussion in 'Electronic Repair' started by [email protected], Jun 25, 2006.

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  1. Guest

    I just worked on a set that had a shorted diode that took out the 1.5
    ohm 7 watt fusible resistor. I have a 10watt resistor in stock but I
    really don't want to use it in place of a 7. In general it seems that I
    never seem to have the correct size of these things around when I need
    them. I was wondering if there is any reason that I couldn't substitute
    an appropriate size fuse for this application? The current in theory to
    open the resistor would have to exceed 3.24 amps. So I was thinking
    perhaps a 4 amp fuse would do. Anyone have any thoughts on this?
    Thanks. Lenny Stein, Barlen Electronics.
     
  2. Simoc

    Simoc Guest

    How did you calculate that? P=I^2R --> I=sqrt(P/R) -->
    sqrt(7W/1.5ohm)=2.16A
    2 amps, rather :) and I wouldn't use one with any higher rated current
    than "the original one (the resistor)" had...
    It depends on that did the resistance of the resistor have any purpose,
    or was it just to heat it up so that it will work as a fuse. That is of
    course highly possible in case of such low resistance...if you can tell
    it, your idea might work, although it sounds a little crazy :)
     
  3. Franc Zabkar

    Franc Zabkar Guest

    I've always wondered about how fusible resistors were rated.

    Here is the datasheet for Vishay's CMF fusible 0.25W/0.5W metal film
    flameproof fusible resistors:

    http://www.vishay.com/docs/31031/cmffuse.pdf

    The average fusing time for the 0.25W versions is 1 sec at 15W, 16s at
    5W, and 24s at ~4W. That's a lot slower than a typical glass fuse. The
    fusing specs also state that lower resistance parts take longer to
    fuse than higher resistances.

    Here is the datasheet for the LCA...SI carbon fusible resistor series:

    http://www.vishay.com/docs/20137/lcasi.pdf

    The LCA0207SI has a power rating of 0.25W but requires a 4A overload
    before it will fuse. At this current the maximum time to fuse is 15
    secs. So it seems that the equivalent circuit is ...

    o---- R ---- F ----o
    1/4W 3A ???

    I'm conFUSEd ...

    - Franc Zabkar
     
  4. Jim Land

    Jim Land Guest

    Use a SLOW BLOW fuse. They have the same characteristic you're describing,
    blowing faster for bigger currents.
     
  5. Guest

    Lenny:
    If it were me and I was able to pinpoint the exact fault that caused
    the 7 watt resistor (like a shorted semiconductor or capacitor,etc.) to
    burn out I would without hesistation use a 10 watt replacement (forget
    the fuse substitute) and let the set run for an hour or so and test for
    overheated components and excessive current through the resistor.
    Usually this type and value of low ohm resistor is used for current
    limiting and/or inrush current control and in light of another drastic
    fault condition, the 10 watt resistor would perform it's task and open
    up just as the original value did.
    electricitym
    - - - - - - -



    snipped:
     
  6. Franc Zabkar

    Franc Zabkar Guest

    Oops, that should be 4W, not 4A, so the fusing current is 4x rated
    current.
    - Franc Zabkar
     
  7. Franc Zabkar

    Franc Zabkar Guest

    True, but it would have to be a *very* slow fuse.

    For instance, these slo-blo fuses require only 0.6 sec before they
    rupture at 4x rated current:

    http://www.littelfuse.com/data/en/Time_Curve/385_multipletccurve.pdf

    OTOH, the fusible carbon resistors require 15s to rupture at 4x rated
    current.

    - Franc Zabkar
     
  8. I think I'd look for a 1 ohm 5 watt fusible resistor. The lower resistance
    will roughly accout for the lower wattage rating - yet still give the
    protection.

    Mark Z.
     
  9. M Berger

    M Berger Guest

    This might be a liability issue for a commercial servicer.
     
  10. Guest

    Thanks for all the responses. I found a .82 ohm7W fusible and used it.
    It is not getting hot as I'm sure this device is only to address as
    others have said inrush current. I ran the set for a few hours and it
    seems fine. I'll have to add some of those fusibles to my next parts
    order. Thanks again. Best regards. Lenny.
     
  11. Simoc

    Simoc Guest

    The explanation to that confusing thing might be that the fusibility of
    fusible resistors is intended just for preventing fire hazard if the
    res is overloaded, rather than to protect other components on the
    circuit. So the only thing that matters is that the "fusing point" is
    below the resistor's ignition point :)
     
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