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Who Has Used Resistors as Fuses

Discussion in 'Electronic Design' started by D from BC, Jun 24, 2007.

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  1. D from BC

    D from BC Guest

    Sometimes I burn too many regular fuses with prototypes.

    Could I use a resistor as a temporary fuse substitute?
    It's ok if the fuse bursts into flames..
    Affer debugging, I'll used a proper fuse.

    Has anybody sacrificed resistors like this?
    If so, which resistor type makes for a good fuse?
    Carbon? Thick film? Thin film? Wirewound?

    I was blowing 2amp fast blow pico fuses.
    D from BC
     
  2. ian field

    ian field Guest

    Most consumer electronics manufacturers use a variety of fusible resistors
    which are specially designed not to burst into flames, seek out component
    catalogues aimed at the service trade.
     
  3. I suggest you use thermistors that switch from a low
    resistance to a very high resistance at a critical
    temperature. They reset back to low resistance when they
    cool off.

    For instance:
    http://www.epcos.com/inf/55/db/ptc_03/00740083.pdf
     
  4. Eeyore

    Eeyore Guest

    Fusible resistors
    http://uk.farnell.com/jsp/search/br...1&Ntt=fusible+resistor&Ntx=&_requestid=194422

    Graham
     
  5. Just do not use metal film, I did, and it burned a hole in the peeseebee.
    Carbon is OK.
    But the energy dissipated in it when a sort circuit or overload
    happens must be high enough to evaporate the carbon.
    And the size must be enough so the electrons do not take a shortcut and flash over.
    Old tube TVs used wirewound reistors in ceramic tubes that heated a soldering
    joint holding a spring breaking the contact.

    There are also microfuses that are re-usable for a variety of volatges and currents.

    I have also split apart the strands of flatcable and soldered that over the fuseholder
    clips... when no fuse at hand.
     
  6. Meat Plow

    Meat Plow Guest

    But, more expensive than a fuse no?
     
  7. D from BC

    D from BC Guest

    Digikey only has stock of a 100mA 230V version of the above link.

    An LVR200S with 2A 240V looked good...but no Digi stock.

    Digikey doesn't always list the voltage rating for PTC thermistors.

    I'd like >170Vpk. So when the PTC trips it doesn't internally arc,
    burn or decompose or whyever it's got a voltage rating.
    I"ve notice on Digi that prices are about $1.00 each.. Maybe shoe-in
    resistors are much cheaper and easier to get and therefore easily
    expendable..
    D from BC
     
  8. D from BC

    D from BC Guest

    http://www.phoenixpassives.com/resistors/fusible_resistor_nfr25_nfr25h.pdf
    I quickly found this as an example..
    I also did a quick average at about 20centsCAD each from Digikey..

    I'll stare at the data sheets for awhile. I'm not familiar with these
    devices..
    I don't know how it compares to a cheapo resistor..
    D from BC
     
  9. Eeyore

    Eeyore Guest

    They don't catch fire. Instead of a flamamble lacquer coating, it's typically
    cement.

    Graham
     
  10. D from BC

    D from BC Guest

    I've seen the wire used in glass fuses.. I've been tempted to source
    that wire to make custom fuses or temporary fuses.
    It might be just a matter of selecting a wire length or winding for
    different fuse currents.
    D from BC
     
  11. ian field

    ian field Guest

    For prototyping only - just solder a length of fuse wire to the end caps
    outside the glass tube, just remember to replace with proper fuses when its
    finished.
     
  12. Eeyore

    Eeyore Guest

    The wire in fuses may be lead or have a high lead content. You can't buy it any old where.

    Graham
     
  13. Polyswitches (PTC resistors) work in that range as long as your voltages
    aren't too high. You might even consider them in place of the pico
    fuses.

    Robert
     
  14. These look like new parts. You might be able to get samples from Tyco
    or maybe just parallel up several smaller devices.

    Tyco has a sample link on this page

    http://circuitprotection.com/lvr/

    Robert
     
  15. MooseFET

    MooseFET Guest

    I have used a resistor as a fuse in a PCB that went into production
    equipment. Once the unit makes it through test and is installed, the
    system fuse protects the fuse resistor. The fuse resistor is only the
    protect against shorts during testing. It worked out quite well once
    I explained for the 15th time "No, we don't need a higher wattage
    resistor there".

    Fuses have the advantage that they are clearly labeled as fuses.

    Polyswitches are a good way to go but you may have to still have a
    fuse to protect the PTC device. Don't trust the ratings you see on
    the first page of the data sheet.

    Later in the data sheet you will often find the real limit on voltage
    is much less than the first page says. The first page's rating
    assumes that you gently take the current up until it trips and then
    apply a short pulse of the voltage. You can't take a 600V PTC and
    hook it to a 600V supply and have it survive.
     
  16. D from BC

    D from BC Guest

    ??? The system fuse protects the fuse resistor??
    Does this mean the system fuse is more sensitive and the fuse resistor
    can never blow..??

    Was that fuse resistor in series with the true fuse?
    But wouldn't a dummy fuse be needed during testing? The dummy fuse is
    a short circuit. Assuming a removable fuse in clips.
    The dummy fuse is replaced with a real fuse at the end..

    I'll be looking carefully at those PTC voltage specs.. thanks..
    D from BC
     
  17. The voltage rating is an upper limit. They work at lower
    voltages.
    What current limit are you needing?
     
  18. colin

    colin Guest

    ive used fine copper wire before, I have an old book wich gives fusing
    currents for copper and fuse wire etc.

    I have an old multimeter wich has a repairable fuse, it has a tiny spool of
    some wire wich you just pull and insert into the fuse space, then press the
    catch down.

    Colin =^.^=
     
  19. Guest

    Carbon is not okay. It has a negative temperature coefficient of
    resistance and if things go wrong, you can find your 10k carbon film
    resistor carrying about an ampere at a voltage drop of a few volts or
    less - all the current is flowing down a narrow, red-hot channel
    through the carbon film

    Metal fim and metal oxide resistors do at least have a positive
    temperature coefficient of resistance, They are designed to run hot
    when dissipated their rated load - somewhere around 250C - so if yu do
    want to use them as fuses, bend the leads so the resistor body sits a
    couple of millimetres above the printed circuit board.
     
  20. D from BC

    D from BC Guest

    Approx 2A... It's inline with the household line voltage.
    D from BC
     
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