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fuse blown up

Discussion in 'Electronic Design' started by [email protected], Nov 25, 2007.

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  1. Guest

    I connected a piece of nichrome wire( heating element) the one u'll
    find in old electric stoves ~4cms (don't know its resistance exactly)
    to Ac current. when I switched on, fuse was blown up in my home.
    Does the fuse blown due to

    1. short circuit or
    2. overloading

    If what I did (connecting ni-cr wire directly) was wrong then are
    there any circuits for connecting it AC mains. Any suggestions
     
  2. Fred Bloggs

    Fred Bloggs Guest

    don't go near anything electrical, you're not there yet...
     
  3. D from BC

    D from BC Guest

    I'm no expert on fuses but here's what I know (or think I know : ) )

    A fuse is like a light bulb. If it's overdriven with current...it
    heats up, melts and/or oxidizes? . That's the burn out.
    P=I^2R where R is the fuse element. R increases with temperature like
    most conductors.
    When P is sufficient for making the fuse element reach the melting
    temperature ..it blows.

    Your fuse blows because it can't sustain the current for your circuit.
    D from BC
     
  4. Um, its important to know the resistance. The resistance tells you how much
    current is going to be pulled and hence you then can determine if the
    breaker will blow/fuse will go.

    Since the voltage in AC mains is ~120Vrms AC, by ohms law

    120Vrms / R = I

    So if R = 0.001ohm then your pulling 120k Amps. The average outlet in a
    house only lets about 15A through.

    If R = 1 ohm, then I = 120 Amps and this is still to large.

    I believe most electrical stoves run in houses run on a seperate branch that
    allows more current(which means they must have larger wires or better
    insulated wires) to pass.

    In any case you need to measure the resistance.

    If the resistance is large enough(say > 10 ohms) then it should be ok. Make
    sure you don't have any other current hungry devices connected on the same
    branch though. If you have a two friges, a stove, a TV, 2 lamps, a computer,
    etc... all connected to the same socket then your going to trip the breaker
    or burn down your house.
     
  5. Tam/WB2TT

    Tam/WB2TT Guest

    Look at it this way. Suppose the full length of the nichrome wire was 40 cm.
    You cut off 4 cm of that. If the whole 40 cm draws 5 amps, then your 4 cm
    was trying to draw 50 amps.

    Tam
     
  6. How can you know what the resistance is going to be after it gets hot until
    you've tried it? Maybe a variac would help, but measuring the cold
    resistance doesn't reveal much.
     
  7. Robert Baer

    Robert Baer Guest

    Are you sure that it was not blown *down* ??
    Methinks that wire was rather short, and most definitely drew at
    least 3 times the fuse rating.
     
  8. Robert Baer

    Robert Baer Guest

    Stoves in the US do take a lot of current; they run off of 220V to
    decrease cut current loading on the power line.
     
  9. Robert Baer

    Robert Baer Guest

    With a short piece, that would be an excellent way to ruin the
    variac, if it is not fuse protected.
     
  10. And if the fuse did not blow, that short piece of nichrome would have
    gone blooey, spitting a few drops of molten nichrome in the process. Or
    the wires to that piece of nichrome might have done that instead. This
    experiment sure sounds like a fire hazard to me!

    If you use 1/10 the length of a heating element, you will be dissipating
    into it 10 times the amount of power that the full length takes (give or
    take due to variation of resistance with temperature). 100 times rated
    power per unit length (or whatever in this ballpark it turns out to be) is
    going to blow *something*!

    - Don Klipstein ()
     
  11. Jamie

    Jamie Guest


    And if the fuse did not blow, that short piece of nichrome would have
    gone blooey, spitting a few drops of molten nichrome in the process. Or
    the wires to that piece of nichrome might have done that instead. This
    experiment sure sounds like a fire hazard to me!

    If you use 1/10 the length of a heating element, you will be dissipating
    into it 10 times the amount of power that the full length takes (give or
    take due to variation of resistance with temperature). 100 times rated
    power per unit length (or whatever in this ballpark it turns out to be) is
    going to blow *something*!

    - Don Klipstein ()[/QUOTE]
    Something I found on the NET for all to look at on this subject..

    http://www.chemical-ecology.net/papers/heater-p.htm
     
  12. The OP never exactly said if he used the whole 120V length or not.
     

  13. It doesn't matter. Resistance doesn't decrease with temperature so it will
    pull max current when cold. Also, Nichrome has a very small coefficient so
    there is very little change.
     
  14. ian field

    ian field Guest

    Yes he did - he stated a 4cm length.
     
  15. ian field

    ian field Guest

    Yes - if you do any more experiments, video the event and post on youtube
    with the link here.
     
  16. Replace wimpy Fuse with Nail
    Try Again
    Perhaps try bigger nail

    .... remember to video your result for YouTube.
     
  17. Eeyore

    Eeyore Guest

    LOL !

    I saw a remarkable electrical display once in Bombay that could have been caused
    by such an experiment.

    Graham
     
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