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    Nichrome Wire Circuit

    Hi, all.
    i want to make a small heater. so i bought 0.1mm x 5mm nichrome strip whos resistance is 2.2ohms per 12 inch.
    The length i am using for making the heater is also 12 inch. But when i connect this nichrome wire directly to 12V 5amps SMPS, the SMPS gets shorted.
    so i connected 4.6 ohms resistor in series with nichrome strip, but still the nichrome strip does not get hot.

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    Re: Nichrome Wire Circuit

    12v / 5A = 2.4 Ohms and you are connecting 2.2 Ohms load, so you are trying to draw more current than what the supply is capable of delivering.

    The strip is rated 2.2 Ohms but at what temperature? Also did you measure the resistance to confirm the value?

    That 4.6 Ohm resistor in series is going to dissipate a lot of heat - are you sure it didn't burn out?



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    Re: Nichrome Wire Circuit

    The resistance of a Ni80Cr20 strip 0.1x5mm is 2.2 ohm/m rather than 2.2 ohm/12 inch.

    I presume you calculated wrongly. Temperature dependency of NiCr is moderate and doesn't explain the differences.



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    Re: Nichrome Wire Circuit

    the specific heat of nichrome is 450 J/(kg K)
    you power supply is 60 W or 60 J/second
    the mass of of your nichrome wire is probably about 10 grams

    so 60 J * 1/450 kg K/J / 0.01 kg = 13 K
    so the temperature of the wire should rise about 13 degrees K

    and if we take into account the cooling of the wire in air
    (which I do not have enough information to estimate)
    the wire will not get very warm

    i'm going to ballpark the temperature rise of the wire at 10 K, or about 18 degrees Fahrenheit
    not enough to heat much of anything



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    Re: Nichrome Wire Circuit

    @G, would a small, low-voltage incandescent bulb, or a pair in parallel, be a better solution ?

    Socket mounted, visible Go/NoGo function, run slightly below rated voltage for extended life-time...



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    Re: Nichrome Wire Circuit

    You need just enough NiCr to get near, but not over,
    the power supply's current limit. You might reduce the
    voltage if it's adjustable, use a series fat rectifier(s) to
    drop just a bit of voltage (apiece), etc.



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    Re: Nichrome Wire Circuit

    Quote Originally Posted by Nik_2213 View Post
    @G, would a small, low-voltage incandescent bulb, or a pair in parallel, be a better solution ?
    A dome light for automobiles, typically uses 1A at 12V. Gets hot enough so I feel it burns my fingers.

    There are 12V lamps which use a bright bulb, 20W to 60W.



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    Re: Nichrome Wire Circuit

    The OP wire does not get hot.
    The light bulb glows. Lot of heat dissipated by the filament.

    Primary difference is lack of cooling (no air surrounding filament) and a smaller resistance in the filament of the bulb.



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    Re: Nichrome Wire Circuit

    so the temperature of the wire should rise about 13 degrees K
    Without going into details, you can see that a soldering iron rated at 60W can get quite hot indeed (surely around 300C at ambient temp).

    With a smaller wire (straight or coiled) the temp rise can be significantly larger.

    i'm going to ballpark the temperature rise of the wire at 10 K,
    More accurately, 10C/s, and the temp will continue to rise till the heat loss becomes equal to 60W



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    Re: Nichrome Wire Circuit

    as c_mitra and BradtheRad point out, my comment was wrong

    re-reading the thread, trastikata and FvM were likely on the right path
    2.2 ohm /m means your 12 inch wire is about 0.8 ohm. the power supply behaved as if it was shorted because it was shorted.

    try a car headlight bulb. the H7 is rated at 12V 55W
    available at most auto supply store



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