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Nichrome Heating Coil

Discussion in 'General Electronics Discussion' started by MoonMaster, Mar 18, 2014.

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  1. MoonMaster

    MoonMaster

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    Mar 18, 2014
    I am planning on making a circuit using a NiCr coil to heat test tubes to 360-400 degrees Fahrenheit. I picked out a 110v AC to 12v DC converter on Amazon, which says it should supply up to 6w: (EDIT: amps, not watts)

    http://www.amazon.com/gp/product/B003TUMDWG/ref=ox_sc_act_title_4ie=UTF8&psc=1&smid=A26WNKDWKDQIE8.

    I would like to use 22awg, Chromel-C wire:

    http://www.amazon.com/Nichrome-Wire...qid=1395182448&sr=1-13&keywords=nichrome+wire

    I plan on making the circuit a simple series circuit, with a toggle switch and another resistor, the resistance of which I calculated as follows:

    Using Wikipedia, I found that my circuit should run at 2.76A for 400 degrees, so 2.5A for more in the 360-400 degree range

    The resistance/foot of 20awg Chromel-C wire is 1.015 Ohm/ft, so a 2ft coil would have a resistance of 2.03 Ohm

    The current in the circuit would be V/(R1+R2) = I
    V/I-R1=R2
    (12v)/(2.5A)-2.03Ohm = 2.77Ohm, let's call it 2.8Ohm

    The power dissipated by the coil would then be (2.5A)^2*2.03Ohm=12.6875W

    Would this work? Would it be enough?
     
    Last edited: Mar 18, 2014
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    What do you think?
     
  3. davenn

    davenn Moderator

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    hi moon master
    welcome to electronics point

    you have a bit of a lack of power problem there

    you said ....
    then you said .....
    see the problem ?
    you cannot generate more power than you supply

    Dave
     
  4. MoonMaster

    MoonMaster

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    Mar 18, 2014
    In the original post, I meant to say 6 amps, not watts, it would have a maximum output of 12v *6a=72w.

    Thermodynamics, man. If it were actually 6w this wouldn't work at all.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK. But let's look at that again.

    12V and 2.8 ohms

    Power = V^2/R = 12*12/2.8 = 51.4W (it's 4.2A, not 2.5A)

    As long as that current isn't going to melt the wire, it will certainly heat stuff up.

    For 2.5A, you need a resistance of 12/2.5 = 4.8 ohms, and at 1.015 ohms per foot, you need about 57 inches.

    Correct math and units man, otherwise it won't work at all.
     
  6. davenn

    davenn Moderator

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    @Steve

    SNAP ;)
     
  7. MoonMaster

    MoonMaster

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    Mar 18, 2014
    I might have been a bit unclear as to what the 2.8ohm resistance means, my original post is not well organized.

    The 2.8ohm resistance is for a second resistor added in series with the nichrome coil, not for the coil itself, which has a resistance of 2.03ohm

    The voltage drop over the coil should be 12*2.03/(2.8+2.03)=5.04v

    So the power would be (5.04v)^2/2.02ohm=12.53w

    This is a little bit off because I rounded, but shouldn't that still work?

    This other resistor would dissipate the other however many watts of power.
     
    Last edited: Mar 18, 2014
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Unless you want something else that's VERY hot, why not go with an appropriate length of nichrome wire and keep the heat where you want it?

    There's nothing magic about a certan current heating the wire up to a certain temperature. That depends on the rate at which energy is pumped in to the wire and the rate it can escape. That will vary due to a large number of variables.

    You will probably need some sort of thermostat or lots of trial and error and a controlled environment.

    A better way of manipulating the current would be to use PWM to reduce the time the 4.8A flows. Making this adjustable would allow a sort of crude temperature adjustment. There are many LED dimmers that can switch 12V at reasonably high currents which might be able to do this for you at low cost.

    This one is rated at 8A, so using it for 4A would be nicely under-rating it.

    And to reiterate, this won't regulate the temperature, but it will allow you to change the power being dissipated in the wire without things getting *really* hot.
     
  9. MoonMaster

    MoonMaster

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    Mar 18, 2014
    Thanks, I didn't even think about how that would affect a second resistor. So this basically switches on and off really fast, to regulate the amount of power going to the coil? That seems like the way to go then.

    Electronics/electricity is sort of new to me, thanks (*steve*)
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Yeah. In this case if constantly on gives you 50W, you can back it off so that it dissipates 50W for 1/4 of the time leading to an average dissipation of around 12.5W.

    The switching happens many times per second so you don't see flicker from the LEDs. This is less important for heating, but works fine anyway.

    From an efficiency and control aspect, you will have a lot more.

    I would still tend to make the wire longer rather than shorter but remember that adjacent turns can't touch because that wire isn't (generally) insulated)
     
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