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Beginner electrical question

Discussion in 'General Electronics Discussion' started by czucker, May 18, 2010.

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  1. czucker

    czucker

    7
    0
    May 18, 2010
    Hi folks,
    Essentially, I'm trying to make a heating pad and I'm becoming terrible confused. . .

    Let me start by offering some background on my project.

    I'm building a fermentation cabinet that will be cooled by a window A/C unit. The cabinet will be split into two halves. The first half, with the A/C unit, will be for lagering beer and cold conditioning. Temps will be around 40° F. The beer fermenting in the second half show be kept at about 60°-70° F.

    The 120mm PC fan will turn on and blow cold air into the second chamber when the temperature drops below a set point.

    I would like to have heating belts wrapped around the containers in the second chamber to bring the heat of the liquid up when it drops too low.

    This is where I'm getting very confused. I'm basically an idiot when it comes to electrical work. . .

    I would like to use about 7' of 30 AWG Teflon coated copper wire as my heating element.

    I've calculated the resistance of the wire at .722 ohms (this is the part where you ought to start speaking up and correcting me).

    Can anyone help me figure out what type of resistor I would need to drop the voltage down enough to get a 20W heating element? I'm starting with 120V from the wall.

    I'd appreciate it.

    Thanks,
    Chris
     
    Last edited: May 18, 2010
  2. czucker

    czucker

    7
    0
    May 18, 2010
    Here is a sketch of the cabinet:


    [​IMG]
     
  3. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    without some form of resistance greater then .722 ohms you won't make anything but a short circuit.

    with the numbers you got 120V/.722 Ohms = 166 Amps x 120 V = 19944 Watts. I don't know of to many devices that can handle that sort of power. If any...
     
    Last edited: May 18, 2010
  4. czucker

    czucker

    7
    0
    May 18, 2010
    That's the problem. No matter how I run the numbers it just won't work. Is there a simple circuit that would allow me to do what I'm trying to do? Heating pads ought to be pretty simple, no?
     
  5. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    if you take some high power resistors and wire them in series, attach a piece of metal and you have a heating pad. You might even find a piece of highly resistive cord wire or something. The approach is all the same, add some more resistance to control the current and power output.
     
    Last edited: May 19, 2010
  6. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    A transformer with a secondary voltage of squareroot(0.722*20W)=3.8V is what you need for making it work with the wire you got. Use a 6A (at least) relay for switching.
     
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