Making a Heating Pad

Discussion in 'The Projects Forum' started by bby13, Feb 2, 2013.

  1. bby13

    Thread Starter New Member

    Feb 2, 2013
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    A project for college I am currently doing is making a heating pad. I am using a 5V power supply (Arduino Module) I know that Nichrome wire can be used as it heats up nicely and its sort of a cheap product but its exposed. The gauge is 26AWG and is 2.670 Ω/FT at 68°F and I have 25FT of this stuff.

    My Question is can 5V give me enough current to create enough heat for a heating pad? I have no circuit diagrams as of yet.

    Any info would be helpful. Thank you
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You need to specify how big the pad needs to be, and what is it heating.
     
  3. bby13

    Thread Starter New Member

    Feb 2, 2013
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    The size of the pad will be 15 x 15 inches. It just a heating pad that could go on a chair. such as a heated car seat how it is has a element in it. But it will be for my computer chair.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    A typical heating pad uses 60-90W on the high setting. You may require less for a seat pad. 25W would require 5A @ 5V. (Can your module provide that much current?)

    he total resistance would need to be 1 ohm, thus your nichrome wire has too high a resistance for your application. 10 feet of 30 gauge copper wire would have 1Ω of resistance. That would give about 10 crossings of the pad with the wire, which seems a reasonable spacing.
     
  5. thatoneguy

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    Feb 19, 2009
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    All 25 ft. would result in about 66Ω resistance.

    At 5V, using Ohms Law, the max current it would draw is about 75mA.

    Power is I²R, so (75mA)^2 * 66 = ~330 mW of heat.

    That's not much heat, about the same that a 2N2222 transistor runs at.

    What is the maximum current rating of your power supply? I'm unsure why a microcontroller is involved, but if that's your only source, it probably won't work.
     
  6. bby13

    Thread Starter New Member

    Feb 2, 2013
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    No reason for the micro-controller. Just thought it would be a nifty way to control it. Any other ideas? I do not want to use 120V

    I believe the arduino only puts out 40-50mA

    So if I use 12FT of my wire, the arduino could handle it?
     
    Last edited: Feb 2, 2013
  7. thatoneguy

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    It would conduct, but it wouldn't get warm, assuming you are using the full 25ft with 65 ohms or so.
     
    Last edited: Feb 2, 2013
  8. bby13

    Thread Starter New Member

    Feb 2, 2013
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    so i need more current? If I use 12VDC Power Supply, how much current would i need to make a proper heating pad to create enough heat for me to feel. I want to use the lowest voltage possible. I have no problem creating the circuit with 12 or even 24V.

    I put 9V straight to a 6inch piece of Nichrome wire and it got very hot and melted my platic bag. I did not hook up 5V straight to it from my arduino as i did not want to burn out the controller. The 9V Battery got hot, but i I do not think that was a effeciently way of producing heat.

    :confused:Help is appreciated. Thank you.
    (I have access to 26,24,22AWG of Nichrome wire) if there is another type of wire I could use to make my heating pad I would like to know

    Thanks. Enjoy the Superbowl tomorrow!
     
  9. THE_RB

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    Feb 11, 2008
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    Why not buy a cheap "pet heating pad" from ebay?

    Re the choice of 9v battery, what is way too small. A 15" x 15" sized heating pad like you mentioned probably needs 10-20W which is much more than you can get from a little 9v battery!

    Where is the heating pad going to be used, in what ambient temperature, and how long does it need to heat, and at what temperature?
     
  10. strantor

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    Crutschow says you need 60-90W, maybe less.

    RB says you need 10-20W.

    I have no idea how many watts you need, so I'll defer to their numbers.

    Your 25ft wire is 66 ohms. For 10W, you need 26V. For 20W, 36V. For 60W, 63V. For 90W, 77V.
     
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  11. kubeek

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    You can cut shorter lengts and run them in parallel.
    For example if you have 8 sections in parallel, you get 24W at 5V.
     
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  12. bby13

    Thread Starter New Member

    Feb 2, 2013
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    This heating pad will just go on my computer chair, its a project I want to build. I want to build it on my own as much as possible
     
  13. THE_RB

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    If you are going to sit on it you probably need LESS than 10W, as it will get uncomfortably warm after a while. The best setup would be to add a temperature sensor and controller to keep it at the temperature you like, that will greatly reduce the power when you sit on it.
     
  14. bby13

    Thread Starter New Member

    Feb 2, 2013
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    So i bought some wire already insulated. it is rated at 60Ω/meter and I have 4.4Meter of it. concluding its a total of 268Ω.

    I had a variable power supply and put 63VDC through it and it would not heat up as high as I wanted it to. By doing ohms law. I had .23A out with a power of 14.17W. I know that the current is what makes more heat. I want more current but less voltage.

    What voltage/current will i need to get a good temperature out of it so i could feel it. Thanks.
     
  15. crutschow

    Expert

    Mar 14, 2008
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    You can cut the wire and run several lengths in parallel as kubeek noted. But note that the wire will seem a lot warmer when it is wrapped in a pad and you sit on it for awhile. 10W or so may be quite sufficient in a pad, as THE_RB stated.
     
  16. #12

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    Nov 30, 2010
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    I think I need to remind you that a regular heating pad will feel way too hot in five minutes if you can feel its warmth in a matter of seconds. Slow down your expectations.
     
  17. KJ6EAD

    Senior Member

    Apr 30, 2011
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    I recently disassembled an old heat pad that had a bad controller and saved ~20 feet of wire from it. The wire has a resistance of ~20Ω/ft.

    For your application, wire with a resistance of 2Ω/ft. and a 24V supply would be in the right range and would require 700mA to produce 20W from 20' of wire. You could use a shorter wire with a higher specific resistance but that might require excessive amounts of thermal buffering material to counter the spot heating effect and you risk overheating the wire insulation.

    Seat heaters for cars are already built to run on 12V and can be purchased as an aftermarket pad.
    http://www.amazon.com/gp/aw/d/B000ANOUHQ
     
    Last edited: Feb 13, 2013
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