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Adding heating elements in series. Will this work?

  1. Nov 13, 2008 #1
    I want to make a heated steering wheel for my car using hand warmers from a snowmobile.
    The power source is 12 VDC and the resistance is 10.0 ohms using 25 watts on the low setting and 19.6 ohms using 35 watts on high. That is for one handwarmer, I figure I will need about 20 of them.

    Will they still heat up since there are so many of them?
    Will the increased resistance affect the wiring(getting to hot) to and from the steering wheel?
     
  2. jcsd
  3. Nov 13, 2008 #2

    russ_watters

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    Well, first, no, you can't put them in series. Resistances are additive, so via ohms law, the current will drop as you add more.

    But second, 20? No, one or two should be plenty of heat.
     
  4. Nov 13, 2008 #3
    Power is 25 or 35 watts??...to get a feel for that, hold you hands on a 25 watt bulb....
    One should be enough....

    If you must, put two in parallel...
     
  5. Nov 13, 2008 #4
    If it's not a secret, why do you want to do this?
     
  6. Nov 14, 2008 #5
    It's 25 watts on the low setting and 35 watts on the high setting. I've got a feel for it, I've been snowmobiling for decades. I have a spare warmer element and I've been using it to size this up. Two elements wouldn't be worth my time and effort. They are 2 5/8" X 4" long and as thin as paper. A snowmobile alternator can handle four of them plus a thumb warmer, no problem. The alternator on my car will be able to support more than that.
     
  7. Nov 14, 2008 #6
    It's no secret, the -40 degree weather in northern Canada is brutal. It takes along time for the steering wheel to warm up in these conditions. I am aware that I could just put on gloves, but I'd rather have a heated steering wheel. I'm always up for a challenge and I've wanted to do it for a long time.
     
  8. Nov 14, 2008 #7
    Check you car owner’s manual for the maximum current load you can place on the battery/alternator with the engine running. I think you will find it is about 15 amps and you can draw that by plugging into the cigarette lighter outlet. At 15 amps and 12 VDC, the maximum power available is 180 Watts. That would correspond to five of the heaters working at 35 Watts as the maximum number you can wire up. They need to be in parallel, not series. But I don’t know how you will find a practical way to wire them up to the steering wheel without having wires getting crossed up and tangled. It might be better to use battery powered hand warmers instead. I spent a couple years up in Alaska, both flying my plane and driving and I found the hand warmers useful.
     
  9. Nov 14, 2008 #8
    How did you determine the ohmic values?

    (There's an alternative solution, or course. Slaver the stearing wheel with sterno. Set it on fire, and move south.)
     
  10. Nov 14, 2008 #9

    russ_watters

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    With 500 W wrapped around your steering wheel, it'll be hot enough to burn your hands in a matter of seconds. Ever try to touch a 100W light bulb? (please don't!)
     
  11. Nov 14, 2008 #10

    I used a multimeter.
     
  12. Nov 14, 2008 #11
    It's going to be on a potentiometer, so it won't be on all the time.
     
  13. Nov 14, 2008 #12
    Usually the way you would want to calculate the power is to apply a voltage and measure the current.

    If you're applying voltage across the heating element, waiting a few seconds for it to obtain thermal equalibrium, opening one side of the element and quickly measuring the resistance, I suppose you could get close to the right numbers. You've got maybe half a second to catch it.

    The temperature coefficient of nichrome wire is 0.4 10^-3 C^-1 according to
    http://personal.tcu.edu/~zerda/manual/lab17.pdf

    Not as much as I though, by the way. It's 1/10th that of copper.
     
    Last edited: Nov 14, 2008
  14. Nov 14, 2008 #13

    Redbelly98

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    I don't think we need to worry about the room-temperature resistance, since we have the operating power specs of 25 and 35 Watts.

    bud415, how many of these do you use when snowmobiling? In a car you shouldn't need any more than that. You and the heaters will be protected from the wind inside the car, so things should be even warmer in a car vs. a snowmobile.
     
  15. Nov 14, 2008 #14
    which is why the current should be measured, or the unpowered hot resistance.
     
  16. Nov 14, 2008 #15

    Redbelly98

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    He could do that if he really wants to know the resistance or current at operating temp, but it should be roughly 2 or 3 amps (for 25 or 35 W settings, at 12V). But that information doesn't tell whether it's enough to keep his hands warm, only trying it out on a cold day will do that.

    25-35 W per hand seems like it would be enough, but then again I've never lived in -40 weather, so for all I know doubling the power (2 heaters per hand or 4 in total) is required. Trying it out is the best way to find out.
     
  17. Nov 14, 2008 #16
    Right you are. I missed it, that the hand warmers normally operate at 12V. Doh!

    bud, you can run, say, 10 strings. Each string has two warmers in series. You should get a total of 10 * 35/2 Watts on the high setting.

    If that's too hot, run strings of three warmers each.
     
  18. Nov 15, 2008 #17
    I think something is wrong here. A a resistor with higher resistance should use less power when attached to the same voltage.
     
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