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Homemade 10 ohm resistor

Discussion in 'General Electronics Discussion' started by Xephera Geeb, Nov 18, 2014.

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  1. Xephera Geeb

    Xephera Geeb

    Sep 6, 2014
    So I wanted to see if I could make a really quick and cheap heater for my pockets by getting a 9V battery to discharge over a small resistor, producing heat. After I did the calculations I realized the best value would probably be around 10Ω, as it allows the battery to last a couple hours, but also produces enough heat to keep a small pocket of air warm.

    My issue is that I don't have any 10Ω resistors. The smallest ones I have are 100Ω, so I was wondering if there's any way to make a 10Ω resistor out of household materials, that can both survive being used at 1 watt, and will convert most of the electrical energy to heat energy.

    If there's anything that would come to mind that would help!
  2. chopnhack


    Apr 28, 2014
    Well, you could put ten 100Ω resistors in parallel - what watt ratings are the resistors you have?
  3. BobK


    Jan 5, 2010
    Your 9V is not going to last more than about 15 minutes discharged into a 10Ω resistor.

    E = I R
    9 = I * 10
    0.9 = I

    That is too much current for a 9v. At 0.5A they have 300mAh, and it gets worse at higher currents, so say 200mAh at 0.9A, or 0.22 hours or 13 minutes.

    If you use the very expensive lithium one, you might get 3X that.

    People keep thinking little 9V batteries can power anything they want. The reality is that they have less energy than a single AA cell and less than 1/4 of the energy in a single D cell.

    Last edited: Nov 19, 2014
  4. davenn

    davenn Moderator

    Sep 5, 2009
    nicely put, Bob :)
  5. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    "People keep thinking little 9V batteries can power anything they want. "
    A 9v battery has enough energy to start a car.
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Not the ubiquitous 9V block battery (PP3). Unless you're talking about model cars :D
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    According to someone who has done some back of the envelope calculations it might take between 300j and 18000j to start a car (depending on engine capacity, compression, and other factors.

    At the very low end this is similar to the energy storage of a 9v battery.

    The only issue is that it is not possible to discharge such a battery in a reasonable time without losses due to it's internal resistance.

    Then again, if we could convert the entire mass of the battery into energy we could probably send the car to the moon
    KJ6EAD likes this.
  8. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    "People keep thinking little 9V batteries can power anything they want. "
    "A 9v battery has enough energy to start a car."

    This highlights the difference between ENERGY and POWER.
  9. BobK


    Jan 5, 2010
    Yes, and I did not say it did not have enough power to run his heater, in fact I calculated that it could run it for 13 minutes. It is more energy that is needed to run it longer since energy = power * time.

    If he had asked if it had enough energy to start a car, I would have recommended (theoretically) that he charge a very large capacitor with the battery over several hours, then try to start the car from the capacitor.

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