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Trying to see if I understand Ohms law

  1. Dec 18, 2009 #1
    I'm trying to get my head around ohms law, and have a question.
    If I have a motor that draws 12volt 100amp, and I would like to drop the amps down to 30, would I put a 360ohm resister in line.
    12 * 30 = 360
    V * I = R

    Side note can you get a resister that is 480 watt?

    Thank you

    P.S I've got some nicchrome wire that I could cut to that ohm, but it would glow red hot. Would that happen to all types of reisiters that could do these.
    Last edited: Dec 18, 2009
  2. jcsd
  3. Dec 18, 2009 #2


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    The motor has a resistance of 12 / 100 or 0.12 ohms. Assume this is constant.
    If you wanted to put 30 amps through this you would have a voltage across the motor of
    30 amps * 0.12 ohms or 3.6 volts.

    This would leave 12 - 3.6 or 8.4 volts across a series resistor which would have 30 amps flowing in it so it would be 8.4 volts / 30 amps or 0.28 ohms.
    The power of the resistor would be 8.4 * 30 or 252 watts.

    Note that this would not be a good thing to do to a motor. It may not even rotate with only 3.6 volts across it.

    Incidentally, V * I = R is not correct. V * I = power, or V / I = R
  4. Dec 18, 2009 #3
    Thanks for ther quick repley.
    If a powermosfet = 50 volt 17 amp, and is in serial with a 2.2ohm reisiter, will it make 17amp at 12 volt, or will a resisiter at .7ohm in serial make 17amp at 12 volt(V/I=R).
    Last edited: Dec 18, 2009
  5. Dec 19, 2009 #4


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    If you put a 2.2 ohm resistor across a 12 volt supply, it will draw a current of 5.45 amps (12 volts / 2.2 ohms = 5.4545 Amps.)

    Putting an extra 0.7 ohms in series would drop the current to 12 /(2.2 + 0.7) or 4.14 amps.

    Putting just the 0.7 ohm resistor across the battery would draw a current of 17.14 amps, as you suggest.

    A MOSFET does not generate voltage. This one can stand 50 volts when it is not conducting and it can conduct 17 amps when it is acting as a short circuit. The 50 volts and the 17 amps cannot happen at the same time or the MOSFET would be dissipating 850 watts. (50 volts * 17 amps = 850 watts). This is a lot of power for a small MOSFET and may blow it up.
  6. Dec 22, 2009 #5
    Please do not try any of these ideas in practice until you are sure of what you are doing. It seems likely that your task involves vehicle electrics, or a vehicle-type 12V battery.

    Be aware that such batteries can pass enormous currents in the case of a short-circuit.
    This is a very serious fire hazard, especially when petrol (gasoline) is nearby.
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