Simple Electronics?

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  • keyboardcowboy
    Senior Member
    • Apr 2006
    • 73
    #1

    Simple Electronics?

    i am pretty confused on this, suppose that i have a 6V DC motor that consumes lets say 5W (supposition),now what happens when i connect it through a resistor to the same 6V battery off course it will run slower due to the resistor but is the power requirement the same???..ie

    power absorbed by resistor+power absorbed by motor=power absorbed by motor without resistor
    Tags: None
    • etnietering
      Badcaps Veteran
      • Apr 2009
      • 379
      • USA
      #2
      Re: Simple Electronics?

      Nope, it will be lower, because by adding the resistors you're increasing the overall resistance of the circuit.

      To find how much power draw there will be, you need to calculate the resistance of the motor. the power equation is Power = Current*Voltage. you know the power and the voltage, so you can calculate the current. In your example, you have 5w = I * 6v, so I = 5/6, or about 0.8333 amps. now that you know this, you can use the resistance equation Voltage = Current*Resistance to calculate the resistance of the motor. 6v = 0.8333 * R, so you get R = 7.2 ohms. Say you had a 4 ohm resistor you wanted to hook up in series with the motor, like you said. So then the resistance of the circuit would be the sum of the resistances, 7.2 + 4 = 11.2 ohms. So using the resistance equation again you can calculate the current: 6v = I * 11.2, and you get I = 0.536. then use the power equation to calculate the power draw: P = 0.536A * 6v, P = about 3.2W.

      Hope this helps!

        Comment

        • PlainBill
          Badcaps Legend
          • Feb 2009
          • 7034
          • USA
          #3
          Re: Simple Electronics?

          Originally posted by etnietering
          Nope, it will be lower, because by adding the resistors you're increasing the overall resistance of the circuit.

          To find how much power draw there will be, you need to calculate the resistance of the motor. the power equation is Power = Current*Voltage. you know the power and the voltage, so you can calculate the current. In your example, you have 5w = I * 6v, so I = 5/6, or about 0.8333 amps. now that you know this, you can use the resistance equation Voltage = Current*Resistance to calculate the resistance of the motor. 6v = 0.8333 * R, so you get R = 7.2 ohms. Say you had a 4 ohm resistor you wanted to hook up in series with the motor, like you said. So then the resistance of the circuit would be the sum of the resistances, 7.2 + 4 = 11.2 ohms. So using the resistance equation again you can calculate the current: 6v = I * 11.2, and you get I = 0.536. then use the power equation to calculate the power draw: P = 0.536A * 6v, P = about 3.2W.

          Hope this helps!
          You math is impeccable, but there is one problem with your facts. You assume that the resistance of the motor is constant. This is not true in many cases. The current draw (resistance) of a motor will vary with load and speed.

          PlainBill
          For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

          Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

            Comment

            • keyboardcowboy
              Senior Member
              • Apr 2006
              • 73
              #4
              Re: Simple Electronics?

              thanx for clearing it up

                Comment

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