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Simple Voltage Question

  1. Feb 2, 2006 #1

    If you have a power source (for example 5v) and you have two wires coming off it would each wire be carrying 5v or does the volatge get split between them and give you only 2.5v on each?


    Matthew Leek
  2. jcsd
  3. Feb 2, 2006 #2


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    Kirchoff's second law states that the sum of potential differences around a loop is equal to the source EMF.

    First off all they would have no voltage unless they were part of a circuit. The voltage accross the wire depends on the potential difference lost as you move from the positive terminal to the negative terminal of the battery/cell. Hope that helps. Your question is kind of vague.
  4. Feb 2, 2006 #3

    I'll explain a bit better.

    Here's a poor drawing of the circuit:


    The AND logic gate needs 3v to work though. So If I was to have another wire coming from the battery into the AND gate would it be supplying 3v to the AND gate or would it be supplying half of that?
  5. Feb 2, 2006 #4


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    Staff: Mentor

    Voltages are differential -- they are the difference in electric potential between two things. In the case of a battery or power supply, the output voltage is between the + output and the - output. You can hook many things in parallel to these two output terminals, and each thing will see the same differential voltage.

    As to the drawing, your AND gate will have 5 terminals. The two inputs, the one output, a power input and a ground input. For an AND gate structure, the output power does not come from the input, it comes from the power supply. So redraw the circuit to show the AND gate horizontal with its output driving the motor + side and the other side of the motor going to ground. And draw the AND gate with power and ground terminals shown. You should take the 2nd input to the AND gate up to the power supply through a pullup resistor, or I suppose you could pull it straight up like you are doing now.

    BTW, logic gates do not have enough power output capability to drive real motors, not even small ones. So I'll assume that the AND gate symbol shown in your figure represents something more than just a 74AC08 type gate....
  6. Feb 2, 2006 #5

    I was assuming that as 3v was going in (i originally thought through the input) that was what would come out. I probably shouldn't be doing stuff like this until I have a better knowledge of the more basic things.

    If you don't mind me asking, is there a way to only have power from the battery go to the motor when a charge is sent down the cable connected to the computer? The only way I could think of was with the AND gate, but as I wont be getting enough power I guess it wont really work.

    Thanks for your help
  7. Feb 2, 2006 #6


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    Staff: Mentor

    Computer control of motors is definitely possible, and is quite common. As you are learning, you need the control signal to somehow enable a switch-like-something that can pass the higher power needed from a power supply to the motor.

    The type of switch-thing that you choose depends on the type and size of the motor. The simplest thing to use is a DC relay. You can turn a relay on generally with a moderate-power logic signal, or you can add a transistor to amplify a lower-power logic signal to turn on the relay. The relay will generally operate at 5V or 12V, and you can pass a fair amount of power through the relay, depending on its size.

    For specialized motors like stepper motors, the computer control signals are sequenced to drive solid state amplifiers to energize the various windings in the proper order to step one way or the other, or to actively hold position.
  8. Feb 2, 2006 #7
    Thanks very much, thats really helpful!

    I'll go looking for DC relays now. I'll have to save my gates for sometime in the future.

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