3VDC control circut help needed

Discussion in 'The Projects Forum' started by robertthred, Feb 1, 2008.

  1. robertthred

    Thread Starter New Member

    Feb 1, 2008
    6
    0
    I am building a toy project and I need a bit of help.
    I have 4AA batteries (2 parallel sets of 2 batteries in series) and I need to find the right size of Potentiometers to use in my build.
    I am taking the 3 volt output and running it into potentiometer #1 to control overall motor speed. from there it runs into the wiper contact of potentiometer #2, from each end of the resistance coil i am running a wire to a motor.
    the overall circuit is to control the overall speed of both motors with the second potentiometer controlling the difference in sppeds on the motors. Then both negative leads gang back to the battery.

    What size potentiometers would i need to use in each location for optimal speed control?

    I realize this is a very simple project not worthy of this forum, but high school electronics was a long time ago and never really been used.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Controlling the speed of a motor using a variable resistance in series with the motor is not a very good way to accomplish your objective.

    The most obvious drawback to such an approach is the loss of torque that will occur due to the resistance in series with the motor. As a motor is loaded it naturally draws more current. As the current increases, the voltage drop across the resistance in series with the motor increases. This increase in voltage across the series resistor due to loading means that the voltage left to turn the motor will decrease and the motor will turn even slower. This is why the torque is so poor when a series resistance is used to control the speed of a motor.

    The most efficient way to control the speed of a motor is by using PWM or Pulse Width Modulation.

    hgmjr
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    Do you know the current and voltage rating for your motors?

    It's going to be pretty tough to get a PWM circuit to work at that low voltage.

    Do you have room to add a couple extra batteries to get 4.5v?
     
  4. robertthred

    Thread Starter New Member

    Feb 1, 2008
    6
    0
    Well, motor torque doesn't really matter in this case.
    they are just vibrator motors.
    i know they are rated at 3vdc but not sure of the amperage due to the enclosre i can't open without destroying it.
    I dont want to spend the money on the pwm controller nor do i want to have the control box that big.
    simple resistance controls are the solution im looking for.
    Like I said earlier, not tech enough question to be worthy, but i dont know anyone with the knowledge.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, Radio Shack has a 25 Ohm, 3 Watt rheostat for $3.99
    http://www.radioshack.com/product/index.jsp?productId=2062299&cp

    You might try one of those, center terminal to a motor lead, an end terminal to a battery lead, and the other motor lead to the battery. It may work OK for ONE motor.

    The other motor you'd need to hook up to another rheostat the same way. You would have to set each motor's speed individually.

    See the attached schematic.

    Keep in mind, since you have not provided any current information about the motor, I have no idea how much power is required.
     
  6. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    Try this one. I posted it in another recent answer, and used it for controlling a 2 volt Mirror ball rotator. Not pretty but it worked for me. Cant remember all the values, but if your keen you'll work it out[​IMG]
     
  7. robertthred

    Thread Starter New Member

    Feb 1, 2008
    6
    0
    thanks all!!
     
  8. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Robertthred,

    As an aside, running batteries in parallel or series-parallel is not a good idea. One of the cell(s) will always discharge slightly sooner than the other(s) and draw the other cell(s) down. If you are doing this for more current capacity at a certain voltage, find a different battery. If space is an issue, look at different battery chemistrys...Alk, NiCd, NMHi, LI...

    Ken
     
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