help : control direction of 12v dc motor using potential resistor

Discussion in 'Homework Help' started by Iraways, Mar 5, 2011.

  1. Iraways

    Thread Starter New Member

    Nov 16, 2008
    3
    0
    i've been searching the web but still no solution

    my objective is to control the direction of 12v dc motor using potential resistor

    let say i use 100k ohm potential resistor

    if i adjust the pot to 0 ohm ==> motor turn right
    if i adjust the pot to 100K ohm ==> motor turn left
    if i adjust the pot to 50k ohm ==> motor stop

    please help me ya..
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    9,666
    1,110
    What world are you from buddy.

    You talk of impossible things.
    To do that you need more than a pot.
     
  3. Heavydoody

    Active Member

    Jul 31, 2009
    140
    11
    How about an H bridge made up of transistors? They would have to be biased such that two were in cutoff and two were in saturation when the pot was at 0, and vice versa at 100k. I am not sure if this will work, and I am not sure how the stop function might work. Its just one idea.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    4,166
    1,125
    If you had +5 volts on one end of the rheostat and -5 volts on the other end, you would find a 'zero' voltage point in the middle.

    give you any ideas?
     
  5. mjhilger

    Member

    Feb 28, 2011
    119
    16
    You will certainly need more than a potentiometer to control the motor. The motor would surely require more current than the pot will supply. So you need a circuit of some type to drive your motor. Is this a home work question or the need for a real world solution? If it is a homework thought problem, then it is just to get you to think about a configuration which might allow control of the motor with the 3 positions you indicated (though 100K will never work).
    If however, it is for a real world solution, you should probably post in one of the other areas, as it will require transistors and bias circuits or at least an IC and a few transistors. H bridge as has been suggested and PWM using a 555 or something would be a start.
     
  6. Adjuster

    Late Member

    Dec 26, 2010
    2,147
    302
    You really do not want to try doing this with just a pot. Yes, theoretically it could be done, using some big expensive wire-wound pot rated at many watts in a bridge configuration with two power supplies. You would not use 100kΩ, by the way, more like 33Ω for a small low-voltage motor.

    That would work, but it would be horribly inefficient. It would also cost a lot. Big pots like that do exist, but they are quite expensive and not that readily available. Take the advice you are being given. Use a small pot to control something like an H bridge.
     
  7. Iraways

    Thread Starter New Member

    Nov 16, 2008
    3
    0
    thanks a lot guys

    i interested about the H bridge idea.
    by refer to this circuit : [​IMG]
    how to make 1 potential resistor turn 'ON' and 'OFF' the transistors of the H bridge??

    so,assume i using 100k pot
    if i adjust the pot between 0k ohm - 49k ohm ==> motor keep turning right (don't care about the speed)
    if i adjust the pot between 51k ohm - 100K ohm ==> motor keep turning left (don't care about the speed)
    if i adjust the pot at 50k ohm ==> motor stop
     
    Last edited: Mar 9, 2011
  8. MisterBill2

    Well-Known Member

    Jan 23, 2018
    1,710
    307
    I have seen and used a circuit that does exactly what is asked for, except that itused transistors and two resistors. The control circuit is a bridge, with the potentiometer on one side and two fixed resistors on the other side. The top is the positive supply voltage and the bottom is the return of the supply. One side of the motor connects to the midpoint, which is the junction of the two 15 ohm resistors, the other side of the motor connects to the wiper of the 50 ohm potentiometer. At the balance point there is no voltage across the motor, while turning toward either end supplies a voltage to drive the motor one way or the other.

    SOit is possible to do what is asked, it is just not very efficient.

    I am going to suggest adding the post date to post titles so that folks become aware of the age before making a response.
     
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