Problem with DC motor circuit

Discussion in 'General Electronics Chat' started by Voltboy, Sep 23, 2007.

  1. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    Hello,

    Im trying to do a DC motor circuit with a potentiometer of 50k.
    If I dial the pot the the left (almost zero ohm) the motor work fine.
    But I want to reduce the current to a small one because its going through some low-current sensors, so I turn the pot to the right to about 15k ohm and the motor doesn't even move a bit.
    Anyone know why this is happening and how I can solve it??

    PS: The connections are alright and the pot to.


    Thanks in advance,
    Yoda
     
  2. ixisuprflyixi

    Active Member

    Sep 16, 2007
    52
    1
    When I used to do this I wondered the same thing and this is what comes to mind now. The pot is an inefficient way to control the speed of the motor because when the motor turns on it needs more power than when it is running so you may notice that you can get the motor to spin slower if you start the pot off to the left (low resistance) and then slowly turn it toward the right. So there is no problem with your circuit and the pot is good you just need to try to redesign the circuit a bit so as to not pass the current from the motors through the low-current sensors. If you post the circuit I am sure someone will have an idea on what to do. Have you thought of using an H-bridge?
     
  3. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    Ideally you want to retain the full voltage, but limit the amount of power going to the motor. Most often this is done with pulse-width modulation control, which is a bit like really quickly turning on/off the motor. Using a pot is fine, until you reach a point where there is insufficient current or voltage to drive the motor coils - I guess this is what's happening to your motor.
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Adding to that, with PWM control you will reduce the velocity of the motor without having the inconvenient of reducind its torque. So the motor may move very slowly without stalling due to friction.

    In your case, what happens is that the motor also looses torque (rotation force), and so the velocity won't be minimally matchet to the pot's dial position. Eventually, you come to a situation where you may damage the motor due to undervoltage, simply because the motor is stalled, consuming much more current and converting all power to heat.

    P.S.: Solar motors are more suitable for PWM control, but they are more expensive.
     
  5. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    Ok guys. Thanks for help.
    This is the circuit, except that the battery is 3.7 volts and the resistor is a pot.
    The point of this is that when the IR phototransistor receive IR light it will let current pass to the motor and turn it on.

    Any ideas to improve this my circuit (or making it work)??
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Use a transistor to control the current through the motor.
     
  7. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
  8. ixisuprflyixi

    Active Member

    Sep 16, 2007
    52
    1
    unfortunately it doesn't look like it will work as the device can only provide around 9mA of current. But, this device would work well if connected to the base of a transistor. You will need to find the maximum current that is used by your motor. Then find a transistor that is capable of sourcing that much current. you might try somehting like this, although I am not sure how well it would operate in your case it should work overall. I couldnt find a normal IR phototransistor in multisim so I used an opto coupler but the diode shown would be your IR signal. [​IMG]
     
  9. Lime

    New Member

    Sep 24, 2007
    1
    0
    If the motor is to be controlled by a 3.3 v source, it is a small low torque motor. I suggest you look a designing a PWM (pulse width modulator) to control the motor. Use the IR sensor to control the PWM circuit.
     
  10. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    This should work for motors up to just shy of 3 Watts:

    photomotor.jpg
     
  11. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    thanx alot thingmaker, i will use that circuit except instead of the 2N2222 I will put a power darlington.


    Thanks alot to everyone!


    -Yoda
     
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