dc motor driver

Discussion in 'The Projects Forum' started by JasonL, Oct 31, 2014.

  1. JasonL

    Thread Starter Active Member

    Jul 1, 2011
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    I am using PWM from a micro-controller to drive a DC motor. I know how to design amplifiers: common emitter, common collector, common base. My problem is, I don't know how the motor will act as a load when I attach it to the final stage. I don't know much about DC motors. I am using http://www.pololu.com/product/2271 and I have a TIP142. My thought process is that, I should make a common emitter to amplify the PWM to 6V and feed it to a common collector to get the current needed to drive the motor. I'm not sure how to make the common collector since I don't think I can treat the motor as a simple resistor.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    A reversible DC motor is normally driven with a H-bridge. Current goes through the motor in one direction for cw rotation and in the reverse direction for ccw rotation. So the PWM will control the average current and thus the torque. You do know that the faster the shaft rotates the less torque it produces -- right?

    http://en.wikipedia.org/wiki/H_bridge
     
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  3. JasonL

    Thread Starter Active Member

    Jul 1, 2011
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    Thanks Papabravo. I don't think I need to use an H-bridge. I am using PWM from a microcontroller to control the speed of the motor. I do not need direction control. The problem I am having right now is that I cannot power the motor directly from the microcontroller and I am trying to make an amplifier using a transistor to drive the motor.
     
    Last edited: Nov 1, 2014
  4. Papabravo

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    Feb 24, 2006
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  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Or like this with the TIP. You will need a good sized heat sink. You could use a FET and get better performance.
     
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  6. JasonL

    Thread Starter Active Member

    Jul 1, 2011
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    thanks Papabravo, I will try the half bridge circuit with some time; I don't have the components now. Thanks Ronv, I got the circuit to work with those schematics.
     
  7. shortbus

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  8. MaxHeadRoom

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    Jul 18, 2013
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    The half bridge will offer braking if needed.
    Max.
     
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  9. Papabravo

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    Feb 24, 2006
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    The half bridge provides active control of motor current. With a low side switch you rely on a passive component to reduce the current to zero, which may take longer. This may be important if you wish to use PWM.
     
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  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Thanks guys. Guess I've (in my ignorance) been lucky in just using a low side with PWM to control motor speed.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    How is that different from a diode across the motor?
     
  12. MaxHeadRoom

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    Jul 18, 2013
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    A freewheeling motor generates same polarity as supply.
    Max.
     
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