Bi-directional PWM Motor Control

Discussion in 'Power Electronics' started by AlbertHall, Aug 10, 2016.

  1. AlbertHall

    Thread Starter Well-Known Member

    Jun 4, 2014
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    I have a device which runs a small DC motor and can reverse the direction. I want add a speed control without completely starting again.
    In the circuit below, if the 'PWM' box (perhaps a '555) generates a waveform which is positive for 75% of the time, will the motor be powered for 75% of the time regardless of the polarity of the input voltage?
    upload_2016-8-10_19-21-29.png
     
  2. crutschow

    Expert

    Mar 14, 2008
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    No.
    That circuit would work only if the transistors were driven by opto isolators.
    As shown, the transistors will not be biased properly when the voltage polarity is reversed (Input 1 negative with respect to Input 2).
     
  3. AlbertHall

    Thread Starter Well-Known Member

    Jun 4, 2014
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    That was the bit I found confusing.
    My reasoning was, when input 1 is negative, Q1 body diode is forward biased, the source of Q2 is a diode drop positive of that negative input and its drain is positive of the source. The negative power to the PWM box is also one diode drop positive of Input 1 (equal to Q2 source). So when the PWM output becomes positive (close to input 2 voltage) it will switch on Q2, and when the PWM output goes negative it will apply roughly 0V Vgs to Q2, switching it off.
    No?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Commonly done with an H bridge for bi-directional control.
    Max.
     
  5. AlbertHall

    Thread Starter Well-Known Member

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    I am familiar with an H-bridge for controlling a motor, but in this case that would mean reverse engineering how the directional control is achieved - that would be difficult. So I already have the polarity switch doing what I want, but I want to be able to control the motor speed.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Or you could have a look at this schematic from Quasar.

    Bertus
     
  7. AlbertHall

    Thread Starter Well-Known Member

    Jun 4, 2014
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    I have realised that it won't work when input 1 is positive. It is trying to use an N-channel as a high side driver so the gate drive is missing.
    Here is the MKII version:
    upload_2016-8-10_21-7-55.png
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I think it could be made to work if all the switchery-do (PWM plus FET) were 'inside' the bridge rectifier.
     
  9. crutschow

    Expert

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    You circuit in Post #7 will work but it does exhibit a one-diode drop in the motor voltage due to the conduction of the current in the OFF transistor through the substrate diode when the other transistor is ON.
    Is that drop acceptable?
     
  10. AlbertHall

    Thread Starter Well-Known Member

    Jun 4, 2014
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    Yes, I don't need it to run as fast as it does now.
     
  11. AlbertHall

    Thread Starter Well-Known Member

    Jun 4, 2014
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    But then I lose the property that the motor direction reverses when the supply voltage is reversed.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Then you need to be sure that the MOSFET can safely conduct the motor current in the reverse direction and that the MOSFET can dissipated the power due to the voltage drop and motor current.

    You could add a Schottky diode in parallel with the source-drain diode of each MOSFET to minimize the MOSFET current and reduce the voltage drop.
     
  13. AlbertHall

    Thread Starter Well-Known Member

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    Thanks all. Now to go select some parts.
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Not so. here's what I had in mind :-
    PWM-bidirectional.JPG
     
  15. AlbertHall

    Thread Starter Well-Known Member

    Jun 4, 2014
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    Ah, I see. That would work. The PWM would have to run from the voltage on C1, but that's OK. This circuit loses an extra diode drop from the maximum motor voltage, and it won't work at 100% (as C1 would discharge) so that loses a bit more from the maximum speed. I can't see any advantage against my MKII version so I think I will stick to that.
     
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