Delay in PWM start for open collector drive

Discussion in 'General Electronics Chat' started by sujith.g, Dec 1, 2015.

  1. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    Hi All,

    I am currently driving the PWM controller of a Motor using open collector darlington pair. Problem is when I switch on the input to darlington pair it takes some time delay to turn on the PWM controller driven motor. When I probed output of darlington pair(AUX_BMS_CTRL) I see that PWM output is generated immediately but still motor starts running only after 7sec delay. Please help. PWM_open_collector.png
    As per the figure, BLOWER_MOTOR is the input from MCU
     
    Last edited: Dec 1, 2015
  2. Johann

    Senior Member

    Nov 27, 2006
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    How did you probe it? Was the motor connected and does the waveform look the same from start till motor runs?
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    1. The V/I spec of motor?
    2. The power for the motor?
    3. The frequency of pwm?
    4. The input voltage of Blower motor?

    If the input voltage of Blower motor is 3.3V or 5V then the R42 can't be 0Ω.
     
  4. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    Yes it was exactly same before and after motor runs.
     
  5. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    Hi Scott,
    1. The motor is driven by an external PWM controller which is 12V pulled up. So I am driving it using open collector darlington.
    2. Frequency of PWM is 33Hz. which lies in range of PWM controller frequency
    3. Input voltage of darlington pair base is 5V.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    The rated current of motor?
     
  7. sujith.g

    Thread Starter Member

    Jul 7, 2015
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  8. ScottWang

    Moderator

    Aug 23, 2012
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  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The delay could be due to the time needed by the controller to power up to a correct working state.
     
  10. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    A- VCC
    B-GND
    C-PWM
    D-MOTOR+
    E-MOTOR-

    IN THIS VCC is 12V and Motor+ and Motor- are the inputs to the motor. We have to only drive PWM pin using open collector transistor.
     
    Last edited: Dec 1, 2015
  11. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    Hi Alec,

    Actually even after the controller is switched on when I turn on the motor by varying PWM from OFF state it takes time delay. Once it is ON it responds correctly in terms of speed variation on various duty cycles.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Technical info for the internals of that controller are conspicuous by their absence :(. Perhaps it's programmed to ramp up the rpm initially, regardless of the PWM duty cycle on its control input (just a guess)?
     
  13. ScottWang

    Moderator

    Aug 23, 2012
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    As the rated Ic current of darlington pair is 500 mA.
    When you using the PD 225 mW to calculate.
    Imax = 225 mW/12V = 18.75 mA

    When you using the PD 300 mW to calculate.
    Imax = 300 mW/12V = 25 mA

    Total Device Dissipation FR–5 Board(1), TA = 25°C, Derate above 25°C, PD 225 mW.
    Total Device Dissipation, Alumina Substrate,(2) TA = 25°C, Derate above 25°C, PD 300 mW

    The Ic of bjt is not enough for motor, and the bjt needs R42 to protect the base, it can't be 0Ω.
     
  14. marcf

    Member

    Dec 29, 2014
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    Is it ever ok to apply (or attempt to apply) 5v to the base of a BJT with its emitter grounded and no base current limiting resistor ? I can see maybe 1.4 VDC to a darlington but 5v seems to be a bit high. I would also feel sorry for the MPU port attempting to supply 5v to the device.
     
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  15. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    Hi Scott,
    The Vbe sat for darlinton is 2V. So if I feed 5V from MCU, then with a 2.2K resistor I can limit base current to (5-2)/2.2K = 1.36mA. Is it fine?
    The MCU can source max 2.5mA from its port.
     
  16. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    HI Marc, Yes I totally agree. So I will try testing with a 2.2K base resisitor. That would give me a base current of (5-Vbe)/2.2K=
    (5-2)/2.2K= 1.36mA. The MCU port can source max 2.5mA.
     
  17. ScottWang

    Moderator

    Aug 23, 2012
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    Now the big problem is that the darlington bjt, it can't provide enough current for motor, maybe you can use irf540 n mosfet or some other logical level n mosfet.
     
  18. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    Hi all,
    Great news! I fixed the problem. The issue was in software logic. I was driving base high to make PWM controller OFF. But in that condition transistor was always ON sinking current continuously. SO everytime I switch ON duty cycle it took time for the Transistor's BJT to sink all that current then take current based on duty cycle. That was causing the delay. Anyways thanks you all for your support!!! Cheers people :D
     
  19. marcf

    Member

    Dec 29, 2014
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    Yeah, too bad the part is only good for 0.5Amp and you require 4 times this amount. The device in your circuit drawing has a current gain approaching 140K (!), you could sink nearly 28 AMPS with it with only 2ma of base current. With the drive you were giving it in the drawing, it probably would have overloaded Grand Coolie Dam.
     
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  20. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    HI Marc,

    The collector current we need is just 8mA for the PWM controller. As I said before we are not directly driving the motor, but driving just the PWM controller. The PWM controller will take care of the motor's drive current
     
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