Modify PWM motor control to limit max rpm

Discussion in 'General Electronics Chat' started by Paul Kruger, Jan 9, 2016.

  1. Paul Kruger

    Thread Starter New Member

    Jan 9, 2016
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    Hi all.

    While I have been into electronics for many decades, I have found an issue experimenting so far has not helped solve.

    I am building a device using a DC motor and an off-the-shelf PWM speed control (Chinese from Amazon).

    It works fine but the only gear motor I could find at reasonable cost turns too fast when at maximum.

    I would like to be able to modify the board or the POT to impose an upper RPM limit below the motors maximum RPM.


    I prefer not to try a series resistor in line with the motor. Heat and may limit torque at lower speeds.

    I have tried various combinations of adding series resistance with the pot, the wiper etc. I cannot seem to find anyway to get this to work.
    555 Based timer. 100K pot. ~4 AMP 12 VDC motor.

    Any suggestions are very welcome. Thanks !
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    Do you have a schematic available to post?

    Bertus
     
  3. Paul Kruger

    Thread Starter New Member

    Jan 9, 2016
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    0
    Not really. I have found some generic schematics but cannot confirm they are the same used on the Chinese board.
    It uses 555 IC so assume it is pretty basic. I just did a Google search for diagrams and found a bunch but any could be similar to what I have.
    ( The pot crapped out so I am waiting for some replacements to arrive Monday )
     
  4. Paul Kruger

    Thread Starter New Member

    Jan 9, 2016
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  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Place a resistor in series with the pot on the high side terminal between the high input and the high side of the pot. NOT the slider.
    Find the resistance of the pot and calculate the series resistor for that % of reduction.
    Max.
     
  6. Paul Kruger

    Thread Starter New Member

    Jan 9, 2016
    6
    0
    Thanks...it is a 100K pot. In my experimentation with resisters in series with various positions, one problem I found was that while I could reduce the over all RPM, turning the pot to "0" the motor continued to turn, though at a low RPM. But also as noted above the 100K pot died and I won't have more until mail lady delivers them Monday. Will continue my experimenting then.

    The silk screen on the board says it is 10K but that does not match the pot used.

    [ Going to fix dinner...check back later. THANKS much ]
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Experiment with a selection of fixed resistors in series?
    Max.
     
  8. dannyf

    Well-Known Member

    Sep 13, 2015
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    It is tough since you don't have any documentation on the controller.

    If so, put a voltage on the Vtrl pin to bias the pwm.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I have one of those and it has a 100k pot, very simple to reverse engineer based on the 555. I don't have it documented, just hand drawn.
    Max.
     
  10. sailorjoe

    Member

    Jun 4, 2013
    361
    63
    Looks like a simple one or two sided PC board, should be easy to reverse engineer the timing circuit. just as a thought experiment, though, let's assume the 555 is putting out a fixed frequency, and the pot is changing the duty cycle. The pulses will get shorter as R gets smaller because the RC time constant gets smaller. Max motor speed is when the pulses are the longest, therefore R is maximum. So we want to limit Rmax to limit motor speed, yes? Then perhaps a resistor in parallel with the the pot would do that.

    That would rescale the entire resistor setting, of course, and maybe that's not what you want. In that case we can use a resistor + diode combo to limit the time constant to an upper limit. But we'd need the full timing circuit schematic to do it right.
     
    Last edited: Jan 9, 2016
  11. chuckey

    Well-Known Member

    Jun 4, 2007
    75
    10
    if its a standard brushed DC motor then its speed is determined by the current and the load. So a motor that has no load on it will spin fast, with a heavy load will turn slowly. Your PWM is just a variable voltage source. You need some feedback from the motor shaft, which alters the PWM to stabilise the speed and to limit it.
    Frank
     
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