DC Motor Speed control with forward and revere control

Discussion in 'The Projects Forum' started by RodneyB, Aug 7, 2015.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I want to build a DC motor speed control with forward and reverse control of the motor.

    I found this circuit on the internet and not sure if its suitable. I have measured the motor under load and its 15 Amps. I wonder if this transistor is big enough or there is an alternative way of switching the motor.

    To change the direction I am thinking of swopping the polarity of the motors through a relay. Is this practical?

    Its to control the speed and direction of an electric child's car that I am trying to rebuild.
     
  2. MaxHeadRoom

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  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    There is something seriously wrong with the linked circuit. A 3055 will not take 15A reliably. Better to use a big FET. Read the data sheet.

    I do this with a 1/4hp DC motor in an airplane tug I built. I'm using a PIC, so I implemented a ramp down, pause, reverse relay, ramp up function in code. The PIC generates the PWM and controls the relay.

    You have to wait for the motor to coast to a stop before reversing it, or you will blow up the PWM transistor. If doing this without a PIC, you will have to use a 555 timer lockout (or a back EMF detector) that prevents the kid from jamming the machine into reverse before it coasts to a stop...
     
  4. MaxHeadRoom

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    A step further is to implement dynamic braking, placing a low value resistor or short across the motor after removing power will cause braking action.
    The ebay versions are good for 20amps.
    Max.
     
  5. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Thank you, because of sanctions we have no access to ebay. I will build the circuit from the link you sent me. I will have to import the 500K pot. Nothing available here. I will use a IRF9540 n Chanel fet to handle the large current.

    Thank you
     
  6. MaxHeadRoom

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    I missed your location,
    Ensure the pot is Linear and not Logarithmic.
    Max.
     
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  7. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Hi Mike

    Thanks once again for the help!

    Shall I use the IRF9540 in place of the 3055 and the circuit from the link in post #2. Or will the circuit I found work. I am going to try and find an old foot pedal from a sewing machine, I don't really know what else to use as the accelerator and hope they either 50K or 500K
     
  8. MaxHeadRoom

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    I personally prefer Mosfets in this application, if no other than the low on resistance.
    Usually a foot pedal from an old S.M. will be fairly low resistance due to the older motors being AC universal.
    Modern ones could be higher if a controller is used.
    Max.
     
  9. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Would the IRF9540 be suitable?

    I am trying to find a suitable method of putting the 500K pot into an accelerator?
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The IRF9540 appears to be OK in this app.
    What is the voltage of the motor?
    If not adapting some kind of existing pedal, it would mean designing a lever connected to the floor pedal to operate the pot..
    Max.
     
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  11. MikeML

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    Most foot controls do not contain a three-terminal pot; they have a two-terminal rheostat, and very low resistance.
     
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  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Would you like to control the speed and direction with one potentiometer?
    Then the following circuit (I found some time ago on the internet) might interest you:

    bidir_speed_control.png

    Bertus
     
  13. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Fantastic Thank you
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    The IRF9540 is a P-channel power MOSFET, and its' maximum continuous current at 100°C is -13A, which means that it is not capable of supplying the current required.
    I would not use a P-channel MOSFET if I could avoid it; it will have about 2.5 times the gate charge of an equivalent N-channel MOSFET causing slower turn-off/turn-on times for a given gate driver, causing more heating, and complicating gate drive - and it wouldn't work with the circuits described thus far.
     
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  15. MaxHeadRoom

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