PulseWidthModulation in context with controlling motors

Discussion in 'General Electronics Chat' started by shankbond, Sep 11, 2008.

  1. shankbond

    Thread Starter Active Member

    Nov 4, 2007
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    hello everybody,

    Monday an idea popped up from, my brain regarding controlling a dc motor(i m here talking about the small motors once used in video casette recorders or tape recorder which used magnetic tapes to record audio and video films .)

    i thought that i could somehow controll the motor usin one relay and a comparator circuit which will compare the voltage with a reference voltage when the voltage will be less that the reference voltage then a negative voltage will be given to motor ,it will run in reverse direction else if the voltage is higher than the reference voltage than the motor will run in forward direction when no signal appears from the comparator thaen motor wont get any power.


    i have planned to use a pc parallel port wire to give controlling signals to the relay .


    my question :
    1)how should i compare the reference signalwith that of pc?
    2) can the motor withstand the fluctuation in the duty cycle?
    3)is it possible to control the motor by varying its duty cycle alone?


    with regards
    shankbond
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    63
    Yes you can use PWM to control a motor's speed but i think your motor is a stepper motor so you will a special driver circuit to control it. How many wires come out of the motor?
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,694
    904
    Oops. Posted to wrong thread. Response moved to CMOS Astable Control thread by kvsingh21. John
     
    Last edited: Sep 11, 2008
  4. shankbond

    Thread Starter Active Member

    Nov 4, 2007
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    no it is just only a concept and i want to use only simple dc motor ,i hope i have made myself clear:)
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    As i understand you want to control thr speed of the motor with PWM by using the PC and want to use a relay to reverse its rotation too, right?

    The easiest way to do it with PC is to use the parallel port.

    In addition, to reverse to motor rotation you need only a DPST relay. The motor will rotate one way if the relay is energized and the other way if the realy is denergized or vice versa.

    Also, the PC has to design in which direction the motor will rotate (activating or diactivating the relay). To achieve this you will use one input of the parallel port and when you apply a low signal (0V) to it the motor will rotates forward and if you apply a high signal (5V) to it the motor will rotate backwards or vice versa.

    To control the motor you will a power transistor, bipolar or FET, capable of driving the motor (i suggest FET).

    Also, you have to use protection diodes on the relay coil and the motor to absorb the back emf created by them when you swicth them off because it will destroy your circuit and your PC. I strongly recommend you to use optoisolators to receive/transmit your control signals form the parallel port to be sure that your PC is safe (PC's are quit expensive to destroy them for stupid things).
     
  6. shankbond

    Thread Starter Active Member

    Nov 4, 2007
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    0
    could you please elaborate more on the direction control.
    because ,i know that pwm can be either zero or one ,that means that i can control the speed by sending a longer pulse and decrease the speed or reverse it by decreasing the pulse width.:D
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You can vary the pulse width from amlost 0-100% but you cant reverse the motor by reducing the pulse width unless you have a special circuit which detects the pulse width below a value and reverses the motor with a relay. Normally, you only control the speed with PWM and reverse the rotation by using a relay.

    See this site to learn how to connect a relay to control the direction of rotation.

    http://www.distel.co.uk/DC_MOT_CON1.htm
     
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