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PWM vs Analog Output

Discussion in 'Electronic Basics' started by [email protected], Dec 24, 2006.

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  1. Guest

    Hi all,

    Thanks for everything that you write before i have learnd a lot of from
    this group.

    I am designing a circuit which uses a 18f452 and DAC0800 for analog
    output. Each DAC drives an TIP31 type BJT. Could we use this analog
    signal to control a DC motors speed? will this be efficent as PWM ? If
    not so why not ?:)

    Thanks for reply?
  2. Andrew Holme

    Andrew Holme Guest

    It would not be as efficient as PWM. Power would be dissipated in the BJT.
  3. To a first approximation, the speed of a DC permanent magnet
    motor is proportional to the voltage across it. The two
    most common ways to provide a variable voltage to a motor
    are linear and PWM regulation. Linear regulation starts
    with the highest voltage you will need, and burns the extra
    voltage, at slow speeds, as drop across some variable
    resistance device (i.e. transistor). A PWM regulator lowers
    the voltage by switching it to zero part of the time, and
    averaging the result over time. It is noisier and more
    complicated than linear regulation, but involves no
    intentional energy waste.

    The PIC18F452 includes two PWM pulse generators
    (programmable total cycle time, and programmable duty
    cycle). All you need to add to those outputs is a switch
    (mosfet of transistor, catch diode, and possibly some filter
    components and isolation between motor supply and
    processor supply) capable of translating the pulses up to
    the required voltage and current needed by the motor.
  4. Jamie

    Jamie Guest

    You can use it how ever, it won't be efficient in power use wise and
    it will generate some heat when throttled back.
  5. pLaNiX

    pLaNiX Guest

    Ok i got it. i have some coding/compiler difficulties using multiple
    pwm outputs (sending the PID output to a 8 bit port seemed a bit more
    easy to me :)) but the using PWM output (CCP1 CCP2A) is the best
    sollution as you said.

    Thanks for *all* replies you cleared my mind. :)
  6. If you run out of PWM outputs, there are simple ways to take
    the analog voltage output from a DAC and use a comparator to
    produce a PWM output that produces a motor voltage
    proportional to the DAC voltage. But it certainly is not a
    simple as using the firmware PWM output.
  7. Dorian

    Dorian Guest

    .. Could we use this analog
    An added benefit of PWM control is smoother starting of motors. Usually a
    motor driven by an analog voltage has to be overdriven to overcome starting
    friction and then throttled back to prevent over-speeding. This is not a
    problem in PWM control.

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