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Simple PWM Circuit for motor control

Discussion in 'Electronic Basics' started by [email protected], Dec 21, 2006.

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  1. Guest

  2. Chris

    Chris Guest

    Hi, James. Yep, it'll work pretty well. I'd assume you're driving a
    transistor or MOSFET with the comparator output, right

    6mA sinking output is pretty much the most you can ask of an LM393.
    You should probably expect somewhere around 0.3V output for a logic
    low. Unless there's a good reason, I'd replace the 2.2K with a 10K
    resistor. That'll pretty much ensure that your logic low will be less
    than 0.1V.

    Now you really should run the PWM of the motor at more than 37Hz, which
    is the frequency shown on the link. You can do this by replacing the
    ..05uF cap on the 555 with an .01uF cap, and replacing the 1 meg
    resistor with a 180K. Circuit operation will remain almost exactly the
    same, except the output frequency will be multiplied by 30 or so. This
    will give you a roughly 1KHz PWM output.

    Make sure not to replace the LM393 with another comparator, or use a
    lower output voltage, without looking things over carefully.

    Good luck
    Chris
     
  3. James_sgp

    James_sgp Guest

    Chris,

    Thanks for a quick, concise response. I`ll make the changes as you
    suggested, yes it will be powering an H-bridge IC to run my motor.
    I now feel much more confident, with a second opinion.

    Thanks again.
     
  4. kell

    kell Guest

    And don't forget to put a diode across the motor.
     
  5. Are you aware that many microprocessors have PWM firmware?
     
  6. John Fields

    John Fields Guest

     
  7. James_sgp

    James_sgp Guest

    Charles,

    Yes I am, but as I`m using firmware in the micro that was written by
    someone else; using the PWM function isn`t currently possible.
     
  8. Chris

    Chris Guest

    Hi, James. You might have a bit of a problem with range of your
    control signal. The 555 oscillator goes from 1/3 Vcc to 2/3 Vcc, and
    your output has to be within that range to be effective.

    Also, since you're a Google Grouper, you should know that it's
    considered good
    form to bottom post. On Google, click "show options", then "reply".
    Put your response to the post below the previous post -- that gives
    greater continuity, and makes it easier to answer the previous post
    with minimum scolling. Also, it helps for continuity -- most
    newsreaders don't act like Google Groups. Please check out Google
    Groups Help Topic "What's good 'netiquette' when posting to Usenet?"

    http://groups.google.com/support/bin/answer.py?answer=12348&topic=250

    Good luck
    Chris
     
  9. kell

    kell Guest

    the circuit has the voltae at the positive input of the comparator
    ramping up and down between 1/3 Vcc and 2/3 Vcc. The control voltage
    to the comparator's negative input adjusts in the same range, 1/3 Vcc
    to 2/3 Vcc. So far I don't see any problem.
     
  10. jasen

    jasen Guest

    what about bridge rectifier, AC side to the motor, DC side to the supply
    rails?

    Bye.
    Jasen
     
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