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Alternate methods for driving DC motor

Discussion in 'Electronic Design' started by Jon Slaughter, Mar 11, 2008.

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  1. I'm curious at to the alternate methods to use for driving a DC instead of
    PWM.

    Could one use power BJT's as an amplifier also? I was thinking that I could
    use 1 BJT and 4 relays where the BJT would control power to the motor and
    the relays would control polarity(direction which doesn't change very
    often). Not sure if this means that that the power delivered would be
    uniform or not though. (although I can't see how it would be that much
    different from PWM)

    Just curious because I have a bunch of BJT's and relays laying around but no
    full bridge drivers that are large enough.

    Thanks,
    Jon
     
  2. Tim Wescott

    Tim Wescott Guest

    Build an H-bridge from discrete BJT's. Use the relays in case you need
    a hammer.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  3. Mook Johnson

    Mook Johnson Guest

    make ser you put a diode across the collector (cathode) and emitter (anode)
    assuming you have NPN transistors. This will steer any inductive currents
    when the motor is turned off.

    Using linear control is doable but will dissipate a lot in heat in the
    transistor depending on the current and voltage levels involved. You will
    need a power transistor and a large heatsink and maybe a fan to keep it
    cool.
    Check the Safe operating area for the BJT before using in this application.

    You can do linear control in a few ways. You can make a emitter follower
    to set a constant voltage (lower than the supply voltage) that will control
    the speed (with poor load regulation) or you cna use the BJT as a constant
    current source to control the max torque (with pretty good torque
    regulation)
     
  4. There are a couple of disadvantages in using linear control. The first
    and most obvious is the power loss (and resulting heat in the linear
    components). Another related effect is the possible torque loss at low
    speeds. The PWM technique has a current multiplication effect at PWMs
    < 100%. For example the current in the motor at 50% PWM will be ~2X the
    current from the power source. Of course these are more pronounced at
    higher powers.

    Of course PWM comes with it's own disadvantages such as noise from the
    PWM switching and the need for caps to handle the ripple current.

    Robert
     
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