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Driving a DC motor

Discussion in 'Misc Electronics' started by ptek, Jan 4, 2004.

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  1. ptek

    ptek Guest

    Hi,

    I pretend to control a small dc motor (about 5 - 6V) using a
    microcontroller (PIC) and I think I need some precautions since the
    motor may be an inductive load...

    What I want to do is this : the PIC connects to the base of a
    transistor working in saturation zone, so that is behaves like a
    switch turning on and off the motor's power supply (you know, the
    motor is wired to the power source and connected to the transistor
    colector, the usual stuff.)

    By doing PWM on the signal outputed by the PIC to the transistor's
    base, it's possible to control the motor speed (the motor is "quickly"
    beeing turned on and off).

    Since the motor is an inductive load (all dc motor are? correct me if
    i'm worng) each time I power it and "unpower" it, a spike of current
    is generated so the life of this circuit would be seriously shortened
    ....

    What is the easiest yet functional way to protect it agains these
    spikes ? Some diodes ? Where and how ? I'm know very little about this
    matter ... A small schematic or a detailed description of an example
    circuit is really welcome.

    Thanks a lot

    Pedro Duarte
     
  2. Jerry G.

    Jerry G. Guest

    You can use a transistor to drive the motor. Use a series resistor on the
    base of the transistor to limit the base current. You would have to work
    out the value according to the amount of required base current required.
    This will isolate the base a little, and thus give the transistor some
    current limiting.

    Put a small capacitor arrangement across the motor. Take care for the
    polarity of the cap (as normal). You can typically use a 10 uF / 25 V in
    parallel to a 0.1 uF / 100 V cap. This is done very often in many types of
    devices using small DC motors. These are very common type voltage ratings
    at the values indicated. The idea of the smaller cap is to suppress any
    high frequency noise that the larger cap cannot deal with because of its
    structure. The larger cap is the main suppressor for the lower frequency
    noise or spiking factor.

    Across the transistor you can put a silicon type rectifier diode. Take
    regard for the polarity of the diode. You want to have the diode arranged
    so that it only conducts across the transistor inversely. This will suppress
    any reverse polarity noise from going across the transistor. Only when the
    transistor is forward conducting the motor should run.

    The PWM effect should still work. The cap arrangement across the motor is
    not large enough to cause any delay effects, or cause excessive loading.


    --

    Greetings,

    Jerry Greenberg GLG Technologies GLG
    =========================================
    WebPage http://www.zoom-one.com
    Electronics http://www.zoom-one.com/electron.htm
    =========================================


    Hi,

    I pretend to control a small dc motor (about 5 - 6V) using a
    microcontroller (PIC) and I think I need some precautions since the
    motor may be an inductive load...

    What I want to do is this : the PIC connects to the base of a
    transistor working in saturation zone, so that is behaves like a
    switch turning on and off the motor's power supply (you know, the
    motor is wired to the power source and connected to the transistor
    colector, the usual stuff.)

    By doing PWM on the signal outputed by the PIC to the transistor's
    base, it's possible to control the motor speed (the motor is "quickly"
    beeing turned on and off).

    Since the motor is an inductive load (all dc motor are? correct me if
    i'm worng) each time I power it and "unpower" it, a spike of current
    is generated so the life of this circuit would be seriously shortened
    ....

    What is the easiest yet functional way to protect it agains these
    spikes ? Some diodes ? Where and how ? I'm know very little about this
    matter ... A small schematic or a detailed description of an example
    circuit is really welcome.

    Thanks a lot

    Pedro Duarte
     
  3. cpemma

    cpemma Guest

    You can put the same inverse diode across the motor instead, which will also
    protect other electronics on the supply line, as shown
    http://www.cpemma.co.uk/555pwm.html
     
  4. Myron Samila

    Myron Samila Guest

    Actually, some brushless DC fans are impedance protected.

    Also, in professional stage dimmer applications, we use opto-isolators and opto-couplers
    to isolate the sensitive electronics from these types of problems you mention. ie: MOC
    3021.
     
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