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Driving a DC motor

Discussion in 'Beginner Electronics' started by Pedro Duarte, Jan 4, 2004.

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  1. Pedro Duarte

    Pedro Duarte Guest


    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. UncleWobbly

    UncleWobbly Guest

    assuming the transistor is an NPN device, put a diode (capable of taking the
    shunt) across the collector and emitter with the anode on the collector...
    see the diagram below, if this doesn't look right, copy the lines into and
    editor (notepad) and use a fixed pitch font.

    -------------------------+---+---------------- +power
    | |
    | |
    / |
    |/ -
    |\ ^ 1N4004 or similar
    | |
    | |
    -------------------------+---+---------------- -power
  3. Yeah, diodes. But a TTL level power FET would be a better bet than a
    transistor. They're better suited to this application. What's the
    power requirement of the motor? John Crighton might make your day if
    he's reading this and post the link to a clever little circuit that
    would probably do the job for you just fine...
  4. The inductance tries to reduce the rate of change of current by
    producing voltage. When you first switch the supply in to the motor,
    it produces a voltage equal to the supply to slow the rise of
    current. This makes it very easy on the switch, since it does not
    have to deal with current rising while it is in the process of turning
    on. But when you try to turn the switch off, the motor inductance
    generates a large and sudden voltage than adds to the supply voltage,
    to continue to pull current through the switch as it is turning off.
    It is this high voltage spike (much like the high voltage from an
    ignition coil primary) that damages the switch. So you need to give
    the motor current somewhere else to go (other than through the switch)
    with the motor voltage rising a modest amount. The most common
    approach for single direction motors is to just put a diode across the
    motor, so that when it adds a single diode drop to the supply voltage,
    the motor current detours through the diode while the switch turns
    off. That way, the PWM circuit alternates the motor voltage between
    the supply and a diode drop in the other direction, with the motor
    inductance and generated EMF averaging these two and acting as if the
    average voltage were applied.
  5. Luhan Monat

    Luhan Monat Guest

    Try these, you can drive them with a PIC and they take care of

    FAN8200 or TC4426
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