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PWM A Dc Motor W/Seperate Supply

Discussion in 'Electronic Basics' started by Chris, Nov 23, 2006.

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  1. Chris

    Chris Guest

    Hi, I'm using a pic a npn resistor and diode to drive a small dc motor,
    everything works fine if i feed the motor the same + supply as the pic
    but when i change the + supply for the motor it's always on, all the
    ground is the same basically i want to power the motor before my 7905...
  2. Doesn't make sense that it works as you describe. Something
    is miswired. Did you reconnect the motor diode to the
    higher positive supply? Do you have a bypass capacitor
    between the NPN emitter and the positive supply connection
    of the motor and diode?

    Got a schematic?
  3. Chris

    Chris Guest

    Nope don't got this .... what would you suggest?

    Do you have a bypass capacitor between the NPN emitter and the
    positive supply connection of the motor and diode
  4. Chris

    Chris Guest

    2n3904 base wired to ground on breadboard, emitter to ground on
    breadboard collector to motor gound

    motor wired to +12v before regulator diode towards + supply

    if i wire it after the regulator it works, but if i wire it before the
    regulator it doesn't

    really strange
  5. I think it should be perfectly workable if you connect a
    diode across the motor) cathode end to the supply).

    The purpose of the bypass capacitor is to provide a small
    local charge storage right at the point where the PWM driver
    will consume current pulses, so that the wiring inductance
    back to the source (especially the ground leg) doesn't
    develop high frequency ringing on each edge, that could
    upset the micro. I would probably use a 100 uF to 470 uF
  6. Do you have access to a digital camera? Email me a photo of
    the circuit when it doesn't work.
  7. Are you sure you have not reversed the emitter and
    collector. This will work (at reduced gain) at 5 volts, but
    the emitter to base junction breaks down around 6 volts, and
    won't hold back higher voltage. Looking at the flat side,
    with the leads going down, the order is E B C.
  8. Chris

    Chris Guest

    Humm I thought

    collector -> motor -
    emitter -> ground
    base res pic

    according to the 2n3904 datasheet if yur looking top down with the curve
    facing you you have collector base emitter, but if i flip it around it
    works as it should ?!?
  9. Jamie

    Jamie Guest

    you have it wired incorrectly.

    Connect one pole to the voltage source you want to run
    the motor on. Connect the other pole to the collector of a
    driver transistor.
    The Pic will PWM the transistor by pulling the series load
    via the motor to common. This way, your percentage of duration
    from the PIC will be the percentage of your source voltage for
    the motor.
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