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MOSFET and talking dog

Discussion in 'Electronic Design' started by samchristy, Jun 28, 2007.

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  1. samchristy

    samchristy Guest

    Hello,

    I have a question about a difference between controlling a motor with
    a MOSFET versus directly. The motor in question controls a toy dog's
    mouth and is part of an animatronic display. When powered, the motor
    opens the mouth. When power is removed the mouth shuts. The shutting
    is caused by a return spring. This works perfectly when I connect the
    motor directly to a power source such as a battery or transformer. It
    works fairly well when controlled from a MCU through a logic-level
    MOSFET. The only difference is that the mouth does not quite return to
    a complete shut. It does close enough for the purpose of creating a
    convincing effect of the mouth being shut, but I still would like to
    know what causes the difference.

    My guess is that when the mouth is fully open the spring has enough
    power to close the mouth completely as long as the motor is completely
    unpowered or in freewheel mode. It appears that the MOSFET is not
    turning off as quickly as when I simply disconnect power and as a
    result the mouth does not have enough force to completely close,
    though I could be way off base here.

    Any thoughts would be appreciated.

    Thanks,
    Sam
     
  2. What mosfet, what mcu, and EXACTLY how are they connected, what power
    supplies are you useing, and what about grounding?


    martin
     
  3. samchristy

    samchristy Guest

    The MOSFET is a IRL2703. The MCU is an Atmega48. I have 100ohm
    resistor between the MCU and the gate. I also have 10K pull-down
    resistor on the gate to limit any false starts while programming the
    chip. I have an additional Schottky diode across the MOSFET and one
    across the motor. The motor is on the high side, going from power
    directly into the drain. The power for the motor comes from an
    unregulated source, either a battery at 7.2V or a transformer at ~8V.
    The MCU is powered by a 5V regulator. The source is tied directly to
    ground. All the components share a common ground.
     
  4. the Schottky diodes across the fet and motor sound dubious, try 1n4002
    instead. What happens when you short the gate directly to ground,
    while the mega is sending out a 0?


    martin
     
  5. john jardine

    john jardine Guest

    I'd be thinking that on the spring driven return, the motor is running as a
    generator and forcing current through the diode load across it. Suggest you
    put a 10V Zener in series with diode.
    The motor will generate a few volts but not enough to overcome the Zener
    voltage. Hence it will return unloaded and at a normal speed.
     
  6. Jamie

    Jamie Guest

    put a pull down resistor on the gate.
    or program your uC with pull Up/down..
    Some uC use open collector outputs..
    etc..
     
  7. Jamie

    Jamie Guest

    put a diode by pass from the uC output to the gate via some small
    resistance to not over current the uC output so that the off state of
    the uC will have greater influence on the gate discharge time.
     
  8. Fred Bartoli

    Fred Bartoli Guest

    samchristy a écrit :
    First get rid of the diode across the mosfet. It serves no purpose.
    Second, what you see is due to the diode across the motor which shorts
    it when it returns.

    Your power sources suggest that the motor current isn't so high. That
    plus the fact that your mosfet is avalanche rated suggest that you
    simply use it as is, without any diode.
    What are your peak current (when the mosfet is switched off) and it's
    duration and also the motor cemf voltage when it returns (open circuit)?
    Compare this to fig 12c diagram and also the cemf + Vsupply to the 30V
    V(BR)DSS and you'll know what to do.
    My bet is that you can simply go with the mosfet alone. If you want to
    protect something, then clamping the gate voltage to GND and +5V with a
    pair of small sig diodes (1N4148/BAV99 or something) will be more useful.
     
  9. samchristy

    samchristy Guest

    Removing the diode from across the motor did the trick. The motor
    spring return now works perfectly. I also removed the diode from
    across the mosfet as suggested. The motor's max current draw is far
    less than the rated current draw on the mosfet. Thanks!

    In what situations does it make sense to use a diode across the motor
    terminals and what kind of diode should be used?

    Sam
     
  10. Fred Bartoli

    Fred Bartoli Guest

    samchristy a écrit :
    For example if you want to PWM the motor to reduce it's speed. Here the
    diode is mandatory and serves as a 'free wheeling' diode (allow the
    current to flow during the mosfet of period, thanks the the motor
    inductance). Use a fast recovery diode, or maybe a shottky if the supply
    voltage is low enough.
    But it is more usual to use bridges (half or full) depending on what you
    need.
     
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