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Current Detecting Circuit

Discussion in 'Electronic Design' started by Matt, Dec 5, 2007.

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  1. Matt

    Matt Guest

    I am looking for some help for my senior design project. I am not an
    electrical engineer, however I will need a simple circuit for my

    This is what we would like to do:
    We are running a 12 volt drill motor off of batteries and we would
    like to detect the current used by the drill motor, and illuminate an
    LED when the motor draws more than some arbitrary cutoff current and
    extinguish the LED when the current is below a the same value.

    I'm guessing this will require an op amp, but I haven't taken circuits
    yet, so I don't know much more than that.

    Thanks in advance for any help you might be able to provide.

  2. D from BC

    D from BC Guest

    Fully integrated Hall Effect Based Linear Current Sensor
    Digikey <$2.00

    ...add comparator.

    D from BC
  3. Guest

  4. Jim Thompson

    Jim Thompson Guest

    Naaaah! You need some bailing wire ;-)

    ...Jim Thompson
  5. And duct tape. Nothing beats duct tape... :cool:

  6. D from BC

    D from BC Guest

    Electronics homebrew style :)

    Resistors : BBQ charcoal
    Capacitors: Cooking foil, paper towels and peanut oil
    Inductors: Coat hanger wire
    Amplifiers: Maybe dual filament automotive bulbs
    PCB's: Cooking foil on Arborite laminate
    Wire: Salt water in plastic tubing.
    Battery: Lemons and dissimilar metals
    Circuit simulator: paper

    D from BC
  7. Jim Thompson

    Jim Thompson Guest

    Extra points if the current flows thru the duct tape ;-)

    ...Jim Thompson
  8. Ecnerwal

    Ecnerwal Guest

    That can be arranged. Nothing like a 50 KV supply and a supposed
    insulator (some are quite good - I recall getting to 30 KV with normal,
    non-high-voltage heat shrink, - probably 3M, not some offbrand. Been a
    few decades.)
  9. Alex

    Alex Guest

    if it do not have to be scientific precision , a resistor in the
    current path could do it, with a transistor base and emitter at the
    ends of resistor and the transistor collector with a resistor to
    limit LEd current could do...
  10. Matt

    Matt Guest

    Thanks for all the suggestions. I'm outta charcoal and my duct tape is
    running low, so I think I might just go with the hall effect sensor.

    I think some of those homebrew solutions might just work ;-)

  11. Paul Mathews

    Paul Mathews Guest

    The Allegro chip provides isolation, which you probably don't need if
    you're going to run your detection and LED circuits off of the drill
    battery anyway. You can monitor current with a series resistor and a
    comparator alone. You may want to slow down the circuit so that it
    doesn't respond to the current spikes that you're likely to have with
    most any motor and speed control circuit. The circuit can be as simple
    as this description:
    1. Comparator open-collector output (*e.g., LM311) drives LED with
    series limiting resistor.
    2. Choose comparator with included reference or generate a reference
    voltage with a zener or a forward-biased junction. A resistor divider
    presents a low reference voltage (say 100 mV) to the + comparator
    input. You may want some positive feedback.
    3. Battery - is system ground. Motor current from Motor - passes
    through Rs. Rs = 100 mV / Ithreshold. For example, for 10 amps, Rs =
    10 milliohms.
    4. The comparator - input connects to the high side of Rs.

    Paul Mathews
  12. D from BC

    D from BC Guest

    Probably the only reason why I like the Allegro chip is that there's
    no hot current sensing resistor in a circuit that depends on it's
    resistance value.

    D from BC
  13. mpm

    mpm Guest

    Don't forget the "hard drive": - Coffee can and paperclips.
  14. Those chips have soberingly-high DC offset specs.
  15. JosephKK

    JosephKK Guest

    Jim Thompson posted to
    But then it would be conduct tape.
  16. D from BC

    D from BC Guest

    *groan* :p
    ..... :)

    D from BC
  17. Combined effort: One *could* probably make some kind of reed-relay out of
    two pieces of bailing wire fixed in the chewing gum and actually get it to

  18. Use barb wire, if you need a latching relay.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
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