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DC Brushed Motor Current Sensing (and Limiting)

Discussion in 'Electronic Design' started by Andrew, Mar 10, 2005.

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  1. Andrew

    Andrew Guest


    I have a quick question regarding current sensing in DC motors. I know
    this topic is beaten to death so I will make this quick.

    I need to get the current reading into my microprocessor A/D (0-5V). I
    want to sense up to 15A. I have heard of many ways, but one seems
    easiest to me, the MAX4372H current amplifier. It seems that with a
    properly sized sense resistor, I would be able to sense the current in
    the fashion that I would like.

    I have one question though. Since the motor I am working with reverses
    direction, will this cause a problem with the current sensing using
    this chip? I want to make sure that having a negative voltage across
    the RS+ to RS- will perform the same as a positive one from RS+ to RS-
    as shown in the typical application here The spec
    sheet is found here, and describes
    the internal working of the chip. It seems that it would function
    properly, but I want to make sure.

    One more thing, just to clarify, if I am using this chip with my motor
    driver, how would it be connected? When looking at the typical
    application, I am guessing that the 24V battery (it is a 24V motor)
    will be connected to Vcc, and the "Vin" will be the output of the motor
    driver (M+), and the "load" will be the motor, and on the other side of
    the load, where in the typical application it is ground, there will be
    the M- fromt he motor driver?

    As I mentioned, the voltage developed across the RS+ and RS- (sense
    resistor) will be both positive and negative depending on motor
    direction, and this is my main concern for this setup, to ensure that
    this is possible. If not, are there any similar circuits that I could

    Thanks for any help!
  2. Luhan Monat

    Luhan Monat Guest

    Put the sense resitor in series with the power feed instead of the motor
    itself. This will give the same polarity regardless of which way the
    motor is running.

    This circuit measures only a threshhold, but an op amp connected to the
    resistor would give a measurable voltage to use to determine current
    thru the motor.
  3. Andrew

    Andrew Guest

    The only problem with doing this through the power source is that the
    motor driver is sourcing two motors (in parallel), meaning that I would
    get the current draw for the total of the two. I really need it to be
    for each motor. What would happen with the MAX 4372H chip if I put it
    in series with the motor? When the voltage at the sense resistor
    swings negative, will this cause a problem?
  4. Luhan Monat

    Luhan Monat Guest

    Sorry, I'm not familiar with that chip.
  5. Guest

    In message <>,

    I take it these are lead-acid batteries you are using. If so, you might
    find the IC dies from over-voltage. The maximum rated voltage is 28
    Volts and a 24 Volt battery can exceed that at times.

    My approach would be to amplify the volt drop across a section of the
    supply cable in a similar way to Luhan Monat's circuit. He is running at
    much lower power so he has used a 1 Ohm resistor. At 15 Amps a short
    length of cable should drop a few milliVolts which ought to be usable.
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