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  1. #1
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    Motor current calculation using current sensor

    Hi,
    Please see the attached schematic,
    I am using a motor drive to drive a brushed DC motor clockwise and anticlockwise using complementary PWM
    I am using a current sensor ACS724LLCTR-05-AB to measure the current of the motor while running
    I am measuring using ADC output and the datasheet of the sensor says the sensitivity of the sensor is 400mV/A
    So when no current flows through the current sensor the output voltage of ADC is 2.5V. The ADC value that I measure is 512 (0 - 1023 range)
    I apply a small load to the motor of around 100mA and I see the ADC voltage rises to 460(Approx) from 512.
    Subtracting the ADC value I get 52 ADC value and multiplying by 4.88mV (5V system) 253.76mV
    If I divide 253.76mV / 400mV I get 0.633A
    My motor max current rating is only 0.8A and I apply only a small amount of load which is 0.1A. I see 0.633A which I dont understand.
    When I say 0.1A I measure it via my external power supply.
    Is it because the current via motor is switching(20KHz) I am doing a wrong calculation in measuring the output current.?
    How do I do the calculation.
    Is it something to do with RMS value?

    Click image for larger version. 

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  2. #2
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    Re: Motor current calculation using current sensor

    Depending on the MOSFET drive pattern, there may be considerable current flow through D210. If so, you don't measure the real motor current.

    The current sensor output should be filtered so that you measure a stable average motor current.



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  3. #3
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    Re: Motor current calculation using current sensor

    I recommend to look at the output waveform of the sensor with an oscilloscope. The sensor can handle quite wide bandwidth (120kHz), not too hard to imagine that the switching caused it. Or try to add a much bigger capacitor parllel with CF, like 100nF, and it will be similar as averaging.
    "Try SCE to AUX." /John Aaron/



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