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Trouble monitoring motor voltage with ADC

Discussion in 'Sensors and Actuators' started by bonedoc, Aug 18, 2012.

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  1. bonedoc


    Dec 21, 2011
    I am trying to monitor voltage to a small motor with an ADC. I am using a 5k pot. One leg is at 3V and the other is on ground. The wiper is hooked to 3 things:

    -The ADC
    -The Motor
    -A 3V LED

    My goal was to have control of the motor speed with the pot. This works.

    I also wanted to monitor the voltage the motor sees with the ADC and use the LED as a visual indicator. This is actually working great....but as soon as I attach the 3V motor, there are problems.

    The motor is making some crazy interference while running. The LED flickers and the ADC doesnt get anything. With the motor removed, everything is great.

    I heard these small motors can be decoupled with a 01uF ceramic cap. That doesnt seem to matter. I also tried a 22uF electrolytic.

    Any ideas? Schematic is attached.

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Try placing a diode across the motor with the cathode (the end with the band) closest to the wiper.

    Using a potentiometer like this is still not going to work well. The motor doesn't act like a resistor, and its impedance is likely to average out to be much lower than the pot.

    If you want to read an average voltage, you are best removing the cap from where it is now and moving it to the input of your ADC. Then connect the ADC up to the circuit via a resistor. The ADC will then see the "average" voltage applied to the meter, not some instantaneous value which will be rapidly changing.

    The size of the resistor and capacitor will determine how smooth the voltage is and how quickly it reacts to changes.

    You will find that the voltage will NOT change linearly as you rotate the pot.

    You will also find that the voltage changes as the load on the motor changes.
  3. bonedoc


    Dec 21, 2011
    Hey Steve, thanks for the advice. It works pretty good. The diode does not seem to affect it as much as moving the cap and resistor like you mentioned. The only issue that I see is that the resistor does lower the readings some.

    The space on my setup is pretty cramped. But, I may try to replace the pot with a button and go digital / PWM for the control of the motor. I will still have to filter the system for distortion, but wont have to do an ADC reading.

    Let me try a couple more things. Thanks for the help! Ian
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