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Measuring motor current on different leads

  1. Jun 1, 2016 #1
    Let's say I have a simple small DC motor. If I measure the current by putting two multimeter leads on the positive will it be a different value then if I were to measure it on the negative?

    My guess is that the current measured on the negative lead will be lower because the energy used in turning the motor will not reach the negative. Please correct me if I'm wrong.

    Thanks in advance
  2. jcsd
  3. Jun 1, 2016 #2


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    The current will be the same wherever you measure it in the circuit.

    The motor energy comes from the electric and magnetic fields made by moving electrons, not from the consumption of electrons in the motor.
  4. Jun 1, 2016 #3
    Great! Basically here's what I'm trying to figure out. I have a servo controller, each servo channel has 3 pins, positive,negative, and signal.
    The board can only handle about 20 amps, however the servos will use way more than that, so I was told to connect them to an external power supply, but I must connect the negative and signal wire for each servo to the controller.

    If I run all the servos and they exceed 20 amps will the board be affected with only the negative and signal wire connected?

    I've been told it should work but nobody has really said why.

  5. Jun 1, 2016 #4


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    Are those inputs or outputs?
    Please link to the data for your make and model of servo controller and motor.
  6. Jun 1, 2016 #5
    Here is the servo controller.


    Each channel can be switched to input, output, or servo. The servos are pulse width modulated which is sent out the signal pin to set a position.

    Here is a link to a servo I'm using which draws up to 9A. I'm using a few of those.

  7. Jun 2, 2016 #6


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    Let's now say that you have a servo with built in electronics. The electronics and motor are powered by the + and – rails. The duty cycle of the signal sets the position that the electronics will hold the servo.

    Disconnecting either supply will stop the servo electronics balancing the position. The servo might then run one way against the stop. You need to experiment to see what will happen when one or both rails are disconnected. There is a possibility that the servo signal input might overload the controller signal output if the supply to the servo was not intact.

    To stop the servo you should disconnect the signal and one supply rail.
  8. Jun 2, 2016 #7
    I have tested a high torque servo with just the ground and signal connected to the board while the servos ground and power were connected to a battery and it worked great. My only fear was that the high amp amount was going through the signal wire too but then again if it was there would be no way for the controller to give the servo a position.
    I'd like to run about 10 of these servos which would set the amp amount way higher than the board can handle but I'm guessing as long as the positive cable is not connected to the controller it won't affect it.
  9. Jun 2, 2016 #8


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    Correct. The controller and servo share the ground connection and need similar positive supply voltages.
    There is no voltage threshold or impedance specified for the digital PWM input signal to the servo.
    It is hard to know just what will happen with slightly different supply voltages.
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