Measuring motor current on different leads

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Discussion Overview

The discussion revolves around measuring motor current in a circuit and the implications of connecting servos to a controller with limited current capacity. Participants explore the effects of measuring current at different points in the circuit and the operational dynamics of servos connected to an external power supply.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant questions whether measuring current on the positive lead differs from measuring on the negative lead, suggesting the negative measurement might be lower due to energy consumption in the motor.
  • Another participant asserts that current will be the same regardless of where it is measured in the circuit, emphasizing that motor energy derives from electric and magnetic fields rather than electron consumption.
  • A participant describes a scenario involving a servo controller with limited current handling, asking if connecting only the negative and signal wires while using an external power supply would affect the controller if the servos exceed the current limit.
  • Another participant requests clarification on whether the servo controller's pins are inputs or outputs and asks for specific data on the controller and motor.
  • A participant provides links to the servo controller and motor, explaining the functionality of the controller's channels and the nature of the signal sent to the servos.
  • One participant discusses the potential consequences of disconnecting power supplies to a servo, noting that it may cause the servo to run against its stop and suggesting experimentation to understand the effects.
  • A participant shares their experience testing a high torque servo with only the ground and signal connected, expressing concerns about high current passing through the signal wire and its implications for the controller's ability to position the servo.
  • Another participant agrees that as long as the positive cable is not connected to the controller, it should not affect the controller, while noting the shared ground connection and the need for similar positive supply voltages.
  • There is mention of uncertainty regarding the effects of different supply voltages on the digital PWM input signal to the servo.

Areas of Agreement / Disagreement

Participants express differing views on the measurement of current in the circuit and the operational implications of connecting servos to a controller with limited current capacity. The discussion remains unresolved regarding the specifics of current measurement and the effects of varying supply voltages.

Contextual Notes

Participants highlight the need for experimentation to understand the behavior of servos under different connection scenarios, as well as the lack of specified voltage thresholds or impedances for the PWM signal.

kolleamm
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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
 
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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.
 
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.

Thanks!
 
kolleamm said:
I have a servo controller, each servo channel has 3 pins, positive,negative, and signal.
Are those inputs or outputs?
Please link to the data for your make and model of servo controller and motor.
 
kolleamm said:
Let's say I have a simple small DC motor.
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.
 
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.
 
kolleamm said:
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.
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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