Interfacing shunt resistor current sensing with dsPIC microcontroller

[variaseng]

Hi, I'm trying to monitor (read) the current in a motor using shunt resistor current sensing. The motor's current can go up to ~5A. I want to do this by using the ADC capability of dsPIC30f4011 microcontroller. Do I connect the voltage from the resistor directly or do I need other hardware to interface the current sense to the microcontroller?

 

[colin55]

Firstly you cannot afford to lose 5v across a resistor.
You have to work out how much voltage you can afford to lose.
This will give you the approx sensitivity of the circuit.
Alternatively you can measure the voltage-drop along one of the leads of the motor.
Or you can measure the noise from the commutation.

 

[Reloadron]

You do not mention if this is an AC or DC motor? You also do not mention the value of the shunt resistor you plan to use? Typically when using a shunt resistance they output a small mV signal that requires signal conditioning (amplification) to get the shunt output to a good useable scalable signal.

Ron

 

[variaseng]

I am using a DC servo motor. The value of the resistor that I am using is 1 Ohm. Thank you very much!

 

[Reloadron]

Enter what Colin stated. If you have a shunt resistance of 1 ohm and a current of 5 amps the voltage drop across the shunt will be 5 volts, that is 5 volts your load (motor) will never see. Per Colin, you can't do that. Just for some example purposes these are typical factory made current shunts. If we look at a typical 10 amp shunt the output with 10 amps flowing through the shunt is 50 mV. Therefore 0 to 10 amps is equal to 0 to 50 mV. That low level signal will need to be signal conditioned (amplified) to be of any value so we use a small IA (Instrumentation Amplifier) circuit. Note in the example the actual resistance of a 10 amp / 50 mV shunt would be .050 / 50 = .001 ohm. Current shunts are typically very, very low resistance.

You may want to give this a read to better understand what I am talking about.

Ron

 

[nickelflippr]

A hall effect IC similar to an Allegro ACS712 come ready made for direct connection to a micro a-d pin. There are folks who package these chips on an easy to interface breakout board too.

 

[Reloadron]

Yeah, actually using a hall effect sensor as mentioned is a good way to go. I only addressed shunts as you mentioned them. I would give a hall effect some thought as it makes for a nice one stop package.

Ron