the shunt resistor for measuring the dc motor current

how can I select the right value of the resistor (ohm) for the shunt resistor for measuring the current of the dc motor?


Question 1: What VOLTAGE do you need at maximum motor current, for a good reading across the resistor?

  • 50 mV (0.050V) used to be a standard. You need a low-range meter to read that well.


Question 2: At the average/max motor current, how much POWER must the resistor dissipate?


Comment: THe "Meter Shunt" resistor (Old Days Nomenclature) is in SERIES with the motor, right?

What data do you have on the motor?
How will you measure the voltage across the resistor?

And of course you want the lowest value of resistance that you can still get a useful voltage measurement from. Too much resistance will seriously affect the current that you’re trying to measure.


I agree completely with Replies #1 and #2

For a quick and dirty result for low currents a 1 ohm resistor has the advantage that every millivolt measured across it represents one milliamp.

Also note that many of the Atmega microprocessors (but NOT the 328) can amplify low voltages before input to the ADC.


thanks a lot(terryking228, slipstick, Robin2), I got it