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### Topic: the shunt resistor for measuring the dc motor current (Read 504 times)previous topic - next topic

#### koronus

##### Dec 12, 2018, 06:40 am
hi
how can I select the right value of the resistor (ohm) for the shunt resistor for measuring the current of the dc motor?

#### terryking228

#1
##### Dec 12, 2018, 06:50 am
Hi,

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.

R=E/I

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

P=E*I

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?
Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

#### slipstick

#2
##### Dec 12, 2018, 10:00 am
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.

Steve

#### Robin2

#3
##### Dec 12, 2018, 10:06 amLast Edit: Dec 12, 2018, 10:09 am by Robin2
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.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

#### koronus

#4
##### Dec 12, 2018, 10:11 am
thanks a lot(terryking228, slipstick, Robin2), I got it

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