Resistor tolerance for shunt current measuring

Other Hardware General Electronics
1

Quick question for the team !!

For a current measuring circuit that is capable for (0V to 12V, 0A to 3A), what would the selection of shunt resistor be?

Knowing 12V x 3A = 36Watts. Any recommendation of resistors and what will be the ohm value?

Thanks, BTFDev

2

That's not a trivial question. The shunt resistor rating depends more upon the circuit that you are using for measuring voltage across the shunt resistor. For example if you use 0.01 ohms then full-scale voltage (at 3A) is 30mV. Does your measurement system measure accurately at 30mV? and properly amplify that up to whatever analog input range you are using?

If the answers are "yes" then at 3A the 0.01 ohm resistor will only dissipate 90mW.

If instead you do all this with a 0.1 ohm resistor then your measurement range is 300mV (easier to amplify with less noise) but now the resistor will dissipate 0.9W.

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3

The shunt resistor rating depends more upon the circuit that you are using for measuring voltage across the shunt resistor.

Obviously precision and accuracy count, so do I assume a 0.1, 0.5 or 1% resistor being used here?

Does your measurement system measure accurately at 30mV?

That leads to the next question, so far we are in concept stage and exact measurement system has not been determined. Here

  1. Range = (0V to 12V, 0A to 3A)
  2. Arduino processing and driven LCD to display current value
  3. Shunt monitor to measure and feed to Arduino, probably from TI

Thanks, BTFdev

4

Obviously precision and accuracy count, so do I assume a 0.1, 0.5 or 1% resistor being used here?

It depends upon the needs of your project!

Do you want to measure 0A-3A at +/-1%? +/-0.1%? etc. The resistor tolerance affects this number, but it is also affected by which TI shunt monitor part you select.

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5

Agreed!

At this point we are look for 1% tolerant and TI shunt monitor INA209 could be a good choice.

Any comment?

Thanks, BTFDev

6

It looks like the worst-case accuracy over temperature of the INA209 is +/-1%, so together with a 1% resistor should get you a tolerance of 2% worst-case. If you're only going to use it at room temperature the INA209 says it is +/-0.5% accurate so together with a 0.5% resistor will get you the 1% specification.

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