Measuring current with shunt and NI-9205

[sean m]

Hi All,

I want to measure current on a set of motors using an NI-9205 and CompactRIO.  I plan to use a 100mV 2A current shunt resistor.  Average motor current is 1.3A.  Does this sound like it will work to you guys?  Should I expect to pick up a lot of noise on the NI-9205 when using it in its 0-200mV range?

Thanks a lot,

Sean

[Ben_S]

Sean,

Using Shunt resistors is a very common method of measuring current as illustrated in our Current Measurements: How-To Guide. To address the question of noise and accuracy on the NI 9205, check out the NI 9205 Operating manual (page 22 or 36) which shows a table of accuracy, noise, and sensitivity for each input range.

[AnalogKid2DigitalMan]

Depending on what type of motor it is (ie brushless, brushed, etc) and what method is used to drive the motor, you may find that the current is not steady and may be mistaken for noise.

Just a heads up.

-AK2DM

[Dave_2]

This is something I plan on doing this with two 0 - 200V DC motors. Current to be about 3A, using a 5A shunt giving 0 - 50mVoutput.

What I'm worried about is that if, for some reason, the motor ground gets disconnected; then I could get 200V directly into the Daq.

Any ideas on how to protect the Daq from certain meltdown?

[apok]

put the shunt on the ground side...........all you have to worry now if the motor shorts

[Dave_2]

Correct, but where the shunt is located there is a lot of cable between the ground side of the shunt and the actual ground.

If any of that gets disconnected -  then there could be problems

Maybe I'm worring too much about this?

Anyway, been looking into using an Isolation Amplifier (TEXAS INSTRUMENTS - ISO124P - IC, AMP ISOLATION) to isolate the signal going to the Daq.