Current Measurement

[Hernar]

Hello!

I am trying to measure current with a shunt. I am working with the DAQ6232, which has an input voltage range +/-10 volts. The voltage in my circuit is 13 volts, at the beggining my shunt was connected in the High Level (+) of the Power Supply, and it was not measuring at all due to the Common Mode Voltage. Somebody suggested me to connect the shunt in the Low Level (Ground), and now it works, I can get the lectures for the voltage, but they are very instable, they go high and low, and this is not very good. I tried to use a RC Filter to see if it was better and it didnt work. I am measuring the same voltage drop from the shunt with another instrument and it measures very good. I dont understand why I cant get the same stable values with the NI Measuring Card.

Thanks,

Ray Hernandez.

[AnalogKid2DigitalMan]

Have you tried a differential anaolog input configuration?

Is your current DC?

What type of device are you trying to measure the current draw from?

[Hernar]

Yes, I am working with the Differential Analog Input configuration.

The load is a Fan Control Module.  This FCM "controls" the motor of the fan that cools the Engine in the car.

I want to measure the current in two diferent points. First I need the current in the Power Supply, so I just connected the shunt to the Ground side. The second point, is the current in the motor, so again I connnected the shunt in the negative side of the Motor. I attached a diagram of the circuit.

I want to measure DC current. This value doesnt have to be very precise, we just want to have an idea of the currents that are flowing through the system to see if everything is allright.

[AnalogKid2DigitalMan]

What you are seeing with the DAQ may be real- depending on the motor construction, current draw may spike in a periodic fashion. Additionally the topology of the FCM (is it a simple on/off or speed control?) may add other current transients.

Is the other instrument you are using a DMM? If so, it probably has averaging and/or filtering built in. An oscilloscope with a differential input should show the true nature of the current by measing the voltage across the shunt(s).

You may have to increase your DAQ sampling rate and do some averaging/filtering in software to suit your needs.

Can you post a screenshot of the current waveform or graph showing the signal?

[Hernar]

Yep, this FCM is a speed control, and the circuit is pretty complex. I measured with the oscilloscope and the current changes a lot, so I guess the DAQ is measuring correct.

You are right, the other instrument was a Multimeter. I need to get a lecture similar to the one in the Multimeter, we dont need very precise info, just an average. I tried using filters in LabVIEW but it didnt work very good. Do you know if there is a tool in LabVIEW  I can use to get an average like the multimeter does? Or can you recommend me something else to do this averaging?

I dont have now a screenshot of the current graph, but I will post it later.

Thanks for the help, appreciated!

[Hernar]

The current for the Power Supply should be around 2,2 A (+/-0,4 A).

The current for the Motor should be around 7,2 A (+/- 0,7 A)

As you can see in the graphs, sometimes they go to negative values or high values.