Read DC motor current using NI PXI-6254

[solaremperor]

Hello,

I would like to measure the current in a DC motor. I placed a 1 ohm resistor in series with a DC motor. The current to my DC motor is controlled with an LDU-S1 which is a voltage controlled current source amplifier. I set the voltage of the amplifer to a low but constant value. So, the motor is rotating slowly. I am trying to use an SCB-68 block connected to a PXI-6254 to read the voltage across the resistor which would be proportional to the current (I am essentially trying to measure current).

The issue is this - everytime I connect the propbes of A0 channel of the SCB-68  across the resistor, it makes a spark at the connection and the motor starts rotating very fast i.e. a huge current is going through the motor. In fact, the current draw is so high that my LCD display flickers for a short interval with a little noise superimposed on the actual image of the desktop. It was the type of noise we see when we hot-plug the connector of the display into the system when the system is running. This is happening even before I configure the DAQ or run a VI - mind you, this is happening even when LabVIEW isn't running.

I am clueless about what is going on and don't want to end up burning the DAQ or worse, even the Chassis housing it. Does anyone know what it means and how to solve this problem.

Regards,

S

[Queue-tip · Accepted solution]

You may consider using a very large resistor to make sure the current draw is not dangerous to your system. Also, the AI range for the NI 6254 is +/- 10 V. You can use this voltage mximum and the input current to select a reasonable resistance value.

~ Q ~