high amp current measurement using ni9203

[im007]

i want to measure and display current flowing to a D.C motor (for my polymerisation project) when loaded using 9203. when loaded the value of current was 4-5 amps (measured with multimeter) but 9203 limit is 20 milliamps. how can i measure this current flowing to the motor. a block diagram of the assambly is being attached. plase help me in this regard.

[Buechsenschuetz]

There are DC current transducers available which can be excited with a very high current and will have an output of 0...20mA. The 0...20mA current loop standard is widely used in instrumentation, I think the input of your system is intended to receive such a signal and NOT to measure current of a load directly. 

If your system has an analog input you can also use a shunt, i.e. a resistor with rather low value (less than 1 Ohm). You should use the resistor such that the current through the resistor will produce a voltage drop which can be measured by your system. You might want to keep the voltage drop as low as possible, however a shunt will always cause a voltage drop and hence a loss of energy.  

[Buechsenschuetz]

PS. Isolation, or voltage levels, between the motor circuit and the analog input are also an issue. If you use a shunt, there will be no isolation, and you have to make sure that there are no significant voltage levels between the motor circuit and the board. The motor circuit has to be completely isolated (except for the shunt connections) against your DAQ system. 

When using a current transducer as described, you will avoid any isolation problems since these guys are always isolated against the measured circuit. Usually you will need an external power supply for the transducer. But I think this still is the easiest (and safest) solution for your problem.