20mA current 9203

[Humada33]

I am trying to use a 9203 to measure current from 4-20mA and my current transducer has one output.  I am using a LEM hall effect closed loop transducer. Should I use the negative lead from the power supply to the transducer or should I use the ground?

Also,  I have another set of measurements that will be in a range of 4 to 100mA.  I know the 9203 has a max of 20mA but is there a way to still use the 9203 or is there another module better suited for these measurements.

Thank you

[Wayne.C]

Whistler24,

Lot's of variation in these kinds of sensors.  What model of LEM are you connecting?

[Humada33]

Wayne,

I am using 5 LF 205-s/sp3 LEM current transducers to measure the current consumption of 5 servomotors on an industrial lathe.  The sensors are supplied with +/- 12 volts.  The connection between the sensor and the 9203 is unshielded.  I did not expect any "noise" to effect the current signal.  The equipment is in an industrial environment with several other large machines operating at the same time. The distance between the sensors and the DAQmx is ~25ft.

Chris

[Wayne.C]

Chris,

I'm pretty sure the 0V would get connected to the common of the +/- supply.  That means that common of +/- supply will need to be connected to common of 9203 module. 

+Vc connected to +V of dual supply.

-Vc connected to -V of dual supply. 

Be sure not to go over 20mA on 9203 modules.  If you need to exceed the 20mA, then I would use a voltage input module

and size the sense resistor accordingly.

Message Edited by Wayne.C on 06-11-2009 11:20 AM

[Humada33]

Wayne,

Thank you for the response.  The supply has an isolated ground on the +/-12 volt side and the building ground on the 120volt input side.  I assume you mean the isolated ground.  Each transducer has it's own power supply so I should connect the ground from each power supply to the common on the 9203. Correct?

Do you think the 25ft wire connecting the current transducer to the 9203 should be shielded? I originally thought not because the signal is a current.

Thank you for your help.

Chris

[Wayne.C]

Chris,

Each +/-12V supply should have a common that is isolated from earth and maybe from chassis.  Because the circuit in LEM is differential, output is referenced to this common.

I generally just use shielded twisted pair and don't connect the shield on either end to start.  All you can do is try it and see if noise is a problem. Distance should not be a problem.

[Humada33]

Wayne,

Thank you for the info. I will try setting the each power supply ground to the common.

Thank you again,

Chris