Need help with connecting 4-20ma Transducer to NI9221

[bricekim88]

Hi

I am having a trouble big time with connecting 4-20ma transducer to NI9221

I saw a similar post but that procedure did not help me in resolving my issue. I checked multimeter also to see the current flow it is not helping me either.

Connection of the transducer I have is shown below

with excitation and output spec shown below

I used Omega meter for power source and configured excitation voltage to 12 Vdc

My current labview DAQ Assist is as shown below

So following the diagram,

1) I first connected +E (12V out) from Omega meter to wire lead from port A on Bendix 6-pin MIL plug then.

2) I wired Port B wire on Bendix 6-pin MIL plug to NI9221 Analog input 7

3) Made a connection from NI9221 Analog Port 9 (COM) to -E on Omega meter.

I also have a question regarding Shunt Resistor, the default is 249 ohms but should I change this? or Do I need to add external resistor (like 100ohm) between port 7 and 9 of NI9221?

also do I have correct setting on DAQ Assist? My multimeter is not reading any current flow either. I made a connection to transducer with pressure gague and varied pressure, but nothing is reading from transducer. Did I make any critical mistake? perhaps I damaged the transducer in the process? I spent many hours on this issue and cannot figure out where I made the mistake

Thank You!

[Stephen_Moore]

Hi,

You need to supply the external resistor.  The 9221 is a voltage input module with an input resitance of 1 Meg Ohmn.  The external resistor is what the 4ma to 20ma needs to pass through and pins 7 and 9 measure the voltage drop caused by the current flowing through the external resistor (in this case 249 Ohmns). 

With a 249 Ohmn resistor 4mA will be 0.996V and 20mA will be 4.98V on the 9221.

Your transmitter documentation should tell you what the maximum loop resistance can be and you should select your external resistance with that in mind.

In your case you need to add the external resistor between pins 7 and 9.  The precision of the resistor will limit the accuracy of your readings.  It is possible to buy precision 250 Ohmn shunt resistors which enable a 4mA to 20mA to be measured as 1.00V to 5.00V.

Cheers

Stephen

[bricekim88]

Hi Stephen

Thank you for your replay

I got 100ohm resistor and 330 ohm resistor and added them between pin 7 and 9 but I am still not reading any measurements from labview. Right now I have

1.) +E from power supply to Positive line of transducer

2.) Negative line from transducer to pin 7 of NI9221 with resistor on one end

3.) Other end of resistor to pin 9 of NI9221 which also leads to -E of the power supply

Have I damaged the resistor during the process? I used multimeter to check the current flow but I am not getting and readings...

I think I should be  getting current flow from pin 7 to 9 but I am not getting any current measurements...

[Stephen_Moore]

Hi,

Can you clarify what you did with the 330 Ohm and 100 Ohm resistors?

Confirm that when you have your resistor connected between 7 and 9.

Confirm that you have 12V on your power supply between E+ and E-.

With a 100 Ohm resistor between 7 and 9 check the voltage across 7 and 9.  It should be in the range 0.4V and 2.0V, corresponding to 4mA and 20mA.

How are you using your multimeter to check the current flow? 

Have you tried E+ to transducer to multimeter in current mode to E-?  This should give you a reading of between 0.004 A and 0.020 A and prove that the transducer is working.

Cheers

Stephen

[bricekim88]

Thanks Stephen

I resolved the problem! There were few wiring connection issues and yes I did and tried plug both 330 ohm and 100 ohm resistor between pin 7 and 9. It is now reading data but I think it's noise prone. I see noticible data fluctuations (by 10~20psi) I am thinking of getting better rating resistor.

Thank you Stephen. I have not thought of adding a resistor!

[Stephen_Moore]

Hi,

You might want to look at the quality of your 12V supply from the  Omega panel meter.  You could also investigate using the 24V supply from the Omega meter as detailed in the Figure 3.9 in the Omega DB25B-S manual.

When I have used 4mA to 20mA Pressure Transmitters with the 9203 module I found that the signal was prone to noise from the power supply so I changed the power supply and added digital filtering to the signal.

Good Luck

Cheers

Stephen