Connecting trandsucer that outputs current to SCB-68

[aye29]

I have a pressure transcuder that outputs 4-20mA current but I'm not sure which wires go where on the SCB-68 connector block since I have never used any NI hardware before.  I know I need to solder a resistor on the board in order to convert the current to a voltage reading.  I've attached a diagram of the wiring instructions that were included with the transducer.  The transducer only has connectors for two wires.  I am assuming that the Load (Monitor) would be the SCB-68.  It looks like I have to connect one of the wires from the transducer and the ground wire from the power supply to the SCB-68.  Is that correct?  Would the current going through that loop be the 4-20mA output current?

Thanks

[devchander]

Which DAQ card is connected to your SCB 68 connector box??

[aye29]

PCI-6220

[Neil_S.]

aye29,

Thank you for contacting National Instruments.  You are correct that to measure the amount of current flowing through your transducer you will need to include a resistor inside the SCB-68.  You will then measure the voltage across the resistor and use this value to calculate the current.  The screw terminals in the SCB-68 corrispond to terminals on your 6220.  The pinout for the 6220 can be found at:

http://digital.ni.com/public.nsf/websearch/EFB197AD62EC3D06862570D2007ABCFA?OpenDocument 

As mentioned in this document you can also access the device pinouts from Measurement and Automation Explorer by right clicking on the device inside the Devices and Interfaces folder in the configuration pane and selecting Device Pinouts.

While I do not know what transducer you are using, from your description I believe you would wire the transducer as shown below:



Note that when wired this way you will need to measure on channel 0 using differential terminal configuration (which is the default).  I would also highly recommend taking a look at the M-Series user manual available at:
https://www.ni.com/docs/en-US/bundle/pci-pcie-pxi-pxie-usb-62xx-features/resource/m-series-user-manu...

Good luck with your application.  Let me know if you have any further questions.

Regards,

Neil S.
Applications Engineer
National Instruments

Message Edited by Neil S. on 03-05-2007 03:44 PM

[aye29]

If I soldered a resistor at R22(A) (I think) and connected the negative wire of the power supply to Pin 34 and the negative of the transducer to Pin 68, would the voltage measured on channel 0 be the voltage across the resistor at R22(A)?

I also have a question about measuring a voltage output.  I have a flowmeter that outputs a 0-5V signal from a single wire.  Could I measure that voltage by connecting the wire from the meter to say Pin 34 and the building ground to Pin 68?

Thanks,
Albert

Message Edited by aye29 on 03-05-2007 11:06 PM

[Neil_S.]

Albert,

When the resistor is connected between pins 68 and 34 I would recommend using a different channel to measure volage output of the flow meter.  You could wire the flowmeter in channel 1 for example (pins 33 and 66).  There may only be a single wire but the flow meter's voltage should be in reference to some ground plane.  You will connect this ground plane into pin 66.  Depending on the specifications of the flow meter it may require a different wiring configuration.  I would recommend taking a look at:

Developer Zone Tutorial: Field Wiring and Noise Considerations for Analog Signals

If you wired the resistor as shown above in the diagram you will measure the voltage by measuring on channel 0.  However, you must use differential terminal configuration with this wiring scheme.

Let me know if you have any addition questions.

Regards,

Neil S.
Applications Engineer
National Instruments

[aye29]

I connected the transducer to the connector box like shown in the above diagram and I was expecting to get a constant reading of 4mA but instead I get a reading shown in the attached image "Transducer Current Test."  The settings I used in DAQ assistant are also attached.  If I try reading a voltage from a power source, I get a reading that looks similar to the transcuder output.  However if I use a multimeter to read the voltage at SCB-68 I get a constant value.  The only case where I get a reading in LabView that looks like what I expect it to look like (a flat line) is if I connect a 1.5V AA battery to the SCB-68.  What can I do to fix this?

Thanks,
Albert

[Neil_S.]

Albert,

Theoretically if you connected your signals as shown in the diagram above you should be able to measure the current.  The behavior you are seeing seems very wierd to me.  First, you specified a max value of 25 mA but the graph seems to go much higher than that.  Second, where does the oscillation come from?  To troubleshoot this problem I would systematically try to eliminate factors.  You can try to measure the current on a different channel by placing a resistor across different terminals.  I would verify that the correct channel in the DAQ Assistant was chosen.  I would verify the wiring and connections.  I would look for possible locations at which crosstalk, noise, or other interference can occur.

Would it be possible to attach a picture of your setup in a post.  I am not sure if it would help but it may be helpful to get another set of eyes on it.

Hopefully some of the suggestions I have made will be helpful in resolving the issue.  Let me know.

Regards,

Neil S.
Applications Engineer
National Instruments

[aye29]

I've attached some pics of the setup.  Right now the pressure transducer is not connected to anything since I'm just testing the Labview connection.  With nothing connected, the output should be 4mA.  I've tried connecting to different channels and I get the same results.

[TheWoost]

Hi Albert,

From looking at the pictures, you have your transducer connected to ai2 of the 6220.  Is this the same analog input channel that you have configured in your DAQmx current measurement task?  Also, the strange signal that you are getting is coming right in at 60Hz, so it looks like the problem may be coming from AC noise from your power supply.  From your picture it looks like you are using the +/- wires from an AC adapter.  If this is the case, do you have a benchtop supply that you can use instead of the adapter?  In addition, have you verified that the voltage that is being supplied to the system from your supply is a stable DC voltage?

Regards,
Andrew W
National Instruments