Output current signal (instead of volts) using Keysight 34970A multifunction module

[girish_jadhav]

Hello,

We have a pressure reduction valve (SP500 Electropneumatic Smart Positioner (spiraxsarco.com)) which takes 4-20 mA as an input control signal. But the multifunction module can only output 0-10 V. And I also noticed in the instrument drivers that there is a vi which only accepts voltage and not current. 

Do we need to buy a different card or is there way around it do it in LabVIEW itself?

[RTSLVU]

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@girish_jadhav wrote:

Hello,

We have a pressure reduction valve (SP500 Electropneumatic Smart Positioner (spiraxsarco.com)) which takes 4-20 mA as an input control signal. But the multifunction module can only output 0-10 V. And I also noticed in the instrument drivers that there is a vi which only accepts voltage and not current. 

Do we need to buy a different card or is there way around it do it in LabVIEW itself?

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Well the easiest way would be to use a programmable current source like a higher end DC power supply.

But you should be able to work something out using the 34970A 0-10V output, a resistor, and Ohms law. 

I get it, that valve is a current control but it also has to have a maximum input voltage rating so you don't' damage it...

1. I would arbitrarily select a voltage that is somewhere near the maximum (Vx)
2. Calculate the resistor value that would equal 20mA of current at Vx
3. Now 0 to Vx should correspond to 0 to 20mA current
4. Put the resistor in series with your valve
5. You might have to slightly adjust the resistor value or Vx max due to the added resistance of the valve itself 

[altenbach · Accepted solution]

There are plenty of commercial solutions. Simply do a google search for "voltage to current loop converter" (random example which I know nothing about or this one in a chip or this one from amazon)

[girish_jadhav]

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@altenbach wrote:

There are plenty of commercial solutions. Simply do a google search for "voltage to current loop converter" (random example which I know nothing about or this one in a chip or this one from amazon)

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I think we will go with a converter. 

But I found this description for the multi-function module DAQM907A:

"The Keysight DAQM907A allows great flexibility for a variety of sense and control applications. It combines two 8-bit ports of digital input and output, a 100 kHz gated totalizer, and two ± 12 V or ± 24 mA analog outputs—all on a single earth-referenced module. The digital inputs and totalizer input may be included in a scan. Alarm limits for the digital and event counter inputs are evaluated continuously, capturing and logging alarm conditions even between scans."

It says that it can either output ± 12 V or ± 24 mA but in the Analog Output.vi, I cannot find a terminal to connect to current output. Do you know how I can configure Analog Output vi to accept a current value? Or is it not possible?