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Configuring a 4-20 mA current loop with NI 9208 Module and CDAQ-9178
labviewjunkie
11-03-2014 05:51 PM
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I have a 500 Ohms potentiometer that I am trying to connect to a 4-20 mA current loop. I want to use a NI-9208 card connected to a CDAQ-9178 USB Chassis to read the current feedback from the potentiometer. The NI-9208 is currently being supplied 24V and is sensing 5 low wattage devices. I am using the potentiometer current feedback to determine the position of a valve that I am adjusting CW or CCW to control refrigerant flow in a sealed system based on certain conditions.
What additional hardware will I need to provide the 4-20 mA current output to the NI-9208 card? I am considering the DRLP-P2 potentiometer input transmitter from Omega which converts a two-wire potentiometer input of 0 - 500 Ohms to 4-20 mA. I was hoping that I could use the same 24V supply to the Vsup and COM on the NI-9208 and that the positive 4-20 mA signal could be connected to one of the available AI positions on the NI-9208. Has anyone used hardware similar to this to create a current loop? If so, can you send me a wiring diagram or describe how the current loop was setup?
natasftw
11-03-2014 06:22 PM
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You just want to see what the current is when you change the potentiometer? The circuit is easy. Just put everything in series.
Here's what you should look at: 24V/500Ohms = 48mA.
24V/20mA = 1200Ohms.
You will want at least 1200Ohms of resistance in the circuit beyond your potentiometer. This way, if your potentiometer is offering negligible resistance, you still stay within the 20mA the device is designed to handle. You can put the 1200Ohms in series with your potentiometer and use that to measure the current. You can either look at the input measurement with the potentiometer set to 0 to see what your measurement is to determine your circuit's baseline or you can use a DMM to measure the resistance of the additonal resistor. Keep in mind, they all have error tolerance so you'll want to have a measure of your specific resistor if you want an accurate measurement.
