How to write 4 - 20mA setpoint to an Alicat PCD series pressure controller

[Gee22]

Hi 

I need help writing the current setpoint to an Alicat PCD series. Normally I should be writing 0 - 5 V for a 0 - 250 PSIG but the input signal is set to 4 - 20mA. Thus it is a 4 - 20mA for a 0 - 250 PSIG 

Note; I am not using a COM connection or Rs-232. I just have an 8pin din connected to the device and the other end are blunt wires connected to myRIO. 

[GerdW · Accepted solution]

Hi Gee,

use a signal converter to convert from your 0-5V analog output of the myRIO to a 4-20mA signal.

Something like this…

[Kevin_Alicat · Accepted solution]

Hi Gee,

With the holidays last week, I was a little slower to find your post regarding the Alicat 4-20 mA setpoint. GerdW's suggestion is definitely a good one, and the linked converter would likely work well in in many cases, but there may be a simpler method and a possible discrepancy in how the Alicat 4-20 mA setpoint inputs operate and how loop-powered 4-20 mA devices operate.

The Alicat devices with the 4-20 mA setpoint input options are actually not loop power 4-20 mA compatible, but rather they sink the input current to the Alicat's internal ground through a 250 ohm resistor. This is done so that inside of the Alicat, the 4-20 mA input is converted to a 1-5 VDC signal that can then be read by the same ADC circuitry that is used for the standard 0-5 VDC input options. While this can cause issues with a loop-powered current input (which generally fix the voltage to a static level required of the supply, such as 24 VDC), it may make your issue actually easier to resolve.

I would believe that if your analog voltage output module of the myRIO you are used to using can support output currents of at least 20 mA, you should be able to send a 1-5 VDC signal to the device instead (if sending a 5 VDC signal, the myRIO voltage provider would need to source ~20 mA due to the 250 ohm current sink inside the Alicat).

Feel free to let me know if you have any questions, and I will be happy to assist.

[Kevin_Alicat]

Hi again Gee,

I did a quick look into the current drive capabilities of various voltage-only NI-DAQ setups, and they seem to cap out at anywhere from 3-10 mA drive capacity. This means that the straightforward DAQ output -> Alicat input method would likely not work due to current drive limitations, though I would still recommend checking if your particular setup can support that since it would be the easiest.

An alternative that could be a decent option would either be something like what GerdW suggested but for connecting to a current sinking device, or even creating a small circuit in-house that does a unity gain on the 1-5 VDC output from your DAQ and allows for the higher 20 mA current requirement (I don't have a particular circuit in mind already, but I would assume some sort of unity gain op-amp buffer could do the trick). The in-house version would of course depend on the requirements of the area in which it's operating (such as needing certified parts for hazardous areas, etc.) and the comfort level with building such circuits.

Figured I'd give a small update. Again, feel free to let me know if you have any questions.