Variability in the 4-20ma Current Loop Transmitter

  1. How precise does the 5V input need to be for the PR33-49? When testing with this circuit board yesterday it seemed like small adjustments in the 5V supply (4.9V to 5.1V) had an effect on the output current. If I was sending a command to output 20ma when applying 5V, then I adjusted the input voltage to 5.1V I would find the current to go up to 21.5ma.

Is this an issue with how I had the board wired or is this an expected behavior of the board?

  1. Also in the sample code you provide it has the full scale range as 290-1500. In your website documentation it has the full scale range as 670 - 3460. It seems to me to be the latter values. And should these values be constant for every board and installation? Say the 5V power supply varied slightly - 5.05V for one installation and 4.95V for another. Would the DAC values still be the same to get 4ma and 20ma?

  2. Can you use the PR33-49 with an external power supply? I know there is a built in 12V power supply and I am curious if there would be any issue using this in series to an external 12V power supply?

Thank you!

  1. This is a DAC. If the supply voltage goes up the output voltage will go up which results in higher current output.

  2. As stated in answer 1 if the supply voltage changes the output values will change. Also there can be slight variation in electronics so not every device will be identical. The DAC values should be tuned to the hardware and the power supply used.

  3. This product is intended to be powered by the I2C bus. Using an external power supply could create ground differentials so I would be careful with that. My recommendation is to use Bus I2C power and tune the DAC values to the hardware.

  1. Ok, so for each installation on a separate I2C bus, this hardware would need to be tuned? Assuming I am powering this device via I2C from another nodeLynk device how much variance could I expect between devices? Could I expect a difference in the 4-20ma output to be different be by 5% or much less? If this difference is super low I am not worried but ideally I would like to have this be plug and play and not need to calibrate the min and max for each installation.

  2. I am a bit confused. Per your documentation for the PR33-49 it says the Loop Supply Range is 7.5V to 36V. If this device is not intended to be connected to an external power supply, how is this range relevant? Also do you know the maximum resistance you can have in the system for this device to work properly?


  1. The typical error rate is around +/-2%
  2. Loop supply voltage is fixed at 12V. its been corrected on the product page
    for more info checkout XTR datasheet

I’m more confused. Where does the 16V loop supply voltage come into play? The PR33-49 has a built in 12V power supply and is powered from the 5V I2C bus. Per the earlier message in this thread, I was told using an external power supply could create ground differentials and to be careful with powering this device with an external power supply. Can you please clarify where the 16V loop supply voltage comes into play?


I meant 12V supply. There i on board buck boost.