4-20mA current loop transmitter: Malfunctioning?

Hello, we recently ordered a PCA9547 8-Channel I2C Multiplexer, and 8x 4-20mA Current Loop Transmitters. The 8x 4-20mA current loop transmitters appear to be malfunctioning.

We are getting successful I2C communication with the onboard MCP4725, and can verify the DAC value by polling the register and it returns what we sent it. The MCP4725 is too hot to touch with nothing connected to the output. With a 4-20mA sensing device connected nothing changes, and there is no current loop regulation. We measured the 0.248V reference on the board and we get random values typically floating close to 0 volts. We also measured the MCP4725 analog output pin (pin 1) and get similar voltage readings no matter what values we send it. We also verified the board has good ground and 5V on the I2C interface, and like mentioned before we have good I2C communication. Per the documentation, it says DAC output of 670 is 4mA, and DAC output of 3460 is 20mA, so this is what we are doing). I also wanted to note that we removed the multiplexer from the equation and are just trying 1 at a time, and still same problem. The symptoms are the same on all 8 modules we received. Any help or advice is greatly appreciated, thanks!

can you share a close up picture of the 4-20mA board?
are all of the boards getting hot?

Hi Bhaskar, thanks for the response. Here is a close up image of the boards. And yes all 8x of the boards get hot.

everything looks good in terms of parts and soldering.
The part which might be getting bit hot could be XTR115( near RNE1 resistor network). Its a current generate so it should be okay as long as its not extremely hot. it should be running around 82-85’F.
regarding the no output
did you add the loop power supply while testing it?

you can checkout how to apply loop power supply in the resources tab.


Hi Bhaskar, that was the problem. Our last 4-20mA current loop transmitters contained the power supply, it was built into the transmitter circuit. So we got an external power supply and everything is working now.

The source of the heat seems to be coming from the MCP4725, it is too hot to even touch it.

Let it run(overnight) and see if the temp increase.
if you have a temp gun, can you measure the temp and let me know what is it.
I will run a test setup over here and will see how this one reacts.

i did some testing over here and the max temp was around 85’F.
if you are still having temp issue i will recommend sending it back for further testing.