Issues with 4-20mA output

[persjere]

I have the Industrial Automation Hat. I have successfully tested the mA inputs and discrete outputs but I can't seem to get the 4-20mA outputs to work. I followed the schematic from the manual (+DC to load, negative of load to +mA output on hat, -mA output on hat to -DC) and seem to be writing the values to the appropriate registers. I included the crude test code that I used below.

The current loads that I have are passive. One thing that I noticed is that the outputs are providing a voltage value even with the load disconnected. As the diagram in the manual shows an external DC supply in series (page 12 of the User's Guide Version 3.2), I expected the outputs to be passive and not provide a voltage. I tried connecting these to a 250 ohm resistor to see if the outputs are active. The output provided well over the 1 to 5V I would expect for a 250 ohm load and the resistor heated up significantly.

Does anyone have any suggestions?

print("Output values:")
i = 1
while i<=4:
set4_20Out(2,i,outputVal)
print(get4_20Out(2,i))
i+=1

if outputVal < 20:
outputVal += 0.5
else:
outputVal = 4

 

[alexburcea]

Hi

Please tell me the card hardware version, or share a picture with the top side of the card.
In the last version we change the 4-20mA output connector. We added +24V (or card input voltage) to the connector so you will not need an external power source. You can connect your load directly between + and - pins.

 

[persjere]

Thanks for the response. I have version 3.2. Rewiring the outputs fixed the issue for two of my outputs. The third output, however, is putting out a reduced current value. The output ranges from approximately 3 to 11 mA when I set the output between 4 and 20 using the set4_20Out command. Additionally, output 4 puts out a fixed 65.0 mA when connected to a 250 ohm load. Do you think that wiring based on the outdated diagram damaged the board?

 

[alexburcea]

Hi,
The connection you describe (the one from the manual) can not damage the card but if you tried different combinations, it might.
The channel 4 looks damaged if the current is fixed at 65mA. You might try to calibrate the channel 3.

 

[persjere]

Thanks for the response. I tried three wiring configurations. The first was based on the outdated manual, with the DC voltage supply in series with the load. In the second configuration, I just added a fixed 250-ohm resistor across the fourth output to try the signal with a passive device. I noticed an excessive voltage across the resistor (something like 19V) when doing so. The final configuration was wiring all the signals without the DC supply in the loop. The final configuration worked as described in the previous post.

The only thing that I can think of that would have damaged the 4th output is if I inadvertently touched the resistor's leads together, thereby shorting out the load.

 

[alexburcea]

This is a possibility since the constant current circuit can not respond instantly, the output transistor might experience some high transient current.