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ADC11 connected to two 4-20 mA instruments

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ADC11 connected to two 4-20 mA instruments

Postby peterb » Fri Apr 18, 2008 10:46 am

I hope some one can help with this !!
I have been using the ADC with one instrument for some time with perfect results. This circuit is from a battery powered unit so I have had to wire in a 24V DC supply.
The additional instrument is 3 channel and mains powered and when I connect it to the ADC Terminal board all the voltages look correct but when it is plugged into the ADC the 3 new lines go to maximum.

I believe it could be something to do with different ground potentials but have not been able to figure it out!

I have attached a pdf of the circuit to make it easier to understand.

Contrec 4-20 mA Output.pdf
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Postby PeterF » Thu May 22, 2008 2:35 pm

After some measuring of voltages the problem was solved:-

I have checked the various voltages and found the culprit.
Measuring between the GND connection of the 120 ohm resistor, and chassis earth I get:
From the Contrec Flow Meter,���� 0 Volts
From the ADC Gas Analyser��� ~12 Volts
And on the GND pins of the ADC11 when connected to our PC, 0 Volts.
It looks like I am going to have to isolate the signals from the Analyser!
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