4-20 ma current loop sensor on the D21G?

i have been using the D21G to measure output from a 4-20 ma current loop sensor. Since the D21G analog input only measures 0-5VDC output, i connected a 230 Ω resistor in series between the sensor -ve terminal and ground. Technically, i should use 250 Ω but since i never reach the maximum output, i used what i had which was 230Ω. i then connect the A4 pin just before the resistor so that, effectively, i was measuring the voltage drop between the resistor which is directly proportional to the current flowing in the wire.

i was using a 12V DC supply for the D21G and the sensor. Everything worked fine for about a month then it suddenly stopped. i notice that there is a short between the A4 pin and Ground.

do you think my setup was responsible for this? i know the IND IO is more appropriate for 4-20 ma sensors but i had to use what i had and i only had a D21G.

any comment will be most welcome

thanks

viv

 

viv padayatchy
viv padayatchy
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Asked on 10/20/18, 10:10 AM
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thanks for your comment below Tom. I prefer to replace the D21G proto with the IND d21G just to be safe. The site where this equipment is deployed is not easily accessible for frequent maintenance

Cybernaptics Ltd, viv padayatchy
on 10/22/18, 6:24 AM

Hi, as responded by email: The I/O on the PROTO is 3.3V level, but 5V tolerant (it has level shifters). So if you need the full scale of the ADC (3.3V) you should change your load resistor for the 4-20mA loop to something like 165 ohms. Probably also a good idea to add a low leakage current 5V TVS diode to protect the input during transients (for example during startup, where your 4-20mA transmitter might output a higher current/voltage than 20mA).

Also make sure the load resistor is always connected before you connect it to the D21G analog pin, otherwise your sensor will try to sink current into the analog pin, and while the currents are small, the voltage may go over the 5V tolerance.

 

Tom
Tom
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Answered on 10/22/18, 3:14 AM
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Asked: 10/20/18, 10:10 AM
Seen: 70 times
Last updated: 10/22/18, 6:24 AM