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
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.
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