4 to 20 mini amp analog input

[kenobe]

  ???Hi
I am using a T100MD 888 plc tring to read an analog  temperature sensor with 4 to 20 mini amp output.
I connect the sensor to analog #1 according to the T100MD888 user manual.
Hower, when I check the voltage at channel 1, it is always 9 volt regardless of the temperature change.
When I isolate the sensor from the PLC, the output of the sensor is 0 to 4 volt and current is well within 4 to 20 mA.
When I connect the sensor ouput to the PLC, the volage at the input channle #1 is always 9v.
Could this be a current loading problem. please help
Thanks

[support]

As mentioned in the T100MD+ manual, to connect 4-20mA to the PLC's analog input you need connect the 4 to 20mA to a 250 ohm resistor. The high side of the voltage connects to the analog input and low side of the voltage connect to the PLC's ground.

If you are connecting two-wire type 4-20mA sensor to the PLC analog input please refer to the following post:

http://www.tri-plc.com/yabbse/index.php?board=2;action=display;threadid=347;start=msg979#msg979

[kenobe]

I have done exactly what you have advised, and that does work
Thanks, the voltage at analog input is 9V.

[support]

If you have connected a 250 ohm resistor and you measure 9V across the two terminals of the resistor, it means the current = 9/250 = 36mA. This excessive current probably caused by  grounding issue.

Do the PLC and the sensor share the same power supply? If so it could mean that the current return path to the analog device is not the same ground level as the 0V of its power supply. In this case you should use a separate power supply for the PLC and the sensor.

[kenobe]

Hi
Thanks for your advise, I will try using seperate power supply for the sensor and the PLC. and make the common ground
Thanks