Sensor connection

[Tom749 0]

Why do we make a way to connect the moisture sensor to PLC io card with external 24Vdc source? Could you explain the polarity direction (+/-) among components in the attached file? Is there any theory or term for this connection? Thanks for your help in advance. Edited 5 Mar 2013 by Tom749

[OkiePC 14]

The moisture sensor appears to be a passive 2 wire device. You supply it with power at its "+" terminal. It's "-" terminal should really be called a return signal. The return wire number 2001 signal passed through terminal number 28 and then is applied to the positive input on the PLC analog card. This moisture signal return will be a voltage or current that is proportional to the moisture content and interpreted to a value by channel 0. The circuit is completed by connecting the power supply low side to the common for the analog card with wire N11. The draftsman could have kept the two sensor wires shown individually. Showing them in a cable makes it a little harder to follow. Edited 6 Mar 2013 by OkiePC