JF-1A Conductivity Sensor
A440-009
Page 42 of 56
FEBRUARY 2009
Example 2
JF-1A Conductivity Current Device Loop, with a secondary
Temperature Current Loop:
As in example 1 a 24 VDC isolated power supply supplies loop power and a 250
ohm sense resistor is used to sense the current in the conductivity loop (convert
the current to a voltage in the PLC). As in example 1 the JF-1A derives its
power for its operation from the loop current supply. Note both the supply and
loop current detector are normally isolated from ground.
A second fully isolated 24 VDC supply is needed for the temperature current
loop. This is required as the conductivity loop which powers the JF-1A has
established a “virtual” common voltage point. If the user is to establish another
connection between the two loops then the virtual loop ground is corrupted and
neither loop current will be correctly set by the JF-1A, and, under extreme
circumstances may result in damage to the JF-1A current output devices. A
common method to avoid this problem is to install a 4 - 20 mA to 4 – 20 mA
isolator in the Temperature loop ( Example http://www.api-
usa.com/pdf/api/api_lpi-1_lpi-2.pd).
