Automating Manufacturing Systems - Process Control and ...

discrete sensors - 4.9
PLC Input Card
for Sourcing Sensors
00
+V
two wire
sensor
-V
01
+V
power
supply
-V
com
V+
Note: These sensors require a certain leakage
current to power the electronics.
PLC Input Card
for Sinking Sensors
00
01
+V
two wire
sensor
-V
+V
power
supply
-V
Figure 4.8
Two Wire Sensors
A two wire sensor can be used as either a sourcing or sinking input. In both of
these arrangements the sensor will require a small amount of current to power the sensor,
but when active it will allow more current to flow. This requires input cards that will allow
a small amount of current to flow (called the leakage current), but also be able to detect
when the current has exceeded a given value.