Interfacing Sensors


1.       All 2-wire and 3-wire proximity sensors drop some voltage in their ON state. This voltage drop determine show close the PLC input can get to the supply voltage. If the supply voltage minus the voltage drop isn’t high enough, the input won’t turn ON. The ON voltage spec is different for different PLC inputs.

2.       The PLC input will draw a certain amount of current when it is ON, depending on its internal resistance. For 2-wire proximity sensors, this current must be higher than the minimum current requirement of the proximity switch for it to work reliably. The internal resistance spec is different for different PLC inputs. However, PLC manufacturers try to keep this value as high as practical to minimize the total current that might have to be supplied to a lot of inputs in a big system.

3.       All 2-wire (not 3-wire) proximity sensors leak a small amount of current through the load when they are OFF. This current flowing through the internal resistance of the PLC input produces a small voltage at the input. If this voltage is greater than the value that will allow the input to turn OFF, the input will remain in the ON state.

 

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