FMD88 Dim output light

[mvincent]

Hi,

I am trying out a new prox sensore from Keyence.  It is a two wire NPN 24 v DC prox.  When it is in its off state, the input it is tied to, leaves a dim light on.  I am using the blue wire to the PLC and black to 0v from it and it is wired properly according to the wiring diagram.  Any ideas?  I am also pulling 17v in off state vs 20v on my other outputs.

Thanks

[support]

Two wire proximity sensors don't  work too well connecting directly to the PLC's digital inputs. This is because two-wire sensor require some minimum current to operate and these will cause the current to flow via the NPN input 5.6K pull up resistor ->green LED -> sensor->0V. The current will light up the input LED slightly. If the sensor draw enough current it could even turn on the PLC's digital input and therefore it is not recommended. A 3-wire sensor will work better with these PLCs with NPN inputs.

If you must use the two-wire sensor, try to attach an external pull up resistor (say 1K) between the PLC's input and the 24V power. As long as the current that flows through the sensor when it is OFF does not cause more than 1.5V drops over the external pull up resistor the LED will not light up.