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General => Technical support => Topic started by: k1w1 on November 14, 2008, 11:52:02 PM

Title: PNP proximity sensor
Post by: k1w1 on November 14, 2008, 11:52:02 PM

Hi

Has anybody used  PNP  type proximity sensors to switch a T100MD input and if so what issues did you have ? also what value resistor did you use to hold the input low ? Normally I use NPN type but in this case the machine already has PNP type fitted , cost to change is a problem.

  ::)

Thanks for your time
Pete

Title: Re:PNP proximity sensor
Post by: support on November 16, 2008, 09:07:31 PM

You can use a 1K ohm pull down resistor between the PLC's NPN input and the ground (0V). In this case, the PLC input is turned ON when the PNP sensor is OFF. When the sensor turned ON, the NPN input will be pulled up to 24V by the sensor and therefore the input is OFF.

You can work on the input directly using negative logic, or if this is confusing, simply invert the state to a relay and use the relay in place of the input contact to the logic:
     IN1                 CR1
|--|/|-----------(RLY)

Title: Re:PNP proximity sensor
Post by: k1w1 on November 16, 2008, 11:16:53 PM

Thanks for the prompt reply      
As the input will be held low for long periods, will 1k be high enough to prevent any input power dissipation problems with a 28v power supply.

Title: Re:PNP proximity sensor
Post by: support on November 17, 2008, 12:07:13 AM

The NPN input is pulled up to the positve supply via a 5.6K ohm resistor and LED. LED forward voltage drop is about 2V.

So if you connect PLC to 28V power supply, the voltage at pull down resistor R

V(R) = (28-2)*R/(R+5600)

For the input to be guranteed ON, we recommend that V(R) < 3V

So you should choose a lower value of R since your power supply voltage is higher.

E.g R = 680 ohm. V(R) = 2.81V

Power that consumed by R

Power(R) = V^2/R = 2.81^2/680 = 0.01W

So any 1/4 W resistor can be used.