PNP Inductive Prox Switch Signal Always active on the pmdx-126 BOB

[robert.beal]

I have aquired several pnp contrinex sensors 100$ each (DW-AD-623-04) the current leakage is =<.1mA  when i hook up the sensor to the pmdx-126 board the light is on really dim when the switch is triggered the light gets bright on the board but in either case the signal to mach 3 is always on. How do i fix this problem. i heard npn sensor can have this this problem but no one has mentioned anything about pnp sensors. Any advise would be appreciated. Thanks!

DW-AD-623-04 
 Application Type Standard 
Rated operating distance sn 1.5 mm 
Mounting Embeddable 
Hysteresis ≤ 20 % sr (10 % typ.) 
Repeat accuracy norm ≤ 5 % sr 
Supply voltage range UB 10 ? 30 VDC 
Permissible ripple content ≤ 20 % UB 
Output current ≤ 200 mA 
Voltage drop ≤ 2 V 
No-load supply current ≤ 10 mA 
Off-state current ≤ 0.1 mA 
Switching frequency ≤ 3000 Hz 
LED Built-in 
Ambient temperature range TA -25 ... +70 ?C 
Ambient temperature range TA US -13 ? +158 ?F 
Temperature drift of sr ≤ 10 % 
Short-circuit protection built-in 
Polarity reversal protection Built-in 
Power on reset Built-in 
Degree of protection IP 67 
Housing material Stainless-steel V2A 
Connection Cable 
 

[Bob at PMDX]

I found the data sheet here:
http://www.contrinex.com/xml/productSheet.aspx?src=ps_DW-AD-623-04&langage=English&typeFolder=IND

Looking at the PDF, the PNP sensor has an internal 47K res from the output to gnd.  The PMDX-126 presumes that the PNP sensor will be either "open circuit" or pulled to +24V.  The sensor's internal 47K resistor to GND appears to be fooling the PMDX-126's input circuit to make it think there is an NPN sensor (or mechanical switch) that is "active".

Are you powering your sensors from an external 24V DC power supply?

I can see two ways to work around this:

(1) Add an external diode between the sensor's black wire and the PMDX-126's input terminal.  The cathode of the diode (the end with the stripe on it) should go on the PMDX-126 side.  This will prevent the sensor's internal resistor from sinking current from the PMDX-126 and falsely triggering the input.  This can be any 1N4001, 1N4002 or 1N4148 style small signal or switching diode.

**OR**

(2) Add a resistor from the PMDX-126's input terminal to the "+5V" terminal on J11 or J12.  The resistor could be a 2.2K ohm, 4.7K Ohm or maybe even 10K ohm.  This *should* keep the sensor's pull-down resistor from drawing enough current to trigger the PMDX-126's input circuit.  But please note that I haven't tested this solution yet (I'm working from home this morning).

I'll see if Steve has any other suggestions later today.

Bob

[robert.beal]

I added the external diode and it worked great. (No false triggers)
Thanks again for the help!

[Bob at PMDX]

Glad to hear it!

Bob