Two wire Proximity connection to PLC input

[rigicon]

Hi all I did a quick search and couldn't find this specifically.
I always buy 3 wire 24vdc proximity's and wire them (fused terminal) direct to my PLC inputs - It just works.
Our client has specified 2 wire NOT Namur as there is no zoning issues.
Had the Turk sales guy here and he says it can be wired direct as there sensor needs 3mA. My argument is that there is no load.
The input on a PLC is an opto-coupler which needs 4mA to operate taken from a Mitsubishi DI datasheet.
I am sticking to my guns 3 wire straight into Input - I know it works. 2 wire needs an interface relay.
Only referring to 24VDC not AC as this is the control voltage.
Any body got any comments or am I too old school and need to change.

 

[nonuke]

I always use 2 wire 24vdc PNP proximity switches.
The +24VDC of the switch goes to +24VDC, and the output of the switch goes to the PLC Input.

 

[JesperMP]

2-wire sensors for 24V.
I am thinking "reed switch", not "PNP".

Do you have a link to a 2-wire 24V sensor with PNP output ?

 

[Lancie1]

I have used lots of Turk 2-wire proximity switches with various brands of PLCs. I never had any problems with getting them to work. The important point is to match up your PLC input with the sensor type.

 

[JesperMP]

Well, I just learned something thanks to this site.

It seems that there are 2-wire 24V PNP proximity sensors. And there are 24V PLC inputs that are "2-wire compatible".
I just checked that for the typical ET200S 24V input module, the off-state current from the sensor must be max 1.5 mA. And a Turck 2-wire sensor has an off-state leakage current of 0.8 mA or less. So putting these together should work.

 

[rigicon]

Thanks I have learn't something new.
So Like JesperMP mentions I need to match the off-state current of the PLC input card which is deep in the database somewhere(on state readily available)with the off state leakage current of my sensor being less example as JesperMP mentions a Turck 2-wire sensor with 0.8mA or less.
I honestly thought a 2 wire Proximity needed more of a load.

 

[Jeff23spl]

let see what is the part number but it should work.

with ohm law, when the prox is open, there is almost 24vdc across the prox terminal and when it is closed, it keeps just enough voltage (Ohms) to keep 3mA but if plc input use 4mA when ON. IT is very tight but should work.
At worst the OP could install a 5K resitor or less parrallele to input term and 24v- to double the current draw to ensure the voltage drop is enough at the prox

Edit: yes the off state leakage need to be checked too but it will basicaly become a matter of voltage. For your 6K ohm input if you have a 0.8mA leak from the prox, the plc will see 4.8V

 

[JesperMP]

Just for comparison, an ET200S input draws 7 mA in the on-state.
So yes, 4 mA sounds like it is a bit close.
Maybe you should make a test.

 

[rigicon]

from the PLC input card datasheet:-
Rated input current - approx 4mA
On voltage/ON current 19V or higher/3mA or higher
OFF voltage/OFF current 11V or lower/1.7mA or lower
I think this will work with the Turk JesperMP mentioned.

Here is a typical Turk inductive 2 wire sensor:-
operating voltage - 10...65VDC
DC rated operational current <= 100mA
Residual current <= 0.8mA
Voltage drop at I <= 5V
Smallest operating current I <= 3mA

Just to confirm this would work - Unfortunately I don't have the hardware to try out.
Design spec stage.