Wiring 2-wire sensor to sinking input

Cunk

Member
Join Date
May 2004
Posts
3
I'm trying to connect a Turck 2-wire inductive prox sensor (Bi1.5-G08M-AD6X-H1341) into an NPN input (on a Triangle Research T100MD888+ PLC if it helps to know).

When the sensor is triggered I'm measuring 4.6 V across it. According to the PLC's manual, 0-5 V is considered active but I suspect that I'm cutting it too close with the 4.6 V.

Is there any way I can get this to work? Sorry if I haven't supplied enough info or if I've confused you with my description. I'm fairly green when it comes to this stuff. Let me know if you need more info.
 
across?

When you say you measure 4.6 volts across it, exactly where are you measuring from. From the input terminal to the common terminal on the card? Or from one side of the sensor to the other? Depending on where you are measuring from will tell us whether the switch is working, or not, or whether you have a bad input, etc.

Russ
 
It's measured between the input terminal and DC ground which is actually the same as measuring across the sensor in this case. Triangle's PLC inputs only have one terminal and they're internally connected to the positive rail of the power supply (24 V in this case).

I think I mistakenly visualized the Turck sensor working like a simple mechanical switch that when triggered provides a nice path to ground for the input but I guess it's not that simple.

edit:
I should mention that I have two of these sensors connected to two inputs and I also connected a third to another input (a loose sensor for convenience) and all exhibited the same behavior. Turck's catalog states that the voltage drop across a conducting sensor will be <= 6.2 V so the 4.6 V I'm measuring is in line with that.

Here's the relevant section of the manual if that helps:

tplcinputs.png
 
Last edited:
I always use three wire proximity switches into PLC's....If I had to use a two wire one then use an interposing relay. This should work, be careful of the resistance of the coil of the relay though, it might not be enough to trigger.... I hate 2 wire DC proximity switches...
 
Yeah, we'll defintely be switching to 3-wire NPN prox switches but I have an immediate need to get these two-wire switches working.

Unfortunately I don't have room for additional relays at the moment -- we're stuffed into the designated area as it is.

I was hoping for a temporary kludge that would involve some simple passive components (similar to solving leakage issues using a current limiting resistor).

*sigh* I suppose not every situation lends itself to such simple fixes.
 
This is how it is for connecting a 2 wire DC prox for an Omron PLC.
Now you can see why i always use a three wire prox....

2wire prox.gif
 

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