2 wire prox switch

escoplcguy

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escoplcguy
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Jun 2010
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I have a prox switch going to a wago IO system. Blue wire connected to 24v common brown wire connected to 24v input card. trigger the prox and red light turns on and i get a 1. pull the trigger off and red light turns off, i still get a 1. I seen an old thread that said...

"Your 2 wires will leak and your PLC input will always be energised... unless you add a load to common between the IO and the device."

Where does the load come from, can anyone provide me with a wire diagram?? what is the load exactly I'm getting confused.

i'm so used to 3 wire switches.
 
What are the exact make/model and part numbers of the proximity switch and the Wago module ?

2-wire switches always require some current to flow through them so they can operate. That means when they're used with sensitive PLC inputs they often make the input appear always on, unless you also use a load resistor to "drain off" that small amount of current.

Because you connected the Brown wire to the Wago module terminal, that suggests that the Wago module is providing +24V and is a "Sourcing" type module. Is that correct ?

I am facing a similar problem today with a customer application that showed an ordinary 3-wire sensor in the wiring diagrams (circa 1997) but when we actually got onsite there's a 2-wire sensor with no labels.

(P.S. Thanks for creating a new thread !)
 
The wago module is a 753-431. The Prox I don't remember the partnumber off hand but is a Turk barrel type Two wire Prox. For the wago module, sending (+) 24v from the device to the module input turns on the led or a 1 on the controller... I believe this is sourcing correct? Because the module is looking for a positive (+) 24v signal. Sinking would mean the module is looking for (-) 24v right? Just to make sure I do understand sinking vs. sourcing.
 
You can put two automation engineers with 20 years experience apiece in a room and ask them to explain Sinking and Sourcing and get two different answers. I've done it.

In general, a "Sinking" input module expects to have +24V DC provided at the input terminal when the logic state is True. The device "Sources" +24V DC, and the input module "Sinks" it to DC common.

A "Sourcing" input module expects to have 0V DC provided at the input terminal when the logic state is True. The module "Sources" +24V DC and the device or contact "Sinks" it to DC common.


The Wago 753-431 module calls itself a "High Side Switching" module, and the wiring diagram is consistent with a "Sinking" input module where you provide +24V DC to the Input terminal when you want the input state to be True.
 
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Try connecting the Brown wire to +24V DC, and connecting the Blue wire to the Input terminal of the Wago module.

I think the problem here is the leakage current of the 2-wire sensor versus the very low input current required by the Wago input module. The datasheet says the Wago module input point only draws 2.8 mA.

Before you can say for sure how a drain resistor might work or how it will be sized, you have to figure out the characteristics of the Turck sensor and how much power it needs and how much it can allow to flow through the circuit.
 
Turck shows a minimum 3 mA for their 2 wire prox's. For this application, I would use a 4900 Ohm resistor. Low wattage. 1/4 should be fine. That will add about 5 mA to the load.

As a general rule when properties are unknown, use a 2400 Ohm, 1/4 watt resistor. That will draw an additional 10 mA. PLC inputs are in the 2-4 mA range, and any prox can handle a minimum of 50 mA or so. I would likely use 2400 Ohms with the Turck too, for the added noise immunity.

You should also be aware that 2 prox's need a constant load to remain powered and have proper switching speed. See the link from Turck. See page M18.
 
You're right to be concerned; I have damaged a few proximity sensors over the years by inadvertently shorting their outputs to ground.

But the Wago module will present a load in either polarity.

The Turck datasheet is a little surprising to me; its this really meant to be an AC or DC sensor ?

It shows the Brown wire connected to "L1", with an AC wave symbol.

That suggests to me that if you're using 24V DC that the Brown wire should be wired to +24V DC, and that the Load is the Wago input module.

The sensor datasheet says that the off-state leakage current is less than 1.7 mA, while the Wago datasheet says its typical on-state current is 2.8 mA.

That *ought* to work together, but from what you've seen it does not.

I would try it in opposite polarity, and then continue from there.
 
Ken Roach said:
The sensor datasheet says that the off-state leakage current is less than 1.7 mA, while the Wago datasheet says its typical on-state current is 2.8 mA.
Click to expand...
A 2 wire prox needs to "steal" it's power from the circuit. The important spec is minimum load. Turck shows that to be >= 3.0 mA. Same for AC and DC prox's.
 
ok so which direction should i head first... switching polarity or using the 4900 Ohm resistor?

Now if i do go with the 4900 Ohm resistor do i solder it in series or just bend the ends and put both into the same terminal block. also does this go on the current side (blue wire) or the positive side (brown wire) or across both.

If i am to put this on the blue wire, i currently have 2 prox switches, one for up one for down, the commons (blue wires) are twisted together, do i need to double the 4900 Ohm resistor or will the one still work. of course if this is to go on the positive side that will not matter as it would then be one each.

Thanks again guys for all the help!
 
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Your prox is AC/DC. Polarity doesn't matter. Swapping leads won't help.
Following convention, Brown should be +24. Blue to module.
In addition, add a resistor from the blue wire to 24 V return.
Parallel with the Wago, not series with prox.
You need 2 resistors - one for each prox. If you have a choice, go for something smaller than 4900 Ohms to give a better margin for error (noise). 2400 Ohms isn't common, but 2700 Ohms is.
 
Could you draw up a wire diagram, I'm a bit confused cause you said to wire
It parallel, the cut sheet I posted earlier says "always wire load in series with the sensor" this load I assume is the resistor correct? Please forgive my ignorance, I don't disagree with you I just am having a hard time picturing this all in my head and where to put the resistor.

To clarify the resistor is the load right...looking at the wire diagram on the cut sheet 24v (+) goes to brown wire. Blue wire goest to load. load goes to ... Wago..
 
output current is >3 mA and <400 mA
input current of wago is 2.8 mA
so it will always be ON.
use a resistor to ground giving 20 mA (nice standard value use 1000 ohms
20 mA*24V = 480 mW so use 1 Watt minimum. Or a LED on a MIMIC panel at 10 mA with resistor of 2200 ohms giving 250 mW (max rating of a 1/4 W resistor so also use a 1 Watt type.
btw as it is a AC type any connection is possible.
 
escoplcguy said:
To clarify the resistor is the load right...looking at the wire diagram on the cut sheet 24v (+) goes to brown wire. Blue wire goest to load. load goes to ... Wago..
Click to expand...
Connect the blue wire to two loads - the Wago AND the resistor. Resistor is in parallel with Wago.
Wago isn't enough of a load by itself.
Shooter is correct about wattage. Use 1/2 Watt or 1 Watt (preferred). It will get a bit warm when prox is made.
 
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