2 wire digital capacitive level sensor problem

[joaco1993]

Hi everyone;

Im having some problems with a digital level capacitive sensor.

I wired it in the NO configuration and the input of the digital input module turns on, while it should be off.

If I touch the sensor, simulating it has material on it, the input turns off.

Looks like it works the other way around.

How ever if I wire the sensor in the NC configuration, the input of the digital module turns on also. and if I touch the sensor it goes off.

So both configuration works the same way, and I believe its not ok.

The input module is 16xDI helmholz: https://www.helmholz.de/en/products...modules/di-dc-24-v/artikel/show/600-210-0ap21

And the sensor is IFM k1000A: https://www.ifm.com/es/es/product/KI000A

Has anyone had this trouble ?

Thanks!!

 

[kalabdel]

How many wires is it?
My first thought, it is miswired.

 

[JTCat]

Have you tried using the selector switch?

 

[joaco1993]

How many wires is it?
My first thought, it is miswired.

2 wires. I am pretty sure it is wired correctly.

Another thing, if instead wiring to the input module,
I wire It to a 220v pilot lamp using 220vac, It works correctly
And as expected

But using 24vdc with input module doesnt work

 

[joaco1993]

Have you tried using the selector switch?

Yes, thats only for regulate the sensitivity of the sensor

 

[BryanG]

This is a common problem using 2 wire sensors with PLC inputs. The PLC input doesn't draw enough current to allow the sensor to work properly. You can try increasing the load by adding a resistor down to Common on the PLC, or add an interposing relay. The relay is probably the better option. If you look at the specs for the sensor it has a minimum load of 5mA, with a 2.5 mA leakage current.

 

[kalabdel]

2 wires. I am pretty sure it is wired correctly.

Another thing, if instead wiring to the input module,
I wire It to a 220v pilot lamp using 220vac, It works correctly
And as expected

But using 24vdc with input module doesnt work

As Brian mentioned the current draw of the sensor, to power it up, is turning on the module input. At 24VDC it shouldn't draw that much, a maybe just above 1mA
If they have a three wire version, that would be a solution too.

 

[joaco1993]

This is a common problem using 2 wire sensors with PLC inputs. The PLC input doesn't draw enough current to allow the sensor to work properly. You can try increasing the load by adding a resistor down to Common on the PLC, or add an interposing relay. The relay is probably the better option. If you look at the specs for the sensor it has a minimum load of 5mA, with a 2.5 mA leakage current.

Hi Brian,

I see what you mean.

In order to size the resistor or to choose correctly the coil of the relay I must know whats the min current for which the input turns on.

Where or how can I check that ? Couldnt find anything in the helmholz input module manual.

Thanks!!!

 

[BryanG]

Get ready to slap yourself on the back of the head
Obviously take all normal precautions when working with electricity.

You measure the input current the same way as you would measure any current. My systems have a +24V supply and a 0V common, adjust my description to match your system.
So to check the current an input is taking, I would: disconnect any other wires going to the input I want to check, set my multimeter to mA, move the leads to the current (mA, A) input position, connect the red lead to the +24V supply, connect the black lead to the input terminal, measure the current.

Some manuals give you the resistance of an input, then you can do all that just by calculation. I couldn't see that for the Helmholz device.