Inductive proximity switches

[Skiroy]

I have a few basic questions. as far as the most commonly used inductive limit switches.

1. Do all DC limit switches have 3 wires and what is the 3rd wire for? Acording to the schematic you have +,- but then there is the 3rd that looks like it is wire in parralell to the + lead and has a load symbol on it which I assumed was the led?



2. Do AC limit switches only have 2 wires? This confuses me because Usually its the opposite on pneumatic valve solenoids. Usually the DC solenoids have 2 wires and the AC solenoids have the 3rd ground.

3. Lastly is there a way to test the limit switches with a multimeter? What should they read in ohms?


Here is the prox switch

McMaster-Carr

[Al_The_Man]

I assume you are talking electronic limit switches? You can get 2 wire or 3 wire AC or DC.
The 3 wire are generally used for a load such as a relay etc, 2 wire have lower current capacity and can be used for PLC inputs etc.
The 3 DC wire can be N.O. or N.C. NPN or PNP.
Not a really conclusive test to use a ohm meter, except test for short circuit maybe.
The link shows various types.
Al.

[Skiroy]

These are DC 3 wires going to a PLC.

The reason I ask about testing them is because. I have a Japanese Tube bender which uses a 3 wire DC prox switch. It lights up. And when I bring it to a Machine with a PLC to test it, triggers a input light. But for somereason its not working in the Japanese machine. We traded it for another prox switch and it works fine. Same wiring,same prox switch but the one that works has a smaller diameter. So this raises a few more questions.

1. Does a smaller diameter prox switch mean a further projected magnetic field to catch a metal object entering the field further away or faster or does the size have nothing to do with it? What specs would indicate this. They both say 200ma?

2. Can a switch led light up and not send an output? And if it does send an output strong enough to trigger a PLC input light can the signal still be too weak for other controller devices, such as the Japanese machine?

3. What signal does the prox switch output ma or Voltage? Whats the PLC looking for, from a prox switch?

4. And finally is there any risk with using a PLC as a testing device for Bad prox switches? I mean if they are bad there is just no connection right?

[Al_The_Man]

Smaller diameter generally means closer detection gap.
With different 3 wire Prox, you need to know whether it is NPN or PNP, gnerally the 3 wire are open collector transistor, with most the LED will normally light though, even if a load is not connected.
See the PDF for sink source, NPN/PNP. input to a PLC.
Al.

[Skiroy]

Originally Posted by Al_The_Man Smaller diameter generally means closer detection gap. With different 3 wire Prox, you need to know whether it is NPN or PNP, gnerally the 3 wire are open collector transistor, with most the LED will normally light though, even if a load is not connected. See the PDF for sink source, NPN/PNP. input to a PLC. Al.

I always thought PNP vs NPN was the difference between NO vs NC.

I was reading the PDF but it seems a little complicated for my level at this point. Could you maybe give a situation why you would choose a PNP vs NPN limit switch in a real world situation?

[Al_The_Man]

NPN and PNP can both be either N.O. or N.C.
Usually the selection of PNP or NPN is when interfacing with the like of a PLC etc that has either a Source or Sink input, see the PDF for an example, if the Prox is operating a device such as a relay, or similar physical device, it does not really matter, as it can be set up either way.
Al.

[Skiroy]

Originally Posted by Al_The_Man NPN and PNP can both be either N.O. or N.C. Usually the selection of PNP or NPN is when interfacing with the like of a PLC etc that has either a Source or Sink input, see the PDF for an example, if the Prox is operating a device such as a relay, or physical device, it does not really matter, as it can be set up either way. Al.

Okay thank You. Last 3 questions on this topic.

1. So PNP vs NPN is only a option for DC switches which explains 3 wires on DC switches or 4 wires on DC switches that allow the option to wire as PNP or NPN,right?

2. So for a typical say PNP Prox limit switch that has 3 wires, is it one wire for +, one wire for - and the 3rd for the PNP lead?


3. Finally I have the following phoyo sensor I want to wire to a timer relay so the timer receives the control voltage when the photo sensor's beam is not being broken.

Allen Bradley (AB) Photoswitch 42GRU-92LO | eBay

The documentation says the 4 leads are as follows:
Pin 3 = (+)
Pin 4 = (NPN)
Pin 1 = (PNP)
Pin 2 = (-)

The sensor outputs voltage when the beam is unbroken with a mode switch, but how would I wire the photo sensor to the control input of the timer and the 24VDC PSU?

[Al_The_Man]

You show a rather specialized universal item, the run of the mill prox or photo switches generally only have NPN or PNP and are ordered as such as you usually only use one or the other.
But if you can set it as you say so it is operate on light or operate on dark, then what are the spec's of the timer relay you intend using?
Al.

[Skiroy]

Originally Posted by Al_The_Man You show a rather specialized universal item, the run of the mill prox or photo switches generally only have NPN or PNP and are ordered as such as you usually only use one or the other. But if you can set it as you say so it is operate on light or operate on dark, then what are the spec's of the timer relay you intend using? Al.

Just a standard repeat cycle timer with control voltage of 24VDC. Just uses a typical 2 contact 24VDC coil to trigger the timer.

[Al_The_Man]

You should probably be able to use either of the PNP/NPN outputs, more details of a model number of the timer would help to confirm it.
A load resistor in the collector may be needed depending on the characteristics of the contact input.
Al.

[Skiroy]

I want to make a limit switch tester. I have had situations where the limit switch LED lights but still doesnt get signal.

Im going to make a little box that has a 24vdc and 120VAC output leads. Then I can hook up the limit sxwsitch and look for the voltage or current output. I could also use this PSU box to test solenoids easily as well.



1. Okay what does the PLC look from a limit switch, current or voltage?

2. When the limit switch has a current rating of 3-300ma does that mean thats how much current it can accept before malfunctioning or is that the amount of current it passes to the PLC? What determines this current amount? I would think the limit switch has a set resistance so if you apply 24VDC to it,it should have the same current everytime?

3. How do you tell if the PLC inputs require PNP vs NPN limit switches?

[Al_The_Man]

The input impedance of the PLC decides the current, as to the type of solid state switch needed, the pdf in post #4 will tell you, you need to know the configuration of the PLC, i.e. sink input or source input.
If rigging up a stand-alone tester you need to provide a series resistor that limits the current to less than the current rating of the switch.
Al.