Please School Me On Proximity Switches


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Thread: Please School Me On Proximity Switches

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    Default Please School Me On Proximity Switches

    Hi All,

    Another thread has got me thinking about proximity sensors for my current build.

    I already have a control box with everything wired from my first build, Gecko 201X, and the top of the line breakout board from CNCRP (well it was the top of the line several years ago when I bought it, I honestly don't remember the model number, I'd have to do some digging)

    So I will be using the same control box on this build. Previously I used the little square mechanical limit switches. Now I am considering proximity sensors. I have bulkhead connectors, nice and neat, on my control box, for audio cable jacks. In the past I have found these work well for 2 wire mechanical limit switches. I have a bunch of good quality audio cables. I really don't want to redo the bulkhead connections for 3 wire proximity sensors or figure out all of that......

    Also, my limit switches are Normally Open.

    So I found these on EBay:

    DC 2 Wire 6-36V 300mA NO 8mm Inductive Proximity Sensor Switch LJ18A3-8-Z/EX | eBay

    2Pcs 2-wire DC6-36V NO Approach Sensor Inductive Proximity Switch LJ8A3-2-Z/EX | eBay

    Looks like these are two wire proximity sensors. Why do some senors need three wires if these can use only two? I just want to find something with two wires that works and is inexpensive, and not think about it too much. If the simple answer is that they will work for me, really, that's all I need to know.

    They list 6 to 36 VDC as an operating range. I don't recall what the limit switch voltage is on my BOB, but I think 5V rings a bell. I have watched some vids of people with 3D printers using 5V on the 6 to 36V sensors and it worked fine for them, but also, in the vids I saw, they were using 3 wire sensors as far as I could tell.

    Has anyone out there used some of these 2 wire prox sensors? How do they work for you? What voltage did you run them at?

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    Community Moderator Al_The_Man's Avatar
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    Default Re: Please School Me On Proximity Switches

    The difference between 2 wire and 3 wire is the 3 wire are capable of switching higher current, for e.g. they have a NPN or PNP transistor output and they can pick up a relay, if needed.
    The 3 wire are either sink (NPN) or source (PNP).
    The two wire is used for low current required inputs, such as a transistor or opto input device, and can be used in source or sink mode.
    The voltage range is shown on the device or the manuf. literature.

    Al.

    Attached Thumbnails Attached Thumbnails Please School Me On Proximity Switches-sinksource-pdf  
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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by Al_The_Man View Post
    The difference between 2 wire and 3 wire is the 3 wire are capable of switching higher current, for e.g. they have a NPN or PNP transistor output and they can pick up a relay, if needed.
    The 3 wire are either sink (NPN) or source (PNP).
    The two wire is used for low current required inputs, such as a transistor or opto input device, and can be used in source or sink mode.
    The voltage range is shown on the device or the manuf. literature.

    Al.
    I assume the current requirement for a limit switch / proximity sensor is pretty low? It says in the specs current output 200 to 300 mA.

    I used to be able to make a 555 timer out of JK flip flops. Now, I have no idea what you're saying. LOL.

    Will a 2 wire proximity switch work for my purposes assuming my breakout board supplies enough voltage? What's your best guess on that? Please keep it simple for me. I'm slow.



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    Community Moderator ger21's Avatar
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    Default Re: Please School Me On Proximity Switches

    What type of breakout board do you have? Is it a PMDX 126?

    Gerry

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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by ger21 View Post
    What type of breakout board do you have? Is it a PMDX 126?
    It was several years ago that I bought it, but yes, that does sound very familiar, I believe that is what I have. I'm looking through a manual I found online just now after reading your post. I don't know what revision I have.



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    Default Re: Please School Me On Proximity Switches

    Most 2 wire will run from 10vdc up and are good for around 200ma.
    That board shows a 12v supply.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

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    Default Re: Please School Me On Proximity Switches

    Thanks ger21 and Al_The_Man for your comments so far!

    So it looks like with my board that I can use the 12V unregulated power for those two wire proximity sensors. Whether the 6V to 36V will work with 5V doesn't seem to be an issue as far as I understand it.

    What I don't yet understand, is the current requirement and the current output listed in the sensor specs.

    For the one sensor I posted a link to from EBay, it says in the title 300mA, but then lower in the description, it says "Current Output 200mA".

    For the second sensor I posted a link to from EBay, it says "Current Output 300mA"

    So I don't know what this means. Does it mean that it needs to draw 300mA to function properly? Or that this is the maximum current that it can handle?

    Thinking about the most basic electronics equation, V = I x R, you could calculate a current based on the resistance, and operating at 12 volts instead of the max of 36, you would only have 1/3 of the amps flowing through the wire. Obviously, this is a bit more complex, and it doesn't give you a resistance, but a "current output" in the specification. No idea how these switches work internally, not do I really want to know all the specifics, because I probably won't understand them without a significant time investment, but it seems that I do need to understand how much current will flow through the wires to see if they are acceptable to use.

    Looking at the user manual for the breakout board, it specifies that the current from all 5V connectors on the regulated power supply can not exceed 350 mA, and from all the connectors on the unregulated 12V supply can not exceed 300mA.

    This looks good at first glance, but for example, if two proximity sensors were activated at the same time, and each draws 300mA, then this would be a problem for me based on the breakout board specs. So someone, please explain what this "current output" specification from the sensor means in practical terms, and if this is only applicable at 36 Volts, with 1/3 of that at 12 Volts?

    Thanks again for all the comments so far.

    Please School Me On Proximity Switches-user-manual-jpg



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    Default Re: Please School Me On Proximity Switches

    The rating on the prox switches should be the max current they can pass. The sensors themselves require very low current to operate so I would think the current rating listed in the ad is how much current they can flow. And if I read it right, I think your board is going to require them to handle 6 milliamps. Just an educated guess. I've never used a 2 wire proximity switch, always used powered hall effect sensors. I thought that prox sensors were active devices that required power to work and that they always had 3 wires; 2 for power and one for the output. But I get lost when terms like PNP and NPN and sinking and sourcing start getting thrown around.



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    Default Re: Please School Me On Proximity Switches

    Two wire pass the current required for the typical input, opto device etc, they retain enough power across them in order to operate the internal electronics, without the need for a +&- supply.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by 1Jumper10 View Post
    And if I read it right, I think your board is going to require them to handle 6 milliamps.
    I don't think so? The board can deliver a maximum current in either 5V or 12V, depending on the pins used, it says don't exceed that current. The amps delivered depends on the resistance of the load. Or, am I misunderstanding the way the board works, and this is current limited, like you can program a VFD to limit the max current regardless of what the load (resistance) demands?

    And thankyou for posting!

    Quote Originally Posted by Al_The_Man View Post
    Two wire pass the current required for the typical input, opto device etc, they retain enough power across them in order to operate the internal electronics, without the need for a +&- supply.
    Al.
    Sir, you are going to have to speak more clearly. I can not understand you.



    Are you saying that I can not use them because they will use too much current given the specs of my break out board?

    Or are you saying that they pass, as in, pass the test, go ahead and use them?

    Or are you referring only to 1Jumper10's comment about 2 wire vs 3 wire and ignoring me all together?

    I'm going to start calling you Boomhauer soon.

    Also, thank you for posting! All posts appreciated! Please post again please (to clarify)!



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    Default Re: Please School Me On Proximity Switches

    Unfortunately many suppliers of these BOB's do not show the nature of the actual input of the board, this often makes it difficult to make an accurate assessment, but the 2 wire consume less total current that the 3 wire, I don't foresee a problem however of using two wire, you may have to try either feeding the + side conductor to the 12v supply and the other to the input, it may also work by connecting the -ve conductor to GND (common) and the other to the input, they will often work either way, again depending on the nature of the input electronics.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

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    Default Re: Please School Me On Proximity Switches

    Quote Originally Posted by Al_The_Man View Post
    Unfortunately many suppliers of these BOB's do not show the nature of the actual input of the board, this often makes it difficult to make an accurate assessment, but the 2 wire consume less total current that the 3 wire, I don't foresee a problem however of using two wire, you may have to try either feeding the + side conductor to the 12v supply and the other to the input, it may also work by connecting the -ve conductor to GND (common) and the other to the input, they will often work either way, again depending on the nature of the input electronics.
    Al.
    Thanks for the Reply Al_The_Man!

    I think I will go ahead and order some 2 wire prox sensors. I can hook them up to a 12 volt battery perhaps and test the current draw before I use them. They aren't too expensive from EBay.

    I need to decide on a sensing distance. Any advice there is appreciated.

    Also, I was wondering if anyone knows how robust these sensors are? If the sensing distance is small, and I crash the gantry into them at a higher speed, well, I might smash the tip of the prox sensor.

    Perhaps I will have to incorporate some rubber bumpers into the design as well, in which case the proximity sensors will need a longer sensing distance so the sensor trips before the machine contacts the bumper and does not run the risk of smashing the tip of the sensor.

    If anyone has some further advice of the subject, I'm all ears.

    Thanks again to everyone who has replied so far!



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