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  1. #1
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    Jan 2013
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    I fried limit switches inputs

    I am a beginner and I know what I did is gonna sound terrible.

    I have a Mk3/4 Planet CNC controller and I connected NC PNP inductive proximity switches to LIM1, LIM2 and LIM3 inputs.

    The problem is that I did that without any optocouplers. I connected the brown wire to +12V, blue wire to GND and black wire directly to the input.

    It seems like I fried the inputs. Is there any way to fix what I did?

  2. #2
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    Re: I fried limit switches inputs

    If I have read your post correctly

    The NC PNP inductive proximity switches will of
    pulled the Mk3/4 Planet CNC controller limit inputs up to +12V
    destroying the inputs

    The only solutions I can see is either
    use the inputs IN1 to IN4 if the PlanetCNC software will let you
    or replace the MK3/4 control board

    Contact Planet CNC to see if they either repair the boards or offer a discount on a replacement

    John

  3. #3
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    Re: I fried limit switches inputs

    Thank you John for your quick response.

    I will contact the support to see whether they can fix the board.

    Can I use this PC817 module to wire the inductive switches: https://www.aliexpress.com/item/32836147946.html

  4. #4
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    Re: I fried limit switches inputs

    yes that PC817 module will work

    the circuit only shows 1 of the 4 opto-isolator circuits on the board

    Attachment 425142

    removing the red jumper will isolate the proximity switch 12V supply GND from the Mk3/4 Planet CNC controller input GND


    when the limit switch is closed
    the PC817 modules LED will be on and the PC817's photo transistor will be switched on
    grounding the Mk3/4 Planet CNC controller limit input



    John

  5. #5

    Join Date
    Feb 2019
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    44

    Re: I fried limit switches inputs

    Most likely one of the buffer chips burned out. SN74AHCT244 (hb244) You can change the chip itself which goes to the conclusions of "LIM".

  6. #6
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    Re: I fried limit switches inputs

    Thank you both for your responses!

    I send the board out for repair to PlanetCNC.

    Your scheme makes perfect sense to me John, thank you!

    There is just one thing that is still bothering me. If you look at the original optocoupler module from PlanetCNC It is also connected to +5V pin on the controller. Please someone explain to me why that is necessary. Shouldn't the module also work if +5V is left unconnected?

    Would this board work as well? This is the board that I can get locally within a few days. I guess that I might need to change the resistors with lower values so it would work with 12V.

    I know it seems like I am making it more complicated than it needs to be, but I am just trying to get a better understanding.

  7. #7
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    Re: I fried limit switches inputs

    the PlanetCNC opto-isolator board has a dual opto-isolator IC that has more than the 2 LEDs and the 2 photo transistors a 2 channel version of the PC817 module from post 3

    Attachment 425420 Click image for larger version. 

Name:	2530.jpg 
Views:	3 
Size:	32.8 KB 
ID:	425424

    HCPL-2503 on the PlanetCNC opto-isolator board data sheet
    6N135-36, HCPL-2503,30,31,4502.pdf

    John

    PS
    the link to the second board you can get locally goes to the planetcnc module the first link goes to

  8. #8
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    Re: I fried limit switches inputs

    Ah, sorry! This is the board that I can get locally: https://www.banggood.com/24V-To-5V-4...r_warehouse=CN

    Thank you for providing the datasheet. This goes beyond my understanding of electrical circuits but I guess the +5V connection is there for noise reduction?

  9. #9
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    Re: I fried limit switches inputs

    using the photo of the board and the HCPL data

    PlanetCNC opto-isolator module circuit

    Attachment 425432

    the banggood board probably will if you get the correct version
    I'll have a look to see if I can find more details

    John

  10. #10
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    Re: I fried limit switches inputs

    this banggood board

    Attachment 425434

    will work when connecting 24V NC PNP inductive proximity switches to the
    5V Mk3/4 Planet CNC controller inputs

    you can see it has 3K9 resistors (marked 392) to limit the current through the LED input indicator and PC817's LED

    assuming the volt drop across the LED is 2V
    22/3900 = 0.0056A = 5.6mA which looks OK



    with the output side powered from a +5V supply the opto isolator output will be safe to connet to the Planet CNC controller limit input

    each PC817's NPN phototransistor has a 10K pullup resistor (marked 103) / collector load connected to the +5V supply
    the output LED's and power indicator has a 510 ohm resistor (marked 511) to limit the current from the + 5V supply

    5V - 2V across the LED = 3V
    3/510 = 0.0056 A =5.9 mA

    John

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