GRBL and hard limit problem

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This topic contains 2 replies, has 2 voices, and was last updated by  Stu H 2 months, 1 week ago.

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    Stu H

    So the MPCNC is built (massive thank you to the folks here at v1!!!) and have got the steppers wired and the spindle (500W chinese spindle) installed.  It works!

    wiring details:

    grbl 1.1f on genuine arduino uno

    shielded limit switch wires with switches all wired in NO position with electrical noise suppression circuit from here:

    The problem:

    f%^$&ing electrical noise! LOL

    I have wired everything up and it works fine but the minute I installed a relay (5V arduino relay) to start/stop the spindle it throws up a hard limit error.

    I have even separated the relay from the stepper drivers and uno by 1ft and it still kicks me out the instant the relay opens or closes.

    Any suggestions for spindle control that doesn’t rely on a relay that must be somehow broadcasting enough EMP to take out a robot invasion??




    Try putting a surge suppressor in parallel with the relay coil – there are some formulas to pick the right resistance and capacitance.  Another option is a “transzorb” or metal oxide varistor (MOV) which limits the maximum voltage kickback from the relay when it deenergizes.  If the problem occurs when the relay gets turned on, you may have an issue with the relay coil inrush current dragging the supply voltage down.  An MOV could also be used on the output of the relay to help limit the peak voltage when the contacts open.


    Stu H

    Thanks for the feedback.  I’ve just disabled hard limits on the grbl since farting three houses over seems to trip the hard limits on this thing.  This is the second uno ive used with the exact same issue and the web is stacked with similar tales.  I wonder if it’s too sensitive a device to use reliably for cnc..

    I’ll revisit it later after I’ve used it for awhile.

    Thanks again!


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