Sunx GL-8 type sensors for end stops with Duetwifi



  • Hello Everyone. New to this forum (but not new to 3D printing). Until now, I have been using MKS boards for my printers but decided to explore my options into the big league due to the unreliability of what I am using right now. The printer that I am thinking of getting duetwifi for is a Tevo Black Widow and the end stops are all Sunx GL-8 type inductive sensors. They are connected to the MKS boards in the same way as a duetwifi accepts end stops. Apparently, Smoothieboard also accepts the end stops in the same pin configuration as MKS/Duet (other than 5V vs 3.3V) but that board doesn't accept inductive end stops that easily (at least that's what I have read online and was told by the black widow group). I am not really sure why it wouldn't accept it since it can work with residual voltage of >3V.

    So I am reaching out to you guys to see if I will be able to get those sensors to work with Duetwifi. It would be an expensive mistake if I can't get my existing setup to work.

    P.S. I really like these end stops and hoping to keep it.

    Thanks 🙂


  • administrators

    I took a look at the data sheet for the Sunx GL8. There are several different models in that series; but assuming that the ones you have are the 3-wire NPN open collector type, you can connect them to the Duet in the same way as an inductive Z probe. See https://duet3d.com/wiki/Connecting_a_Z_probe#NPN_output_normally-open_inductive_or_capacitive_sensor. The purpose of the diode is to protect the Duet if the ground connection to the sensor breaks.



  • Thanks for looking into it. Yes it is an NPN normally open three wire type.

    Since the sensor should work with >3V, can I directly connect it to 3V3?


  • administrators

    @mrhasan:

    Thanks for looking into it. Yes it is an NPN normally open three wire type.

    Since the sensor should work with >3V, can I directly connect it to 3V3?

    The datasheet I looked at says that the supply voltage range is 12 to 24VDC.



  • Yes but that is with a load connected as per the diagram shown in the spec sheet. It shows the residual voltage to be 3V. Alongside, the sensor is working on 5VDC right no (3 wires connected to the end stop pins on the MKS board which gives out 5VDC). Or am I missing something?

    And to add another question: the heated bed peaks around 400W (operates at 24VDC) so can consume around 17A while heating up (initially). Will Duetwifi's power mosfet be able to handle this?


  • administrators

    The residual voltage applies to the 2 wire versions. It is the voltage drop when the sensor is triggered. If the data sheet says your sensors need 12 to 24V, i would go with that.



  • Aha I see. Thanks for clarifying that. It would probably be easier to just go with microswitches and use the + and - pins right?

    And for the heated bed, will the mosfet support 17A peak (bed can consume up to 400W @ 24VDC)?



  • Microswitches using the NC contacts going to Ground and Signal pins. + and - would short the 3.3vdc power.

    https://duet3d.com/wiki/Choosing_a_bed_heater has the specs.



  • @StephenRC:

    Microswitches using the NC contacts going to Ground and Signal pins. + and - would short the 3.3vdc power.

    https://duet3d.com/wiki/Choosing_a_bed_heater has the specs.

    Thanks 🙂


  • administrators

    The heated bed circuit has been tested at 15A. We're hoping to increase this rating, but not until we have done more thermal tests. Meanwhile you could turn the voltage of your 24V PSU down a little to limit the bed current to 15A.



  • @dc42:

    The heated bed circuit has been tested at 15A. We're hoping to increase this rating, but not until we have done more thermal tests. Meanwhile you could turn the voltage of your 24V PSU down a little to limit the bed current to 15A.

    Thanks for your help. I will keep that in mind 🙂



  • @mrhasan:

    And to add another question: the heated bed peaks around 400W (operates at 24VDC) so can consume around 17A while heating up (initially). Will Duetwifi's power mosfet be able to handle this?

    This is the perfect use case for an SSR. That way you're not loading the Duet at the limits of it's capability. FWIW, not all SSRs are created equal and I have been very unhappy with the Fotek models available on Amazon and eBay. (3 years ago) The one I ordered got exceptionally hot and I have significant questions about their durability. Mine lasted somewhere around 6 hours before dying. I switched to one of these and it's been operational with no thermal issues for 3 years:

    https://www.automationdirect.com/adc/Shopping/Catalog/Relays_-z-_Timers/Solid_State_Relays/Panel_Mount_Relays,Hockey_Puck_Style,10A-75A(AD-SSR6_Series)/AD-SSR6M25-DC-200D

    Yes, it's expensive at $40+shipping but it is overrated for your application but that just adds to peace of mind!


  • administrators

    The bed heater mosfet on the Duet series will handle more than 20A easily. At 17A it would dissipate no more than 1.4W. The problem with handling high currents is making the PCB traces wide enough to handle the current.

    When choosing a DC-DC SSR it is indeed very important to get one with a low voltage drop, both to avoid generating a lot of heat in the SSR and to avoid reducing the voltage that reaches the bed. I looked up that Automation Direct relay you linked to. It has a maximum voltage drop of 2.8V @ 40A, which isn't very good. So I don't recommend it. The Auber Instruments and Crydom SSRs that I list on the wiki at https://duet3d.com/wiki/Connecting_a_bed_heater do much better.



  • @dc42:

    I looked up that Automation Direct relay you linked to. It has a maximum voltage drop of 2.8V @ 40A, which isn't very good. So I don't recommend it. The Auber Instruments and Crydom SSRs that I list on the wiki at https://duet3d.com/wiki/Connecting_a_bed_heater do much better.

    Fair point. I forgot you had that documented already. I'm not a huge fan of Automation Direct equipment (I work in industrial controls) but it's generally the most cost effective industrial grade stuff around. I'm not familiar with Crydom and Auber Instruments but the websites inspire confidence of quality products.


  • administrators

    Several people have used the Auber Instruments SSR and report good results with it. The Crydom one is too expensive for most people to consider.

    I think the difference with the Auber and Crydom SSRs must be that they provide voltage for the mosfet gate drive from the control input through am oscillator and a transformer. That would mean they don't need a voltage drop across the load side to provide the gate voltage, and it is only the mosfet on-resistance that gives rise to a voltage drop.

    There are also inexpensive 5-terminal SSRs such as the PowerExpander from reprap.me.


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