Heaters always ON

  • My bed heater is stuck on.

    I have already tried to remedy this by removing the sd-card so that any programming errors are eliminated as per this recommendation.

    "dc42 ADMINISTRATORS 22 Dec 2018, 10:48

    Are you saying that as soon as you apply VIN power, all three heater LEDs on the Duet turn on? If so, please power up the Duet with the SD card removed, to eliminate configuration issues. If the same thing still happens then the mosfet driver chip is faulty or not properly connected, and you should ask your supplier to replace your Duet.

    Duet WiFi hardware designer and firmware engineer
    Please do not ask me for Duet support via PM or email, use the forum
    http://www.escher3d.com, https://miscsolutions.wordpress.com "

    Link: https://forum.duet3d.com/topic/8244/heaters-always-on

    After the removal off the SD-card the bed heater still tries to light the fires of hell but fortunately it barely reaches 120°C which is a more sauna like temperature.

    Bed heater current around 7.5 A which should not be a problem for the duet which is rated for more than twice this current.

    Otherwise it has been a wonderful upgrade from trigorilla board on a delta printer. I especially do not miss the sound of a hot end scraping along the top of the support which happened frequently with the trigorilla board.

    Best regards Stefan

  • Moderator

    @Mr_Xu If you've already tried with the SD card removed, and it's still on, this sounds like a wiring issue. How is the bed heater connected? Are you using a relay? If so, please check how it is wired. If not, it could be a short circuit, or the bed heater MOSFET is stuck on. Please post a good picture of the Duet board and how the heater is wired to it.


  • Hi,

    no relay used, just the heaters wires connected to the duet board directly.

    IMG_2927 2.JPG

  • Have you checked that all runs of your wires have BOTH POWER AND GROUND only connected to their respective power and ground connections?
    Sounds to me like you might have decided to inter-connect ground someplace since "it's only ground"

  • @jens55 The bed heater is connected directly, both wires go directly without any other connections between the board and the heater.

  • @droftarts Is the picture OK or do you need another one?

  • the circled area might be of interest if the problem applies to all the heaters not just the bed

  • Moderator

    @Mr_Xu Is this a new installation, or has it been working for a while and recently failed? Does the LED for bed heat stay on permanently, or only when you turn the bed (even if it is on)? It is not lit in the picture, was the power on or off?

    Looking at the bed MOSFET, I don't see any damage, though this doesn't mean it hasn't failed. The MOSFET switches on the negative side of the bed circuit, which means the bed is 'live', and any short to ground will make it heat up. It looks like quite a tight installation; are you sure nothing is touching the back of the board, and shorting the bed negative pin to ground? Is there any point on the wire that could have chaffed and be touching metal that is grounded? Carefully check through all the wiring.

    To test if the MOSFET is stuck on, the easiest thing to do is, with power off, disconnect the bed wiring from the bed negative terminal, and check for continuity with a multimeter between the bed negative terminal and Ground (on the VIN terminal) , so basically the two screw terminals closest to each other in your picture above. Again, make sure the board is not shorting on something behind the screw terminals.

    For reference, this is the bed heater circuit. The bed is connected between V_BED and BED-. From https://github.com/T3P3/Duet/blob/master/Duet2/Duet2v1.04/Duet2_1.04c_Schematic.pdf


    If the bed MOSFET has failed, and the board is less than 6 months old, it should be covered by warranty. Where and when did you buy it?


  • @droftarts

    Hi again

    it is a fairly new installation but it was running for over a month without any problems. The Duet was bought for this installation so it is 1-2 months old by now. The board was bought in Sweden from RepRapPro.

    The Duet is mounted inside a small 3D printed box with active cooling and yes it is tight. This PLA-box has warped, it was never intended to reach higher temperatures and with a fan it never really reached any high temperatures (below 40°C) before the bed heater which sits a centimeter or two above it overheated. There is a thin isolation between the heater and the PLA box as well.

    So with the warped PLA box I need to remove all the wiring and Duet anyway so let me check a final time if there really is no short circuit that is hiding somewhere and which I have not been able to measure by disconnecting the bed heater and power wires from the duet and checking for shorts with a voltmeter. As for the LED I can't really see it while the bed is in place and everything is connected.

    I foresee that my easter holidays will saved with a new project called "rebuilding my 3D printer again". 😉

    Thanks for all the help and have a nice easter holiday.


    PS I´ll check the MOSFET as well as suggested, sorry have not thought of doing this before.

  • @Mr_Xu

    So, I disconnected the duet 2 wifi totally, removed all the wiring from my printer checked the wiring and could find only one spot where a power cable (the negative one) had been slightly chaffed. I never managed to get a connection between this spot end the end of said wire though.

    Then I tested the duet 2 wifi as Ian suggested, something I should have thought of myself from the beginning and it worked. Only problem was a screw connector on duet that did not open properly but that was an easy fix with the help of a screw driver. After that I rewired every high power cable in the printer and it has been running for to weeks now without any problem. Somehow I must have managed to get a short although I did not find any smoking gun and from the little I know from mosfet they generally do not come back from the dead. Apart from the screw connector being slightly sticky I can't see anything wrong on the duet.

    Anyway printer and duet are both working so thank you for a superb support.

    Best regards Stefan

  • Moderator

    @Mr_Xu thanks for the update, glad it’s working now.


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