24V PSU and 12V Onyx bed!



  • Al
    I am trying to autotune my 12v Onyx bed on a 24v system. I have tried issuing everything from M303 H0 S110 P0.20 (through .5) and keep exceeding the temperature. Is there something that I am missing? Probably need to source a 24v Onyx bed at a point but would like to use this for now.

    Charles


  • administrators

    First, by connecting a 12V bed to a 24V supply you will quite likely overload the 15A rating of the bed heater circuit on the Duet WiFi.

    Second, the system will be inherently dangerous, because if the bed heater mosfet fails short circuit (perhaps as a result of overload) then the bed heater will run at 4x its intended power, and may set fire to itself or something else.

    So your best bet is to use a separate 12V supply for the bed heater, until you get a 24V heater.

    However, if you want to persevere with your current setup, you need to use an even lower P parameter. Try 0.125. You should use an S parameter of 0.25 in your eventual M307 command.



  • Thanks for the input. I know it's a risk, but this thing doesn't get left unattended, it's not even in an operational stage yet. I am trying to figure out where people get the Onyx 24v heated bed. I am currently stationed in England and can source one from here or in the U.S. no problem if I can find one.

    Charles



  • One issue I do have is trying the PWM at a .125, the tuning cancels because there isn't a significant temperature change

    Charles



  • So, after issuing a M307 H0 D60 B0, the autotune worked perfectly. Holds 100C quite well.


  • administrators

    I'm glad you got it working. Perhaps P0.125 was a little conservative. I was basing it on you needing S0.25 in the M307 command to allow for it running at 4x normal power, and tuning happening at half normal power by default.

    Have you measured the resistance of the bed heater?



  • Just to chime in on the safety concern, if the FET on the controller fails in the on position I'd expect that bed to reach 400C, if it didn't catch fire before then.



  • Yup, fully understand. Just an interim testing situation. I have a crydom SSR if to keep it at 12VDC if needed, just don't feel like messing around having two supplies at the moment. I need a 310mm bed to fit my max metal frame and would prefer something of quality. Not having any luck trying to find an Onyx except for some with a funky resistance on seemecnc.


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