Bed overheating



  • I'm running a bed heater at 24v through an SSR relay. The bed is an Onyx v6 or v7 that comes with the Rostock Max v2. I've seen other people having success in doing this.
    It's currently in bang-bang mode.

    When turning the heater on it quickly reaches it's target temperature, then the heater turns off. I've confirmed this by both looking at the LEDs on the relay and bed and a amperage meter. However the heat continues to rise to about 11 degrees celsius over the target temperature, causing a heater fault.

    I don't really understand what's going on here. Could the contact between the thermresistor and the bed be bad, causing the heat to dissipate into the thermresistor too slow, causing a lag?

    I've tried to lover the voltage to 18v and hooked it up to a lab power supply supporting current limit, to no avail. This behavior was not there on 12v on the RAMBO controller.

    I might just hook it up to 12v again but I'm tired of waiting ~30 mins to reach 100 degrees.

    I also tried to hook it up to the Duet directly and running the autotune with PWM reduced to keep the avarage current well below 15A, but I get a error that the temperature is not rising right before it's starting to rise (and does so for some time after the tuning is finished).



  • Sounds very likely your thermistor does not have sufficient contact with the heat bed.



  • If it's overshooting in bang bang mode, then why not just tune it and use PID? You can still run it at 24v through a SSR.



  • I tried to remove the thermresistor but managed to break one of the legs, since it stuck on there with silicon glue. So I can't test it again for now. I might recycle one from an old hotend and see if I can get better results.

    @deckingman:

    If it's overshooting in bang bang mode, then why not just tune it and use PID? You can still run it at 24v through a SSR.

    Yeah, but I'm not very good at manually doing this yet, especially without any starting point and the auto tune fails, because it reacts so slow. I think it will be better to try to fix the possible mechanical issues first.

    Either way I have to wait for a new thermresistor or get an old one for now.

    Edit: Is there a GCODE that will allow the autotune to wait longer for the temperature to rise?


  • administrators

    If you are running a 12V heater on 24V then it will produce FOUR TIMES its design power. No wonder it overshoots to much. Overdriving a 12V heater by turning the voltage up to 14V is generally OK, but I most definitely do not advise running a 12V heater at 24V.

    If you want to try running it at 18V (or as low as your 24V power supply will go), then you will need to tune it and use PID mode instead of bang-bang.



  • I'm very aware and conscious of the power usage being much bigger. I know plenty of other that has done this with this bed and SSRs ( http://sublimelayers.blogspot.no/2015/12/my-current-rostock-max-ultimate.html for example) and my power supply has overload protection and should be more than enough for even 24V (700W, calculated power draw should be around 510w if my multimeter is correct, running at full capacity).

    I will try and see if the lag in the temperature can be fixed before i do anything else.


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