Limiting Heatbed current through M307?



  • I'm looking at replacing my existing MK2B PCB heatbed with a 12V/200W silicon heat-pad and using it with my existing Duet 0.8.5 boards. I noticed the documentation says the Duet 0.6 can handle 10A, and the Duet 2 can handle 18A, but the Duet 0.8.5 was never rated.

    I understand from the documentation that I could add a larger wire to the underneath of the Duet 0.8.5 to help compensate for larger currents, but I'm not changing to the new heat-pad for faster heat-up times, this is a change to my entire heatbed and moving away from the PCB to an aluminum plate.

    My question here becomes, if I use this new heat-pad and know up front it's overpowered from that I'm currently using, could I add the S parameter 0.5 to M307 to limit the PWM output to stay below the current limit of the new heat-pad or would that not introduce some sort of current limit to the heat-pad?

    Edit: The machine is currently using a 12V 30A powersupply.



  • If you are unsure, use an external MOSFET Module for a few bucks like this:
    0_1550609756930_MOSFET Module-1.jpg
    It's good for 25A and to Duet opto isolated on the control line.



  • That's definitely an interesting option, but not exactly the answer to my original question.

    However, do you have a part number or schematic for your picture or website link or manufacture etc? And I'm assuming you would wire the existing bed output to the control lines of the external mosfet, or would it be done directly with pins on the expansion header?



  • With the external MOSFET modules that are intended for heatbeds it is generally a case of running a power supply direct to the MOSFET module, connecting the heater bed to the card, and then connecting the old heater output from the v0.8.5 to the module as a trigger. I use similar units on a v0.6 and RAMPs board. check your currents and cable limits carefully. I think 14 AWG is on the edge for 200W at 12V. I used silicone insulated wire for heat resistance and it tollerated my very poor gas soldering!



  • What you propose would work but is not safe, because if the firmware fails or FET fails, full current will pass through those traces.



  • There will be in a fault state of the mosfet or firmware NO high current on the duet traces, only the mosfet module traces. But yes, you could put a thermal switch under the bed and in line with a bed cable.

    @pdbeal said in Limiting Heatbed current through M307?:

    That's definitely an interesting option, but not exactly the answer to my original question.

    However, do you have a part number or schematic for your picture or website link or manufacture etc?

    Search aliexpress for 'hotbed mosfet'

    And I'm assuming you would wire the existing bed output to the control lines of the external mosfet, or would it be done directly with pins on the expansion header?

    I have mine connected to the heatbed output terminal.



  • @andres; I think elmoret was referring back to the original poster.

    With an external MOSFET module it's best to include a fuse in series with the main supply to the board and seriously consider fitting a relay that kills the mains supply to the PSU that supplies the heaters.



  • Be aware that companies trading from china through sites such as Aliexpress don't have to comply with EU/UK directives and can't be prosecuted in the event of an accident where they are shown to be negligent.

    Use with extreme caution on safety critical components.

    There is little to no insentive for them to build to the same standards as EU supplied parts.



  • Your 200W heated bed will theoretically take 17 Amps allowing for just a small safety factor you, should rate the power supply, any components, PCB track and wiring to at least 25 Amps. In addition you will need to provide adequate cooling to dissipate the heat that will be generated.

    I can't see that M307 will limit the current. It may however limit the power by providing pulses of 12v but the current will still be 17 Amps for each pulse.

    A separate MOSFET or even a relay is the only way to go.



  • @jmshep said in Limiting Heatbed current through M307?:

    I can't see that M307 will limit the current. It may however limit the power by providing pulses of 12v but the current will still be 17 Amps for each pulse.

    A separate MOSFET or even a relay is the only way to go.

    The pulse would not limit the current, however, it would stop the trace on the PCB from maintaining constant current and therefore constant temperature rise. Once the heatbed is at temperature, there is very little output anyway so the limited PWM output would really only come into play during initial heat-up.

    It's the same as what I've done in the past with large industrial motors. It's possible to run them at twice the rated current, as long as the duty cycle of the motor isn't too long. Of course, in that environment the motors are strapped with thermocouples to monitor the internal temperatures of the armature and motor windings, but the same theory applies.

    In my case, I'm not trying to run the heatbed at double current. I'm only trying to limit the current through the trace for a long period of time and most ampacities assume 100% duty cycle at a current level which is why if I did use PWM modification it would be with 50% PWM.

    It's a moot point now anyway I think, as I've ordered some of those external heatbed mosfets from a respectable US supplier. I don't mind paying more when its properly built and supported and the cheap chinese stuff doesn't meet either requirement.



  • @pdbeal said in Limiting Heatbed current through M307?:

    The pulse would not limit the current, however, it would stop the trace on the PCB from maintaining constant current and therefore constant temperature rise. Once the heatbed is at temperature, there is very little output anyway so the limited PWM output would really only come into play during initial heat-up.

    As with most things, there is what is possible, and what is recommended. As long as you're aware of what you're doing and aware of the consequences, you can make an informed decision about what acceptable risk is for you.


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