Heated Bed Wiring question?



  • I have a heated bed with Gnd 12V and thermistor wired in the same circuit and I am concerned that it may short and fry the board. I am using the Duet 2 Maestro Board.

    Would this be safe to wire up the 12V pin 1 and 12V pin 2 Ground to HB power and then the 5V with the Thermistor to the board thermistor plug?

    Additionally Could I add something to protect the thermistor ground from the 12V and current for over-voltage and current protection (such as a polyfuse (or normal fuse) and Zener-diode?

    I have attached what someone did for the HBP spec below.

    0_1542578314904_Heated Bed.pdf


  • administrators

    @ktapper said in Heated Bed Wiring question?:

    I have a heated bed with Gnd 12V and thermistor wired in the same circuit and I am concerned that it may short and fry the board. I am using the Duet 2 Maestro Board.

    No sane electronics engineer would ever design a bed heater like that. You won't get an accurate temperature reading, because the high bed heater current will cause the ground side of the thermistor to be at a potential that is significantly different from VSSA. Also, sharing a ground connection between bed heater and thermistor is incompatible with every open source 3D printer controller that I know of. I presume that bed heater was designed to work with some particular proprietary electronics board.

    I suggest that you either hack that bed heater to provide separate heating and thermistor circuits, or use a different bed heater.

    In principle it could be made to work if you disconnect the lower end of the thermistor R9 from the local ground and run a separate wire from it back to VSSA on the Maestro; and connect pin 3 of J1 to the internal point on the Maestro that drives the bed mosfet. But that internal connection would void the warranty on the Maestro. Also, the bed heaster mosfet circuit doesn't have a gate series resistor; so depending on the cable you use there could be a risk that it would oscillate at a high frequency.



  • Thanks. That's what I figured. I am switching to silcone heater and standard setup now.


 

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