Dual Power (24V Duet VIN/12V Heaters) Circuit Sanity Check

  • Hello, first post on forum, just starting the Duet3D adventure.

    I am upgrading a SeeMeCNC RostockV2 which i had been running on dual voltage with the RAMBo (24V for the stepper motors and 12V for the heaters and fans etc). I would like to keep 24V for the steppers as i am using a Zesty Nimble extruder which has a 30:1 reduction ratio and benefits for the speed afforded by 24V. Also, i have just installed 0.9deg motors on the axis.

    The bed is a 12VDC heater and i have a small fleet of customized hot end/effectors which incorporate the SeeMeCNC accelerometer board which is constrained to 12V. Although the accelerometer probe output will not be utilized, the accelerometer board is integral to the hot ends and i would prefer to not modify them at this time.

    The Duet board is a V1.02 Duet Ethernet and i have a few questions regarding power and connection circuitry (please see attached schematic). 0_1527427927501_V2Max_Duet_Upgrade_Dual_Power_Accelerometer_V1.0.pdf

    1. 24V feed into 12V Circuit: Given that there is potential (i think) to feed 24V back into the 12V power supply through the Bed and E0 indicating LEDs (D6 & 7) when the mosfets (TR2 & 4) are not conducting, should a blocking diode (D11 & 10 on the "Power Management Board") be inserted before the BED- and E0- inputs to the Duet? Or can one rely on the indicating LED 4K7 limiting resistors (R65 & 67) to limit current into the 12V circuitry to about 2.5mA which "shouldn't cause any problems"....

    2. Hot End Thermister Connection: The accelerometer board forces a common ground for the hot end cooling fan (always on), hot end leds and thermistor. Between the fan and LEDs, the ground return current approaches 200mA. Is the correct approach to avoid bringing this current (and potential HE associated noise) back through VSSA by terminating the hot end common ground to the ground bus on the power management board?

    3. JohnSL FSR Board Connection: It was suggested on the SeeMeCNC forum that probe signal be brought in through the Z Probe input. Are there any specific considerations for taking this approach? I am hoping to use the E0 and E1 Stop inputs for user inputs in the future (an "E Stop" of sorts and an "Easy Quit Printing" button for my better half who often takes on late night print monitoring duties).

    Any comments appreciated.

    Thank you,

  • administrators

    1. The 4K7 resistor will limit the current to a safe value. No need for the extra diodes.

    2. Yes, connect the common ground to VIN ground not to VSSA. Expect to get some noise on the thermistor readings. Using a common ground for the thermistor and the fan is not good practice.

    3. Yes, connect the JohnSL board to the Z probe input.

  • Very good, thank you. Replacing those accelerometer boards with a simple connector board is indeed the next item on the upgrade list.

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