Consolidating supply and GND for hotend



  • Hi all,

    Apologies if this has been asked before - I couldn't find anything by searching.

    I'm planning on running 2x laser filament sensors and 2x PWM fans, LEDs and a few other things on the X carriage for my printer. I have run a 40 pin ribbon to the carriage, but once everything is installed I will end up running out of wires. Can I consolidate power and GND pins for multiple devices? If so, to what limit?

    Specifically - Can I run 2 laser filament sensors from the 3.3V and GND of one pin (say E0), then use the input (E0/1 STOP) pins from E0 and E1. Then, can I run a 12V and GND from the Power In terminal to the hot end to power the fans and LEDs, and run the Fan's PWM signal pin to the FAN0- and FAN1- pins?

    I'm reluctant to try for risk of damaging the board.

    Thanks



  • Yes and no,

    Everything switched is low side switched. So for example, you can use common supply for heaters and fans (so long as you are using the same voltage for both) with separate switched grounds for each.
    For sensors it will depend on how exactly they work, and I've not used the filament sensors so I can't advise on them. But as an example, inductive proximity sensors mostly work by connecting a pin to ground, so can use a common 24V and a common ground, but need a conductor for that logic pin. Endstops can be configured different ways, but can use a common ground and a dedicated logic conductor.

    I'm running a heater, RTD, 2 fans, Z probe, stepper motor and endstop, all using a common 24V supply, with individual grounds where needed. It would be 17 conductors normally. By using common conductors I got it down to 14.
    But you also need to make sure that your 24V supply conductors can actually carry enough current if everything is running at the same time. And motors can't share conductors with anything else.
    I'm actually running a 12 pin cable for all the low power stuff (and I'm keeping the temp sensor isolated, I didn't want to risk noise so its got its own 2 conductors). And a separate 2 core 18AWG that is supplying 24V and the switched ground for the heater.

    A 40 pin ribbon cable is going to have such low current capacity per conductor that you need to plan carefully.

    Check exactly how each sensor/device is working, then map something out on paper.



  • Thanks for the advice, most of what you mentioned is what I had in my mind, so that's good.
    I noticed on the duet pinout some of the pins have a current capacity noted, but not all of them. Do you know where I might find the current capacity for all power sources?

    For the filament sensors, they have a GND, 3.3V and "IN" which seems to be some kind of serial data. I imagine sending the data to the respective pins with common power and GND will work, but I can't be sure. The PWM fans are similar.

    Thanks



  • If its not in the wiki, you could try looking at the actual files in the github... or one of the designers will probably happen along.

    That said, I think only the important current limits are noted, everything else should be considered to be logic only. If you try and drive anything more than an indicator LED you'll probably burn out the pin.
    I don't recall what the fan outputs are rated to, for example, but they are not going to work for driving an extra heater or anything. But they work just fine for driving an SSR.


  • administrators

    @Gerrard said in Consolidating supply and GND for hotend:

    Can I run 2 laser filament sensors from the 3.3V and GND of one pin (say E0), then use the input (E0/1 STOP) pins from E0 and E1.

    Yes the filament monitors can both be powered from the same 3.3V and ground connection

    Then, can I run a 12V and GND from the Power In terminal to the hot end to power the fans and LEDs, and run the Fan's PWM signal pin to the FAN0- and FAN1- pins?

    Assuming you are using 12Vs to supply the whole system then you can use a single source of 12V, and switch the -'Ve sides (i.e use the FAN0- and FAN 1- for the two fan 've connections, possibly use fan 2- to control the LEDs.

    caveats

    • You need to ensure you don't exceed the limits of the fan mosfets (1.5A max)
    • There is a potential failure mode where the VIN voltage shorts onto 3.3V. this is more likely if you bundle all the wires up into the same bundle. You mention you are using a ribbon cable. This can be a good solution but be careful to manage the strain relief and flexing to it does not break and short. Also check the max current rating of each of the conductors in the ribbon cable (probably 1A max or less). If you end up putting 12V and the return GND over multiple conductors then if one breaks, the other ones will end up carrying more than they are rated for, and get hot.