External 5Vdc Voltage Input Reporting Bug in DWC?



  • I've built the circuit described in this thread to connect an independent external 5V input to the Duet and the utilize the ATX-like functionality for the printer, and it all works perfectly apart from one small problem, namely:

    When the Duet is running off the 5V external supply (and the main 24V supply is off), DWC is reporting only 1.5V input in the 'Machine Status' window.

    At first as was alarmed at this apparent voltage drop (and surprised that everything was still working as expected, i.e. I could turn the main supply on/off as intended via the ATX button in DWC), but then I checked the actual input voltage to the Duet with a voltmeter and the actual voltage coming in is 5V rather than the reported 1.5V.

    Is this a bug in the DWC voltage input reporting mechanism or perhaps I haven't setup something correctly?


  • administrators

    DWC is reporting the VIN voltage, not the 5V input voltage. So ideally it would read zero with only 5V power applied. In practice it reads a little above zero.



  • @dc42 said in External 5Vdc Voltage Input Reporting Bug in DWC?:

    DWC is reporting the VIN voltage, not the 5V input voltage. So ideally it would read zero with only 5V power applied. In practice it reads a little above zero.

    Thanks for the quick response, @dc42!

    Good to know, but I'm getting a lot more (relatively speaking) than just a few mV... 😕

    Do you think I have a voltage leakage somewhere? Is it something I should be concerned about (short- or long-term)?

    Btw, it might be worth adding a short comment about this in the relevant wiki(? ) to avoid similar confusions about how external voltage input reporting works in DWC.



  • @dc42, it just occurred to me why I thought it was a bug to begin with:

    With the external 5V input (and 1.5V showing in the display), I got a 'low voltage' warning in DWC (that's what drew my attention to the issue).

    That's obviously a false warning and perhaps indicates that some ironing out of the details of how voltage input is checked by the system and displayed is needed.

    (personally, I would have liked to see the actual input displayed, whether via vin or external 5v supply).


  • administrators

    The 5V and VIN supplies are separate, so the VIN monitor has no knowledge of the voltage on the 5V rail. The "Low voltage" warning means the the VIN voltage is too low for motion.



  • @dc42 said in External 5Vdc Voltage Input Reporting Bug in DWC?:

    The 5V and VIN supplies are separate, so the VIN monitor has no knowledge of the voltage on the 5V rail. The "Low voltage" warning means the the VIN voltage is too low for motion.

    Got it, thanks!

    That just leaves my 2 questions above which are easy to miss so I thought I'd repeat them here:

    Do you think I have a voltage leakage somewhere? Is it something I should be concerned about (short- or long-term)?


  • administrators

    @snowcrash said in External 5Vdc Voltage Input Reporting Bug in DWC?:

    That just leaves my 2 questions above which are easy to miss so I thought I'd repeat them here:

    Do you think I have a voltage leakage somewhere? Is it something I should be concerned about (short- or long-term)?

    No I don't think you need worry about that. I just tested a board on my bench with USB power only and I get similar results: 1.7V across the VIN terminals on my multimeter, and DWC also shows 1.7V. My guess is that the TMC2660 drivers leak a small amount of current between their +3.3V and VIN pins.



  • @dc42 said in External 5Vdc Voltage Input Reporting Bug in DWC?:

    @snowcrash said in External 5Vdc Voltage Input Reporting Bug in DWC?:

    That just leaves my 2 questions above which are easy to miss so I thought I'd repeat them here:

    Do you think I have a voltage leakage somewhere? Is it something I should be concerned about (short- or long-term)?

    No I don't think you need worry about that. I just tested a board on my bench with USB power only and I get similar results: 1.7V across the VIN terminals on my multimeter, and DWC also shows 1.7V. My guess is that the TMC2660 drivers leak a small amount of current between their +3.3V and VIN pins.

    Very happy to hear! Thank you very much for taking the time to test this on your machine, @dc42, very much appreciated!


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