Static Electricity



  • Hey I'm building a 3D printer, there's a Duet WiFi and a Mean Well LRS-350-24 powering it, the frame is mostly T-slot aluminum beams with some aluminum plates. The PSU is mounted to one such plate.

    The Duet WiFi is connected to a separate secondary SD card slot, mounted in a plastic panel. This works, I can list the files, all is well.

    Then I tried to remove the card... and I felt a static electric shock. I was sitting in a nylon covered chair wearing the cheapest T-shirt I own. The Duet still works. The card still works.

    Electrical ground of the Duet is not directly connected to the Earth ground explicitly. Should I?

    If my theory is correct, doing so will NOT prevent me from getting static shocks in the future. Am I correct? The charges will want to go somewhere regardless of Earth ground.

    Unless, of course, the charges were built up on the printer. The small plastic panel is a piece of PLA mounted to an aluminum beam. If this is the case, how do I make sure charges don't build up on the PLA panel?

    Thanks



  • It's considered best practice to connect DC negative to the ground of your AC PSU, along with grounding the frame of your printer, as well as the motor housings. For safety's sake, as well as eliminating static buildup.

    Note: it is highly recommended that the stepper motor casings be grounded, especially in belt-driven printers. Otherwise, motion of the belts causes static charge to build up, which eventually arcs over to the windings. If the motors are screwed to a metal frame, grounding the frame is sufficient.



  • Ok thanks. I checked continuity everywhere and all is grounded except DC negative. I have the Duet on brass standoffs on the aluminum panel so this should be super easy to accomplish.

    However, the microUSB connector does not appear to be connected to the ground plane. I have a panel mounted USB extender that is connected to it. I guess it would be grounded to a computer's chassis but that's only when a computer is hooked up. Should I add a ground wire to the microUSB connector? Yes I understand it might make one giant ground loop the size of a room if the computer is also earth grounded.



  • Definitely avoid USB ground loops. It's recommended to use a laptop on battery power if you just connect via USB with DC negative grounded. It should be quite rare to need to connect via USB though.

    https://duet3d.dozuki.com/Wiki/USB_ground_loops



  • wow your wiki has a page on USB ground loops lol what doesn't it have

    thanks

    so the additional ground wire between DC negative and earth/chassis would dissipate charges on the frame, but would the charges on my body still be a problem?



  • I could be wrong, but I think if there is a charge on your body it would drain to ground via the frame when you touch it. The shock you got would have been from charge built up on the frame trying to get to ground through you.



  • Get yourself a desk lamp with a three pin socket and touch it before your printer!

    Other options are the anti-static workstation desk mats and wrist straps. You can get mats for the floor too.

    Edit: Be really careful if you use solvents to clean your machine! 😮


  • administrators

    Large PSUs leak a little current between the mains input and the DC output. That's why you can get a small shock of you touch the output. Grounding the negative DC output solves it, but risks a ground loop if you connect to a PC via USB.

    Makers Muse did a video on the consequences of not having a ground connection, see https://www.youtube.com/watch?v=v1OuYg7AJjw.



  • That video is slightly miss leading. That was due to no mains connection at all, ie only mains and neutral from the wall, not missing off the DC-ground connection on the output of the PSU.

    A connection between DC - and ground/earth on the output side of the PSU would not have made any difference in that example!

    Most important thing for tinkerers like us is to check the mains supply is wired right (£10 plug tester) and use an plug in RCD circuit breaker. Make sure all fuses are sized appropriately for your machine.

    Ground all steppers to earth, ground metal work to earth where possible (inc hot end via heatsink), optionally add a single direct link between earth and negative at the power supply but be aware this may cause noise and creates ground loop issues. There is an example circuit for the best way to tie earth and ground on this forum somewhere that reduces the probability of noise.




  • administrators

    @doctrucker said in Static Electricity:

    A connection between DC - and ground/earth on the output side of the PSU would not have made any difference in that example!

    I think there probably was a connection between the mains ground wire and the DC output in that example. The problem was that the mains ground wire was not connected at the distribution block.



  • As in a multi way mains connector wasn't it? Thought he had some poor quality extension leads?

    I'll watch again later.

    Edit: Think we're on the same page with regards what the video shows:- the danger of not connecting the mains ground.

    As soon as mains ground connection is passed to the earth connection on the PSU the capacitor link works and sinks the leaked AC voltage to the ground. The video isn't a justification for linking earth and the DC- side of the power supply unless the capacitor link is missed in the PSU.

    The circuit on the linked thread on this forum allows DC voltages above a threshold to be dumped to DC- through the chassis earth.


  • administrators

    I agree, we're on the same page.

    Even without the capacitor link in the PSU, there will be a small a mount of leakage between the mains input and DC output, because of the capacitance between primary and secondary of the transformer.


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