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Grounding a plastic chassis

  1. Apr 3, 2017 #1
    Hello

    The Chinese were kind enough to send me a "music mixer" (XH-M164) and apparently these sell for about 8-11 dollars so I thought I might aswell build something cool with it. I found an amplifier aliexpress (http://bit.ly/2nTD8VX) and the recommended transformer for that is a 32v 200VA toroidal transformer. Luckily there are transformers out there that are specifically made for what I'm going to make. They have 3 outputs, 6V, 12V and 32V so the music mixer will be powered by the 12V output and the amplifier will be powered by the 32V output.

    Anyway, this is the first time I'm building something that is powered from the wall and I want to stay safe. How does one go about grounding a 3D-printed plastic chassi? Should I just make every effort to isolate everything and leave the grounding wire alone? The transformer has 3 wires for for every output and both the amplifier and music mixer has 3 inputs, but I don't see how that third wire is grounded. The 6V output is an exception, that one is just two yellow wires.

    Thanks
     
  2. jcsd
  3. Apr 3, 2017 #2

    berkeman

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    Staff: Mentor

    If you are wanting to use AC Mains input voltage for this project, you need to do a number of things to make this safe. What is the safety rating of this transformer? Is it rated as "single insulation" or "double insulation"? Can you post a link to the datasheet?

    If the transformer is single insulated, or if you cannot be sure, you need to use a metal box for the chassis, and connect that metal box to Earth Ground from your 3-prong power plug (assuming you are in the US). The :Line input needs to go through a fuse and then to the power switch, and then to your transformer.

    Is this the first time you've built a project with AC Mains input power?
     
  4. Apr 3, 2017 #3
    Sure
    Yes it is. I've only built low voltage DC driven things before. I'm more on the arduino side of things.
    I didn't know that the transformer rating determines the chassi material but it makes sense now that I think about it. The transformers I have been looking at are from china (aliexpress again) and I don't really know if you'll find either datasheets or safety ratings on these things. Here are two examples: http://bit.ly/2nPxjri http://bit.ly/2oRwqgY

    I do have a metal chassi from a power supply that crapped out on me. I could 3D-print just the front so that I can turn the knobs and all that and ground the chassi.
     
  5. Apr 3, 2017 #4

    berkeman

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    Staff: Mentor

    It's really best if you have a Mentor who can work with you in-person on a project like this, so you can learn how to build such projects safely. Basically you need to comply with all of the UL Safety rules that you would if you were designing a real product and submitting it to UL for approval.

    Here is a good tutorial website (notice the disclaimer about AC Mains Safety):

    http://www.markhennessy.co.uk/articles/power_supplies.htm

    I was lucky enough to build my first AC Mains powered project (a lab power supply) in my spare time while I was working a summer job as an intern at Tektronics in Oregon. Several of the very experienced engineers and technicians in our group gave me hints and watched as my project progressed, and were able to help me build it as if I were going to submit it to UL in the end. That power supply lasted a very long time. :smile:

    For liability reasons, we are limited in how much help we can give you on a project like this. Please look around your area for a hobby group or similar that you can link up with.
     
  6. Apr 3, 2017 #5
    Thanks for the article, I'll read it tomorrow when I have more time.

    An internship like that must be absolutely golden. I'm moving to Gothenburg this summer to start studying computer science (which is not without some electrical engineering but maybe not so much about AC mains circuits) and it would be awesome to do an internship like that.
     
  7. Apr 7, 2017 #6

    CWatters

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    Keep all the high voltage wires short and in the same area/corner of the case. Cable tie wires together so that should one become disconnected (eg from the switch) it cannot reach any other components (such as mounting screws for the switch). Use lots of heat shrink sleeving. If possible put all the high voltage stuff in a "case within a case" so you can work on the rest of the circuit with less risk to yourself.

    One of the hardest things to avoid is introducing mains hum. Avoiding that is a whole other story.
     
  8. Apr 12, 2017 #7
    Really good suggestions. Thanks!
     
  9. Apr 12, 2017 #8

    CWatters

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    No problem. Some years ado I was involved in designing set top boxes and similar IT equipment that had to meet BS EN 60950.
     
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