AC Voltage at USB Ground

Home › Forums › TinyG › TinyG Support › AC Voltage at USB Ground

Viewing 7 posts - 1 through 7 (of 7 total)
  • Author
    Posts
  • November 23, 2015 at 3:02 pm #8972
    mchezz
    Member

    I was researching online about properly shielding limit switch wires when I came upon several websites talking about USB ground issues. In two cases people had a USB cable burn up due to a voltage between the Tinyg USB ground and the PC. I have already powered up my new build with the Tinyg without issue, but decided to check for a potential across my USB grounds. With the usb cable unplugged from the Tinyg I measured 9 volts AC between the ground of the USB cable plugged into the PC and the Tinyg. Both the PC and the Tinyg were powered on when I took the measurement using a DMM. For additional information the Tinyg is powered by a MeanWell 24VDC supply. The supply also powers a motor speed control. The Tinyg ground is not connected to the power supply AC ground.

    Is it normal to see a potential at the USB ground between a PC and the Tinyg? As I mentioned I have not had any issues connecting the Tinyg to my PC, I am just being cautious. Thanks in advance for any feedback!

    November 23, 2015 at 3:26 pm #8973
    cmcgrath5035
    Moderator

    The Tinyg ground is not connected to the power supply AC ground.

    Are you sure? Many transformerles power supply designs have a high impedance coupling to the Output, and some are lower impedance.

    Try powering your tinyG from a 12V-24V wall wart. Not enough currrent to drive motors, but tinyG will be fine. Still see?

    November 23, 2015 at 5:26 pm #8975
    mchezz
    Member

    I don’t have a wall wart readily available to plug in. I will see about setting one up. As for the AC ground, I checked to see if grounds were connected using the continuity function on my DMM which may not have been the correct way. I just checked the potential between the AC ground on my power supply and the Tinyg ground and I see the same 9 volts AC I saw at the PC USB ground.

    November 23, 2015 at 7:18 pm #8976
    cmcgrath5035
    Moderator

    I have seen transformer-less AC to DC Power supplies that are reference to Line (black AC) as well as White (Neut AC), Green Ac of course is connected to white back at the AC breaker panel.
    So you might try measuring AC potential between your PC ground and the Power Supply output, both + and -. Typically this is a high impedance reference, so connecting the ‘- ‘of your power supply to pc ground should not create an ARC event, but make create an ‘ESD’ like spark event.
    You see this sometimes when connecting a Coax feed to a modern TV powered by a transformer-less power supply.

    So, I don’t think the 9V ac you are measuring is a big deal, but cannot be certain from the available information. You might try connecting PC ground to Power supply ‘-‘ with a 1K or 100 ohm resistor, or whatever you might have.

    If it is a ‘high impedance’ reference, the measured AC voltage across that resistor will be ~0 vac.

    November 23, 2015 at 9:54 pm #8978
    mchezz
    Member

    If I understood your suggestions correct this is what I tried. I connected resistors between the power supply “-” DC output on the Tinyg and to the PC USB ground (this is where I normally read 9 volts AC). With a 1k ohm resistor the potential across the resistor read .124 volts AC and when I did the same with a 10 ohm resistor the potential went to 0.

    November 23, 2015 at 10:34 pm #8979
    cmcgrath5035
    Moderator

    Yes, that is what I had in mind – the source of the AC ‘leakage’ is high source impedance and should not be an issue when you connect the tinyG USB to PC.

    If you are unsure of your power supply, always a good idea to tap the shield of the usb against the outer metal of the usb connector on the PC. A slight spark (ESD like) is fairly normal, a ‘snap’ sound or heavy arc means something is amiss.

    The burned USB cable item you reference was, IIRC, a clear power supply incompatibility with this application. I don’t have time to dig it up at the moment.

    One lesson learned from that event, if I recall – don’t attempt to put two transformerless 12V supplies in series to get 24 V, they don’t work that way (and usually indicate that on spec sheets).

    November 24, 2015 at 6:23 am #8980
    mchezz
    Member

    Great, thanks so much for your help on this!

  • Author
    Posts
Viewing 7 posts - 1 through 7 (of 7 total)
  • You must be logged in to reply to this topic.