Beware! Non grounded TV monitors can damage your OLinuXino


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We use cheap TV HDMI monitor to test OLinuXino boards, and we noticed that several OLinuXino boards got burned when connected to the TV monitor and USB-SERIAL-CABLE to the console. UART Tx signal got burned when TV is connected, so we decided to investigate further.

The TV monitor have no grounding on the power supply cable:

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We measure the voltage potential between board ground and HDMI cable ground and see 103V:

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We usually do this sequanece:

  1. attach HDMI cable to the board
  2. attach UART cable to the board console
  3. plug power supply cable

exactly between point 1 and 2 board burns as 103V voltage potential is between HDMI and UART, the USB cable is connected to PC and grounded.

Board is still safe when UART is not connected but power supply jack just you will see sparks when connect HDMI cable with such TV.

What is the solution? Adding USB-ISO between USB-SERIAL cable and PC solves the burning issues.

More safe solution would be to open the TV and to add grounding cable to chassis and replacing the power connector to one who have grounding too, then the TV will have no such big voltage potentials.

The bottom line is avoid TVs which have no grounding on their power supply cable! If you can’t avoid them try to add grounding yourself, if this is not possible please use USB-ISO to save your OLinuXino from these high potential voltages.

15 Comments (+add yours?)

  1. sh
    Jan 22, 2014 @ 18:33:21

    Does it mean that it is not safe to use such TV If the board is powered by USB OTG?

    Reply

  2. Enoob
    Jan 22, 2014 @ 19:16:04

    Please explain for electro noobs how to find out whether a TV power supply cable has grounding or not. Thanks in advance.

    Reply

    • OLIMEX Ltd
      Jan 22, 2014 @ 19:24:46

      power supply plug with grounding have metal plate for grounding like on this picture:

      power supply plug without grounding have just two pins for power like on these pictures:

      Reply

  3. Keeper
    Jan 22, 2014 @ 20:53:55

    For American “electro noobs”, the grounded power supply cable would have a three-prong connector.

    Reply

  4. Stan
    Jan 23, 2014 @ 18:40:42

    I see crown tv, but mine samsung is same – wthout 3th wire – why manufactures make this shit? 1m cable is around 1bg lv????

    Reply

  5. Dddd
    Jan 24, 2014 @ 06:46:48

    what is a USB ISO? How does it look like and how can I get one? Thanks!

    Reply

  6. David
    Jan 27, 2014 @ 11:29:41

    The problem may not be that the TV is ungrounded. I have a similar/same problem here in Indonesia and I traced the problem to unbalanced phases. I had one computer device plugged into one wall socket and the display device plugged into a different wall socket near by, Luckily when I connect connectors for the first time, I measure the ground difference. Mine was a little over 100VAC (mains are 220 VAC/50Hz). I went down stairs and measured across the phases at the fuse panel and found the same thing. Strangely, the wiring looked like it was a delta configuration, not wye (separate grounds for each phase), but the voltage potential between the phases at a little over 100V could be shorted out with very little current drawn. I deem the problem to be the separate grounds that have different series potential accumulated between the fuse panel and the building distribution transformer which at around 100 meters is rather far away for the relatively small/cheap cable gauge used (high copper losses). Once the potential difference was nulled, by adding gimmick shorts, there were no more ground potential problems. I’m still convinced the building is wired improperly, but the building owner doesn’t care what I think.

    Reply

  7. landswellsong
    Dec 28, 2014 @ 17:21:29

    One question in that aspect, will I get my board burned if I use a HDMI-to-VGA adaptor like this?

    Reply

  8. Jakub Piotr Cłapa
    Mar 10, 2015 @ 16:42:44

    The problem is related to two filtering capacitors between the phases (L and N) and the ground (PE) in the power supply. If the device does not have a proper PE (protective earth, ground) connection but the two filtering capacitors are still there, they form a reactive divider that is normally pretty high impedance (not much current flows through ~100nF capacitors at 50Hz) giving you 1/2 AC voltage potential. Unfortunately when you are connecting two separate circuits together the impulse current and voltage can be very high (half of AC voltage with current limited only by the cable resistances). If this does not discharge though a proper ground connection and cable shields shields then no amount of ESD protection on the data pins will be able to withstand it.

    Reply

  9. Mateusz Baran
    Mar 23, 2015 @ 18:41:51

    Hi, I have one questions. You wrote that uart got burned when you connect it to grounded PC. But what happens if I will connect it to the notebook that is not grounded? Can I do it without USB-ISO or extra TV chassis grounding?

    Reply

  10. jpclapa
    Mar 23, 2015 @ 19:04:34

    Mateusz: If you connect the laptop to the power adapter then it is grounded. Otherwise you should be ok.

    In general the discussed TV is designed incorrectly. It should either have the ground wire or not have any filtering caps in the power supply.

    Reply

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