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Connecting TV to PC with HDMI, fried TV mainboards...

Discussion in 'General Electronics Discussion' started by shanyiqua, Jul 24, 2020.

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  1. shanyiqua

    shanyiqua

    6
    0
    Jul 24, 2020
    Hi!

    3 TV mainboards fried by me when trying to connect to PC with HDMI cable, and the shielding part of HDMI cable touching with the other, mostly to the PCs back panel. Also i'm seeing sparks too sometimes.

    The mains voltage is ok, just like ground, and earthing (however none of the TVs have earthed plugs), but the PC have of course. The in house grid have GFCI too.

    Of course i know that i should not connect HDMI cables while the devices are connected to grid, especially not, when both are on. But i did that before many times without problem, while also seen sparks when HDMI shielding touched the other device.

    So after these happened, i started to look for voltages: ALL of the TV chassises (3 different type tv from different era, and manufacturer), have half of main voltage AC referenced to earth, so HDMI shieldin has too, which is connected to chassis.

    On another forum topic they said this is a common issue with LCD tv-s, because the filtering capacitors in these cause half of mains voltage in chassis.

    Is this true? I should connect hdmi safely only if both devices are off the grid, or if running on battery?

    I'm thinking the spark happens because an unearthed device touches the earthed PC chassis. But if i earth the TV chassis (wire from earth to one of the TV's wall screw thread (which are connected to internal chassis, and i measured that half mains voltage between that and earth too), would it eliminate this sparking problem?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,090
    2,157
    Nov 17, 2011
    This happens if the TV chassis is not connected to earth, as is obviously the case here:
    This sounds questionable at best to me. I personally think this is a bad design.
    The TV's power supply is seemingly constructed along this principle:
    [​IMG]
    Source: TDK-Lambda.
    The Y-capacitors to the left are intended to suppress noise. But if the chassis ground is not connected to Earth, the chassis obviously is at 1/2 mains voltage due to the capacitive divider. Connecting the TV and the PC while not connected to mains will prevent a spark from being created, but any residual current through the Y-capacitors will then flow through the HDMI cable's shield and the PC power supply to earth. There's no guarantee that this will work safely and not create problems.
    Worst case: Accessible parts of the PC receive dangerous potential though this connection and pose a danger to the user!

    To circumvent this issue use isolated HDMI interfaces as e.g. these ones. Or get a better TV with properly grounded chassis.
     
  3. bertus

    bertus Moderator

    754
    270
    Nov 8, 2019
    Hello,

    The new TV I have has also no grounded mains connector.
    It uses a mains cord like this:

    Euro-2-pin-plug-to-iec-c7-mains-power-cable-black-p9234-7294_zoom.jpg
    These are quite common in the region I live.

    Bertus
     
  4. shanyiqua

    shanyiqua

    6
    0
    Jul 24, 2020
    same here 5 of 5 tvs at home have same cord like yours. the only lcd tv i seen with grounded connector, is so old that does not have digital tuner, only analog, and have some hdmi connectors
     
  5. shanyiqua

    shanyiqua

    6
    0
    Jul 24, 2020
    It's even more stranger. I've just plugged an USB hub into my pc's back USB connectors, and it's also sparked as touching the metal back panel. But here is the twist: everything is on the same switchable surge, etc protected distributor, which was OFF!
    The only thing that was on grid is WIFI repeater, what is connected to the ethernet switch which was off too.
    I also have a car BT transmitter, which is connected to my (off) speakers on 3,5 jack, and have a small 500mA battery, so also connected to the USB hub which charges it while playing music from BT to jack. So i've disconnected this BT thing from the hub, and there were no sparks anymore.

    How?
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    10,090
    2,157
    Nov 17, 2011
    So maybe the issue is with the PC and not the TV at all?
    Is your PC correctly connected to earth? Is the earth connection in your mains installation in good order?
    On or off doesn''t necessarily mean disconnected from mains as power today often is soft-switched only, meaning there is always some standby power which in turn implies connection to mains.
    This is an indication that the BT transmitter or rather its power supply could be the culprit.

    To clearly isolate the issue I suggest you locate the issue by a methodical approach.
    1. Disconnect everything but PC from mains.
    2. Then for each device in the list of devices attached to the PC:
      - Connect only one device, start e.g. with the device
      - Check for sparks and correct function.
      - If everything is o.k., add the next device, leaving the already connected devices connected
      - If the issue turns up (sparks) disconnect the newly added device and verify that the issue is no longer present.
    You will now have isolated the device causing the trouble. You can now try to use a replacement device or a replacement power supply to that device.
    Keep in mind that a correct mains installation is a prerequisite for a good earth connection. When in doubt, have a qualified electrician check your installation.
     
    Martaine2005 likes this.
  7. shanyiqua

    shanyiqua

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    0
    Jul 24, 2020
    as i wrote the PC wasn't connected at all to grid, nor to earth. as the distributor was switched off.

    How can a 500mA 3.2V battery from an USB cause those sparks?
     
  8. Martaine2005

    Martaine2005

    2,849
    756
    May 12, 2015
    Please try what was advised by @Harald Kapp .
    Disconnect each and every item. Not just turn off at ‘distributor’. The EARTH is not switched. Only LIVE and NEUTRAL are.
    Unplug them as suggested and try again.

    Martin
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    10,090
    2,157
    Nov 17, 2011
    It is definitely not the battery causing the sparks but some high voltage creeping to where none should be present.
     
  10. shanyiqua

    shanyiqua

    6
    0
    Jul 24, 2020
    between house earth and ground (literally, one of multimeter's end pushed into ground) have no ac or dc voltage. Earth and neutral have no voltage between. Live-neutral= stable 228V, live-earth same, live-ground same. What would be wrong with earth?
     
  11. shanyiqua

    shanyiqua

    6
    0
    Jul 24, 2020
    new thing: the failed TV mainboard was checked by the factory, and fried from the LAN simple utp connector!
    But i don't know how it's possible, if the ethernet switch connected to there has an external 5 volts 1A power supply, and works fine with other devices (wifi AP, and a PC).
     
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