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:
We measure the voltage potential between board ground and HDMI cable ground and see 103V:
We usually do this sequanece:
- attach HDMI cable to the board
- attach UART cable to the board console
- 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.