Overvoltage on +5V pin

[fugazza]

Hi Jiří,
on my opentherm board (rev4 + arduino UNO) I met problems with what I suppose is overvoltage that appears on +5V pin when connecting 24V to power supply. It is transient effect that appears only at the very moment of connecting the 24V, but it seems that at this moment the voltage passes through regulator or other path in circuit to the +5V pin. In stable state after power-on the +5V pin shows nice 5 volts. Plus it does not appear every time when I power on the shield, in fact I met two serious cases compared to approx. 50 power-ons when nothing visible happened.

First time when it happened, I connected 24V and imediately overvoltage passed through USB to my laptop, which instantaneously shut down. Luckily both comupter and USB port survived. Since that time during working with board I connected board to UNO by jumper wires and took care not to connect +5V pins in between - in order to protect my laptop.

Second time i finished my work, coupled board to UNO and powered on, which burned the UNO - it is not answering any more, impossible to upload sketch.

From now I will try to power-on the shield different. Up to now I had the power supply connected permanently to 230VAC and powered the shield by (dis)connecting the 24V line, which might be not optimal.

Did you met overvoltage difficulties? Any ideaas how to prevent them? For next revision of the board woudl it be possible to add an overvoltage circuit to +5V pin?