Problem: board seriously fried. Desired outcome: measures to take in future.
Background: This project has been running for about a week on the 2560, and previously for a few weeks on an UNO clone. Board switch a result of program size/expansion. It sits on a shelf with others of the same ilk. Nothing is touched unless I need to plug in USB cable for update. The Arduino is a clone, connected to DS3231, LCD1602 via I2C Serial Interface board, LCD2004 also via 12C SIB, SD card adapter, HC-12 wireless, 5 leds [normally off, used to flag specific conditions, each in series with 470 ohm resistor. The unit receives data from another Arduino and logs to the SD, updating the two LCD displays.
Steps leading to failure: Plugged in USB cable [no other physical changes, nothing else touched, power jack remained connected]. went to Arduino s/w to make minor changes, would not upload to Arduino. While scratching my head, smelled something burning, then saw smoke coming from the shelf.
Post failure: Board seriously scorched around 5v regulator, can provide photo, but seems unnecessary. Visual wiring inspection revealed no obvious problems. Input to power jack measured at 10.06v. Replaced with new Arduino 2560 clone, matching connection for connection, connected USB but not power port [just in case of ?, seen ancedotal posts of possible conflict; a clone after all]. Uploaded my modified program, and all works fine. Disconnected USB, reconnected power jack and all still fine, 16 hours in. Also searched internet for possible problems related to such a failure, but found nothing to suggest probable cause.
Seeking: Words of wisdom from others far more knowledgeable and experienced than I. Could the dual connection of USB and power jack, be the root cause? Never experienced this with UNO clones.
I do not want to put more boards at risk in the future.