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I have a Pro Mini wired up to an LCD via an I2C board. At http://www.arduino.cc/en/Main/ArduinoBoardProMini it says the pro mini can accept up to 12v on the RAW pin as it has a voltage regulator on board. When I plug a 12v power supply into the raw pin, the LCD works fine initially, then slowly degrades into flickering more and more. It also does this with a 9v power supply. At 5v it works perfectly. Any ideas what's going on?

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If the LCD has a backlight you are absolutely over heating the regulator on that board by running it at 9 or 12V: At 12V with 100mA for the backlight and using 175C/W thermal resistance for the regulator: Tj = 25C + ((12V - 5V) x 100mA x 175C/W) = 147C That regulator goes into thermal limit at 125C.

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