I'm purposely dropping AC from its normal levels (230VAC @50Hz) just to analyse the behaviour of a DC motor drive board that I have.
When the AC mains voltage falls below 180V, the 7 segment display of my board goes off. The circuit derives the 5V for the MCU and 12V for the display driver ULN2003 from a 12V step down transformer, a bridge rectifier and a 7805 regulator circuit. ULN's 12V supply is given from the input pin of 7805.
I'm curious about the display turning off problem when the AC voltage drops. As the display needs 12V only, can't it still find the power from the 180V AC? What would be the better power supply solution for such a noisy mains?