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?**