I will appreciate very much if you explain us the function of TL431 in wich the cathode is connected to the ground of 7805 regulator and the reference voltage is conected to a volage divider between 7805 output and ground ; so the 7805 output can deliver between 1 to 5 volts by changing the value of the voltage divider ,This circuir appears in the Forrest Mims Engineer's Notebook Thnak you very much
The TL431 is never connected to the ground via the cathode. The cathode and ref are connected together, then connected to the GND side of the 7805. This creates a 2.5V reference. The anode then goes to the ground.