7805 & Wall-wart Experiencing Voltage Drop-out PUBLIC

A 7805 linear voltage regulator can't quite maintain its output voltage. Can you fix it?

V1, XFMR1, and BR1 model an unregulated wall-connected AC-to-DC adapter ("wall wart").

R1 models a load circuit, such as a microcontroller and LCD and a few sensors, that requires about 50mA.

U1 is a 7805 linear voltage regulator. It does a great job maintaining 5 volts on its output... until its input voltage (node "cap" here) drops below 7.5 volts.

Use the time-domain simulation to experiment.

There are several ways to fix this circuit, all of which you can try and see in CircuitLab:

1. change XFMR1 and specifically the turns ratio. (What's the potential downside to this approach?)
2. change C1's capacitance. (What's the potential downside to this approach?)
3. change C2's capacitance. (What's the potential downside to this approach? Does it take more or less capacitance than putting it on the input side where C1 is?)
4. change U1 to be a low-dropout-voltage model voltage regulator. (What's the potential downside to this approach?)

See also

AC Power Supply (Transformer and Bridge Rectifier)