a circuit that will give a marginal AC to DC linear power supply a voltage boost, so it can
maintain regulation at low line voltages.
I solved this problem many years ago with the circuit shown
below. The added components are wired at the AC input and the + DC output of an existing
bridge rectifier circuit. The components act as a full wave “charge pump” which will
boost the DC output voltage across the main filter capacitor C3. Without the boost
circuit, when the AC line voltage drops to 105vac, the DC output voltage only reaches 11.5
volts, too low for a 12v voltage regulator. With the parts added, the output voltage
increases by about 2 volts, boosting the voltage to 13.5 volts. This is just enough for
many low voltage drop 12 volt regulators. Since a fair amount of current is pumped in
and out of the 220uF caps, they should be the type rated for high ripple current or low
equivalent series resistance (ESR).