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Challenge:
Design 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.
Solution:
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).
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