Low-Cost AC Supply
Applying the filtered and rectified AC input to a high-input-voltage linear regulator is the simplest way to produce low current level from an AC source. However, it will causes the power dissipation in the regulator. The power dissipation can be reduced by using a switching regulator but it will increases the cost if we use a switching regulator. This circuit is designed to solves that problem. This circuit converts 115VAC, 24VAC, or higher levels of AC power to 5.0V or 3.3V. Here is the circuit :
The MAX6330 shunt regulator is used in this circuit. The MAX6330 shunt regulator can supply up to 50mA. The IC1 contain power-on reset and shunt regulator that was connected with coupling capacitor in conjunction will transfer the energy to the regulator with negligible power loss. The IC1 is an active shunt, so before applying it, we must rectify the AC voltage. This circuit uses a simple half wave rectifier which will save the cost. To hold the charge during off cycles, the capacitor follows the rectifier.
In this circuit, C1 act as the transfer capacitor and C2 act as energy storage. As half-wave rectifier, this circuit uses D1 and during negative cycles, the transfer capacitor is discharged by D2. R1 is used to limit surge current during the discharge of C1 and high-voltage transient testing. [Source: MAXIM Application Note]