Figure 3. A Simple Capacitive Power Supply for the MSP430 Energy Meter
Appropriate values of resistor R22 and capacitor C49 are chosen based on the required output current drive of the power supply. Voltage from the mains is fed directly to an RC-based circuit followed by rectification circuitry to provide a direct current (dc) voltage for the operation of the MSP430. This dc voltage is regulated to 3.3 V for full-speed operation of the MSP430. For the circuit in Figure 3, the approximate drive provided approximately 12 mA. The design equations for the power supply are given in the Capacitor Power Supplies section of MSP430 Family Mixed-Signal Microcontroller Application Reports (SLAA024). If additional drive is required, either an NPN output buffer or a transformer-/switching-based power supply may be used.