A microcontroller requires dc operating power in the 2 to 5.5V range, an amount that a battery or a secondary power source can easily supply. However, in certain situations, a microcontroller-based product must operate directly from a 120 or 220V-ac power outlet without a step-down transformer or a heat-producing, voltage-decreasing resistor. As an alternative, a polyester/polypropylene film capacitor rated for ac-line service can serve as a nondissipative reactance (Figure 1). Capacitor C{sub}1, a 2-μF AVX (www. avxcorp.com) FFB16C0205K rated for 150V rms, provides a significant ac-voltage drop that reduces the voltage you apply to a diode-bridge rectifier, D{sub}1. A flameproof metal-film resistor, R{sub}1, limits current spikes and transient voltages induced in the ac-power line by lightning strikes and abrupt load changes.
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