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 C₁, a 2-µF AVX FFB16C0205K rated for 150V rms, provides a significant ac-voltage drop that reduces the voltage you apply to a diode-bridge rectifier, D₁. A flameproof metal-film resistor, R₁, limits current spikes and transient voltages induced in the ac-power line by lightning strikes and abrupt load changes. In this application, the ac current does not exceed 100 mA rms, and a 51Ω, 1W resistor provides adequate current limiting. R₂, a 5W, 160Ω Yageo type-J resistor, and D₂, a 1N4733A zener diode, provide 5V regulated power for the microcontroller, a Freescale C68HC908QT2.

The schematic shows a representative circuit for a microcontroller-based fan-speed regulator in which a thermistor senses air temperature and the microcontroller drives a fan's motor. Figure 2 illustrates a light-intensity regulator based on an inexpensive two-diode rectifier and a bidirectional-thyristor-lamp controller that share a common ground. IC₂, a Fairchild MOC3021-M bidirectional-thyristor-driver optoisolator, separates the lamp-return path from the microcontroller's ground return (Figure 3). In each of the three circuits, the Kingbright W934GD5V0 LED indicator includes a built-in current-limiting resistor (not shown).